Slap MessageBoards
Skateboarding => Shoes & Gear => Topic started by: Roisto on December 12, 2019, 05:36:01 AM
-
Does Friction Depend on Contact Patch Width?
An Experiment - Part 1: Unloaded Static Friction
Abstract (aka tl;dr)
Made a bunch of measurements and calculations of same durometer and material skateboard wheels with different width contact patches to compare how the contact patch width affects grip. Turns out it doesn’t as is to be expected.
Introduction
Some of you probably know me as the annoying fucking guy who constantly argues that wheel width has no effect on grip. Some have probably gotten tired of it. I have too. But instead of giving up on it I decided to go full in.
Materials & Methods
So I got myself a set of 54 mm 99A Spitfire Formula Four Classics and a set of 58 mm 99A Spitfire Formula Four Conical Fulls (Pictures 1 & 2) and devised a bunch of experiments to actually get proof of how it actually is.
(https://i.imgur.com/WESX07t.jpg)
Picture 1 - The wheels
(https://i.imgur.com/x9imFW8.jpg)
Picture 2 - The wheels again
According to the Spitfire website (https://www.spitfirewheels.com/formulafour/) the contact patch width for the 54 mm Classics is 16.5 mm and for the 58 mm Conical Fulls it is 27.3 mm. I measured this myself too and it seem to hold true (Pictures 3 & 4). So the Conical Fulls I tested are 65% wider than the Classics.
(https://i.imgur.com/q6RyivY.jpg)
Picture 3 - Conical Full 58 mm width
(https://i.imgur.com/jWtiRCt.jpg)
Picture 4 - Classic 54 mm width
According to many people here on Slap (& elsewhere) the Conical Fulls should therefore have 65% more grip.
More proper term for grip would be static friction. The force that is required horizontally to impact on the object to make it break into a slide. This can be measured in various ways. I got myself a 800x710 mm sheet of plywood from a local shop (Picture 5). I used birch plywood cuz it was cheap and it really doesn’t matter what it is for this test.
(https://i.imgur.com/L4Jx7z1.jpg)
Picutre 5 - The plank
I placed the skateboard on the plywood sideways and lifted the plywood slowly until the board starts sliding. I measured the angle where the board starts sliding with my iPhone 8 and an app called Bubble Level which shows the angle to one decimal accuracy. To keep things repeatable I taped the phone to the plywood so it wouldn’t slide or budge. Lifting I did by hand slowly trying to avoid vibrations that would help the board breaking loose.
To make sure the board wouldn’t roll off from the side of the plank I tightened the axle nuts so tight that the bearings were essentially locked (Picture 6).
(https://i.imgur.com/4HvUxzv.jpg)
Picture 6 - Nuts cranked the fuck down
I made 10 replicate measurements with each set of wheels.
Both wheel sets used were brand new and in packaging. The plywood sheet was also new and fairly clean. I “broke in” both sets of wheels by tilting the plywood sheet with the skateboard until it slid to the bottom. I did this 10 times after which I rotated the skateboard 180° and repeated the procedure 10 more times. I did the same for both wheel sets.
For the setup I used a symmetrical Real Ishod 8.3 Twin Tail Slick that has been slightly skated, Thunder 149 Titanium Lights which have also been very lightly skated. (Picutre 7) Bearings were very lightly skated Spitfire cheapshots (Picture 8), bolts Thunder Allen 1” and I also used Bones 1/8” risers just cuz they were on the setup to begin with. I didn’t use the axle washers to help me lock the wheels better by over-tightening the nuts.
(https://i.imgur.com/kz6bVWs.jpg)
Picutre 7 - Board and trucks and such
(https://i.imgur.com/wor90ei.jpg)
Picture 8 - The bearings
Coefficient of friction is what is used to describe friction between two surfaces. From the sliding angle the coefficient of friction can be obtained by taking the tangent of the angle. With the coefficient of friction you can determine how much force will be needed to get the object to slide. If we assume an object to weigh 50 kg and the coefficient of friction of being 0.5, on earth the normal force the surfaces pushes back up on the object is (50 kg * 9,81 m/s^2) = 490.5 N. With the coefficient of friction being 0.5 we would need a horizontal force of 0.5*490.5 N = 245,25 N to get the object to slide.
Results
The coefficient of friction for each measurement can be seen in figure 1.

(https://i.imgur.com/H3b2kY7.png)
Figure 1 - Measured static friction coefficients of both wheels
There are slight variations in the coefficient of frictions but overall they are fairly close to each other. This chart alone doesn’t really visualize the situation so I calculated values for both wheels based on the other wheels’ measured data using the assumption that contact patch width is directly proportional to friction. These values along with the original measured values can be seen in Figure 2.
(https://i.imgur.com/oHb0iXa.png)
Figure 2 - Measured static friction coefficients and calculated “assumptions” for both wheels
Conclusions
As can be clearly seen from the bars in figure 2, increasing/reducing the contact patch width does not increase/reduce the friction of the wheel in proportion or at all.
A possible source of error on my experiments are vibrations on the sheet of plywood due to me lifting it by hand. However I did try to minimize bad vibez by taking it easy and listening to some good music while lifting. There were also no significant outliers in the data suggesting the presence of bad vibez.
Another possible source of error is contamination of the sliding surfaces. I made sure not to touch the sliding surfaces with my hands and all materials were clean and new. Nor were there any noticeable changes suggesting contamination would have at some point affected the slide.
EDIT: Another source for variability between wheels is manufacturing tolerances. I have no idea how uniform the batches coming from the Spitfire factory are but I would think there might be slight variations in various properties of the wheels between batches.
This is part 1 of my friction testing of skateboard wheels. This test only takes into account static friction on an unloaded skateboard. Possible deformation of the wheels under load and its effect on friction has not been taken into account in this experiment.
In conclusion it seems that the laws of physics also apply to skateboard wheels. I will continue my tests however just because I’m fed up with wrong information being spread here about this subject.
-
so ur one of them brainy types huh
-
Nice fuckin science nerd
No seriously nice science
But also serious about the nerd part
Maybe the much larger conical full wheels (aside from increasing slightly the weight) changes the center of gravity to some small degree... Which might explain the consistent differences in measurement? I don't fuckin know I'm not a nerd.
-
Thanks for doing this, I'll be following along and look forward to future tests as well.
-
Tldr buy full conicals rite
-
There is a lot going on with wheels, static friction is only a tiny part of it.
If you really want to nerd out on it/give yourself a brainache you can read up on racing car tyres (a skateboard wheel is basically the tyre to your bearing's wheel) here:
http://theracingline.net/2018/race-car-tech/race-tech-explained/tyres-road-interaction-contact-patch-grip/ (http://theracingline.net/2018/race-car-tech/race-tech-explained/tyres-road-interaction-contact-patch-grip/)
and here:
http://theracingline.net/2018/race-car-tech/race-tech-explained/tyres-lateral-forces/ (http://theracingline.net/2018/race-car-tech/race-tech-explained/tyres-lateral-forces/)
and here:
http://theracingline.net/2018/race-car-tech/race-tech-explained/tyres-load-sensitivity/ (http://theracingline.net/2018/race-car-tech/race-tech-explained/tyres-load-sensitivity/)
and here:
http://theracingline.net/2018/race-car-tech/race-tech-explained/tyres-combined-tyre-forces/ (http://theracingline.net/2018/race-car-tech/race-tech-explained/tyres-combined-tyre-forces/)
There's more but those are the most revelant to skateboard wheels as we don't have to deal with effects of driven wheels (unless your an e-board kook) or pneumatics (mountain boarders I guess).
Enjoy!
-
https://www.roadandtrack.com/car-culture/videos/a4835/thing-you-thought-you-knew-points-of-contact-tire-patches/
Width won't do it. Merely increasing the width of a tire doesn't increase the area touching the pavement. It just makes it a wider, shorter patch.
-
What was your control?
-
http://racingcardynamics.com/racing-tires-lateral-force/
Hysteresis
-
"According to many people here on Slap (& elsewhere) the Conical Fulls should therefore have 65% more grip."
- who says this? Are we just assuming that grip/static friction scales exactly linearly with the width of the contact patch? I was not under the impression that the claims being made were that grip/friction increases at the same rate as the increase in width of the contact patch.
If grip/static friction doesn't scale linearly with width of the contact patch, then your results (besides one outlier which can be easily attributed to human error) show that conical fulls consistently have less static friction than classics (of sometimes up to a difference of 0.1, which is not a trivial difference considering this value usually ranges from 0 to 1). Highlighting the major difference in coefficients when you calculate assuming that friction is directly proportional to the contact patch seems to me to be a red herring that tries to make the difference in your measurements appear less significant. Unless I missed the debate where the claim width scales directly with grip/friction was made? It would be good to link also the debates that you've had so people have full context.
Also, I suspect if you had the wheels on a set up on the board and put more weight on it (idk tape a bunch of weights to your board or something), it probably would have made the difference between the two wheels more pronounced. Seems to me that a board without the weight of a person standing on it probably isn't going to give results that differ by a very noticeable margin, making it easier to attribute the difference to experimental variance and human error.
I don't have a background in physics/material science so I'm definitely no expert. Intuitively you'd think a wider wheel results in more grip, even if not by much (this is also my anecdotal experience). I've heard that certain characteristics of the surfaces may not necessarily be caught by the formula defining the friction coefficient, though I'm definitely not willing to die on that hill so not taking any sides for now. Just pointing out some things that don't exactly convince me, yet.
Interesting though, looking forward to the next part.
-
I thought it was common knowledge here that weight not surface area was the primary factor in grip/friction in wheels/tires
-
i appreciate this. more pls.
-
I thought it was common knowledge here that weight not surface area was the primary factor in grip/friction in wheels/tires
Foy Vs Kader for example....? ;)
-
Did you have some thing to weigh the board down?
A grown man on the board is not the same as just the board, maybe put something fairly heavy on and see what happens?
-
Roisto, can you please post the numbers for each of the 20 measurements in figure 1?
-
Nice experiment mate, takes me back to physics class!
As a guy who only ridden 90a wheels the last 10 years (except for some Bones 100, which I never rode much) Id like to see a experiment of duros next 🤙
In my head it makes sense that a 100a ish wheel wont 'grip' the ground much better because of a few mm difference. A lower Duro on the other hand would allow the wheel to dig into the surface more.
-
Nice experiment mate, takes me back to physics class!
As a guy who only ridden 90a wheels the last 10 years (except for some Bones 100, which I never rode much) Id like to see a experiment of duros next 🤙
In my head it makes sense that a 100a ish wheel wont 'grip' the ground much better because of a few mm difference. A lower Duro on the other hand would allow the wheel to dig into the surface more.
That kinda of goes into different formulas rather than different durometers. Spitfire F4s 99a and Bones STF 99a ride and grip much differently because of the formula, not the durometer. Durometer is a whole nother topic that really deserves it's own thread.
As for this experiment, great work. This deserves to be in some sort of book about skate science.
-
Nice experiment mate, takes me back to physics class!
As a guy who only ridden 90a wheels the last 10 years (except for some Bones 100, which I never rode much) Id like to see a experiment of duros next 🤙
The problem with duros is that the coefficient of friction depends on the formula of the wheel. As a general trend, softer is grippier - but a specific wheel may be above or below the trend line. There are 101s that probably have higher static friction than 99s.
Then you have the problem that the A scale is a shit way to measure wheel hardness. The A scale only goes up to 100 and isn't accurate over 95. 101a is just marketing speak and has no scientific basis. One manufacturers 99 may be harder than someone else's 100 or 101. Wheels should be measured on the B scale like Bones does - but we are so used to the A scale that people can't transition over to it.
Lastly, there are two coefficients of friction. Static Friction (how hard it is to break the wheel loose) and Dynamic Friction (how does it grip/slide once the wheel is moving sideways). They don't correlate directly. You can have a formula with a high static and low dynamic, low static and high dynamic, etc...
-
Love the paper outline and the low-key nerdery. Curious to see what's coming next. Would gnar if I could.
-
Grip has more to do with lip profile and deformation. Wheels with a wider contact patch tend to have more square lip profiles, and more deformation.
-
Nice fuckin science nerd
No seriously nice science
But also serious about the nerd part
Maybe the much larger conical full wheels (aside from increasing slightly the weight) changes the center of gravity to some small degree... Which might explain the consistent differences in measurement? I don't fuckin know I'm not a nerd.
That's a good point. It might have an effect. I could test the 58s without risers to see if that makes a difference. 🤔 Overall the difference is very small though and even though it is systematic, I'd say it can be explained by experimental error and manufacturing tolerances.
What was your control?
None. This is not about the absolute values because they are irrelevant. This is about the differences between the two wheels.
"According to many people here on Slap (& elsewhere) the Conical Fulls should therefore have 65% more grip."
- who says this? Are we just assuming that grip/static friction scales exactly linearly with the width of the contact patch? I was not under the impression that the claims being made were that grip/friction increases at the same rate as the increase in width of the contact patch.
If grip/static friction doesn't scale linearly with width of the contact patch, then your results (besides one outlier which can be easily attributed to human error) show that conical fulls consistently have less static friction than classics (of sometimes up to a difference of 0.1, which is not a trivial difference considering this value usually ranges from 0 to 1). Highlighting the major difference in coefficients when you calculate assuming that friction is directly proportional to the contact patch seems to me to be a red herring that tries to make the difference in your measurements appear less significant. Unless I missed the debate where the claim width scales directly with grip/friction was made? It would be good to link also the debates that you've had so people have full context.
Also, I suspect if you had the wheels on a set up on the board and put more weight on it (idk tape a bunch of weights to your board or something), it probably would have made the difference between the two wheels more pronounced. Seems to me that a board without the weight of a person standing on it probably isn't going to give results that differ by a very noticeable margin, making it easier to attribute the difference to experimental variance and human error.
I don't have a background in physics/material science so I'm definitely no expert. Intuitively you'd think a wider wheel results in more grip, even if not by much (this is also my anecdotal experience). I've heard that certain characteristics of the surfaces may not necessarily be caught by the formula defining the friction coefficient, though I'm definitely not willing to die on that hill so not taking any sides for now. Just pointing out some things that don't exactly convince me, yet.
Interesting though, looking forward to the next part.
That's what the common misconception is. Larger surface area meaning larger friction. So if a surface area is 1 unit of area doubling that area would also double the force needed to make the object slide, so the friction coefficient would have to double.
Linking the debates would be best, I agree but they are numerous and scattered all over the forum in several random topics. Unfortunately I can't be bothered to go find those.
Like in my reply to yourbreakfast I think the differences between the two wheels can be explained by various things. While they are systematic they are not very big. I'm quite sure you could not notice the difference, making it completely insignificant.
I am not trying to mask or hide anything. I put those calculated bars there to make the point that the surface area/contact patch width being directly proportional to friction simply isn't true.
I can't see putting more weight on the board changing anything but I will test that later on just not with this exact setup as the plywood would bend and manually tilting more weight repeatedly tens of times with 0.1° accuracy will be too difficult.
I thought it was common knowledge here that weight not surface area was the primary factor in grip/friction in wheels/tires
Are you talking about the weight of the wheels? Because the weight differences of the setups including the rider with different wheels are minute and completely insignificant.
Did you have some thing to weigh the board down?
A grown man on the board is not the same as just the board, maybe put something fairly heavy on and see what happens?
For this experiment, I did not as I explained to tzhangdox. For my future experiments I plan on testing if weight makes any difference.
Roisto, can you please post the numbers for each of the 20 measurements in figure 1?
Sure thing.
Here's the original angle of measurement values I recorded based on which I made all the calculations:
Measurement Classic 54 mm (measured) Conical Full 58 mm (measured)
1 33 32,2
2 33 32,2
3 33,7 33,1
4 32,7 31,7
5 32,6 33,2
6 33,9 31,3
7 33,6 32,3
8 33,2 32,8
9 33,7 31,7
10 33,8 30,3
Formatting is a bit wonky but it's easier to copy from that than a pic if that's what you want. First column is measurement number second angle of friction values for Classic 54 mm and third angle of friction values for Conical Full 58 mm.
Nice experiment mate, takes me back to physics class!
As a guy who only ridden 90a wheels the last 10 years (except for some Bones 100, which I never rode much) Id like to see a experiment of duros next 🤙
In my head it makes sense that a 100a ish wheel wont 'grip' the ground much better because of a few mm difference. A lower Duro on the other hand would allow the wheel to dig into the surface more.
This would be interesting but as others have pointed out after your post grip vs. durometer isn't all that simple and also the durometer values are sadly largely bullshit. Also many new wheels have treads on them which might affect the measurements and measuring worn wheels might give all sorts of values due to dirt on the wheels. Also I don't have the desire to buy any wheels than 99A F4s and maybe some soft cruiser wheels. I'm all set on cruiser wheels at the moment though and with my use they last for years so it is unlikely I'll be doing such experiments. But this experiment is very simple to do and anyone could do it themselves. I'm sure it would be appreciated by many if you were to make some tests between different brands and durometers.
Grip has more to do with lip profile and deformation. Wheels with a wider contact patch tend to have more square lip profiles, and more deformation.
I don't really see the lip profile making any difference in grip between the Classics and Conical Fulls except on extremely rough terrain. Those straight cut longboard wheels might catch on smaller roughnesses easily though so likely that'd kinda provide more grip.
Deformation of a 99A durometer wheel under normal load I suspect to be minimal. I haven't come up with an easy way to measure the deformation at home without spending way too much money on it. I am willing to measure it if anyone has good suggestions on how to do it.
And generally for everyone:
Thank you for your interest in this. It was fun to do and I like civilized and thoughtful discussions about this. I have more experiments planned but those will require buying more stuff to get them done and also require a bit more time to set up so they likely won't happen very soon. I guess I'd have to do them before the summer though as I plan on riding those Conical Fulls once outdoor season starts again.
-
Your test seems like it'd only apply to ultra smooth ground, like marble tile. And even a matte finish is gonna have texture
Concrete and asphalt are way rougher
http://www.mate.tue.nl/mate/pdfs/8147.pdf
6.1 Friction
Friction of polymers is closely related to their viscoelastic behavior. Generally speaking, the co- efficient of friction increases with sliding velocity until a maximum value is reached at a certain speed, followed by a decrease of the friction coefficient. The viscoelastic properties of polymers depend strongly on temperature. Grosch showed the relation between the dependence of the fric- tion coefficient due to sliding velocity and temperature and the viscoelastic deformation on strain rate and temperature.
The friction force for soft rubber sliding on a smooth surface (perfect contact) is more or less constant, independent of the load. Thus the coefficient of friction decreases continuously as the pressure increases. However there is also a coefficient of friction, independent of pressure, for harder rubber compounds, sliding on a rough surface. Apparently in this case contact is incom- plete. An increase in pressure creates a larger true area of contact and hence a larger frictional force. This shows the influence of the temperature, sliding velocity and real contact area.
The current theory of rubber friction and surface roughness, which capture all these properties can be found in the publications of Persson, (see review paper Persson, 2005). The surface roughness is characterized by a fractal description, which exhibits geometrical self-similarity. These results are based on the early studies of Grosch, taking into account that sliding friction of rubber has the same temperature dependence as that of the complex elastic modulus. He states that the friction force is related to the internal friction of the rubber, which is a bulk property. The hysteretic friction component is determined by sliding of the rubber over asperities of a rough surface. These oscillating forces lead to energy dissipation. Every length scale up to the largest particles of asphalt, can be related to an excitation frequency. As a result of energy dissipation heat is generated, in a recent article this local heating of the rubber is also taken into account. Although this theory provides knowledge of the physical origin of rubber friction it has some drawbacks. Exact knowledge of the contribution of each length scale is needed. Therefore it has, at the moment, limited practical use, since variable amounts of unknown foreign materials in the interfacial region makes it almost impossible to derive quantitative estimates of the absolute magnitude of friction from physical properties of the rubber and surface. So assumptions must still be made.
https://www.brachengineering.com/content/publications/Wheel-Slip-Model-2006-Brach-Engineering.pdf
Page 4, mechanical adhesion
http://multiscaleconsulting.com/publications/Rubber_Friction_and_Tire_Dynamics_A_Comparison_of_Theory_with_Experimental_Data.pdf
Check out this experiment
http://www.dimnp.unipi.it/guiggiani-m/Michelin_Tire_Grip.pdf
Page 17, The mechanisms involved in the rubber-road interface friction
-
Expand Quote
"According to many people here on Slap (& elsewhere) the Conical Fulls should therefore have 65% more grip."
- who says this? Are we just assuming that grip/static friction scales exactly linearly with the width of the contact patch? I was not under the impression that the claims being made were that grip/friction increases at the same rate as the increase in width of the contact patch.
If grip/static friction doesn't scale linearly with width of the contact patch, then your results (besides one outlier which can be easily attributed to human error) show that conical fulls consistently have less static friction than classics (of sometimes up to a difference of 0.1, which is not a trivial difference considering this value usually ranges from 0 to 1). Highlighting the major difference in coefficients when you calculate assuming that friction is directly proportional to the contact patch seems to me to be a red herring that tries to make the difference in your measurements appear less significant. Unless I missed the debate where the claim width scales directly with grip/friction was made? It would be good to link also the debates that you've had so people have full context.
Also, I suspect if you had the wheels on a set up on the board and put more weight on it (idk tape a bunch of weights to your board or something), it probably would have made the difference between the two wheels more pronounced. Seems to me that a board without the weight of a person standing on it probably isn't going to give results that differ by a very noticeable margin, making it easier to attribute the difference to experimental variance and human error.
I don't have a background in physics/material science so I'm definitely no expert. Intuitively you'd think a wider wheel results in more grip, even if not by much (this is also my anecdotal experience). I've heard that certain characteristics of the surfaces may not necessarily be caught by the formula defining the friction coefficient, though I'm definitely not willing to die on that hill so not taking any sides for now. Just pointing out some things that don't exactly convince me, yet.
Interesting though, looking forward to the next part.
That's what the common misconception is. Larger surface area meaning larger friction. So if a surface area is 1 unit of area doubling that area would also double the force needed to make the object slide, so the friction coefficient would have to double.
Linking the debates would be best, I agree but they are numerous and scattered all over the forum in several random topics. Unfortunately I can't be bothered to go find those.
Like in my reply to yourbreakfast I think the differences between the two wheels can be explained by various things. While they are systematic they are not very big. I'm quite sure you could not notice the difference, making it completely insignificant.
I am not trying to mask or hide anything. I put those calculated bars there to make the point that the surface area/contact patch width being directly proportional to friction simply isn't true.
I can't see putting more weight on the board changing anything but I will test that later on just not with this exact setup as the plywood would bend and manually tilting more weight repeatedly tens of times with 0.1° accuracy will be too difficult.
Yes, I was under the impression that the common understanding was that larger surface area means larger friction. However my only gripe is that you seem to be taking this claim one step further and asserting that everyone is saying that the relationship is exactly one to one: “if a surface area is 1 unit of area doubling that area would also double the force needed to make the object slide, so the friction coefficient would have to double.” It may well be the case that for 99a skateboard wheel urethane, a 70% wider contact patch only changes the friction coefficient by 5% (I just made these numbers up), the claim that larger surface area means larger friction would still hold true.
This also makes your experiment more difficult since its hard to detect such small differences with wheels that are, in the grand scheme of things, not too different. I never thought that the relationship (if there is one) is exactly linear or directly proportional, it may well be a logarithmically proportional or whatever. I think you get what I mean. What is your position on that?
If we are talking about a strictly one to one, linear proportionality then sure I definitely agree with you. But that claim even at face value seems a little silly (especially with no formula that says so), to me its somewhat like saying when a person doubles in height, they have to also exactly double in weight or something, thats obviously not true. Though the overall idea that ones weight increases with their height, regardless what the specifics of the proportion is, isn’t anywhere near as silly. All I’ve seen is people say that as you increase the width, the grip increases to some degree. I’ve never actually seen anyone claim that the relationship was one to one, or exactly doubling the area means exactly a doubling of the coefficient. Can you find an example of such a claim?
"Like in my reply to yourbreakfast I think the differences between the two wheels can be explained by various things. While they are systematic they are not very big. I'm quite sure you could not notice the difference, making it completely insignificant." - What are some of these things? Couldn't one argue that the same thing could potentially be said about the width: systematic differences but not very big? I mean we are picking at minor differences anyway aren't we?
-
Your test seems like it'd only apply to ultra smooth ground, like marble tile. And even a matte finish is gonna have texture
Concrete and asphalt are way rougher
http://www.mate.tue.nl/mate/pdfs/8147.pdf
Expand Quote
6.1 Friction
Friction of polymers is closely related to their viscoelastic behavior. Generally speaking, the co- efficient of friction increases with sliding velocity until a maximum value is reached at a certain speed, followed by a decrease of the friction coefficient. The viscoelastic properties of polymers depend strongly on temperature. Grosch showed the relation between the dependence of the fric- tion coefficient due to sliding velocity and temperature and the viscoelastic deformation on strain rate and temperature.
The friction force for soft rubber sliding on a smooth surface (perfect contact) is more or less constant, independent of the load. Thus the coefficient of friction decreases continuously as the pressure increases. However there is also a coefficient of friction, independent of pressure, for harder rubber compounds, sliding on a rough surface. Apparently in this case contact is incom- plete. An increase in pressure creates a larger true area of contact and hence a larger frictional force. This shows the influence of the temperature, sliding velocity and real contact area.
The current theory of rubber friction and surface roughness, which capture all these properties can be found in the publications of Persson, (see review paper Persson, 2005). The surface roughness is characterized by a fractal description, which exhibits geometrical self-similarity. These results are based on the early studies of Grosch, taking into account that sliding friction of rubber has the same temperature dependence as that of the complex elastic modulus. He states that the friction force is related to the internal friction of the rubber, which is a bulk property. The hysteretic friction component is determined by sliding of the rubber over asperities of a rough surface. These oscillating forces lead to energy dissipation. Every length scale up to the largest particles of asphalt, can be related to an excitation frequency. As a result of energy dissipation heat is generated, in a recent article this local heating of the rubber is also taken into account. Although this theory provides knowledge of the physical origin of rubber friction it has some drawbacks. Exact knowledge of the contribution of each length scale is needed. Therefore it has, at the moment, limited practical use, since variable amounts of unknown foreign materials in the interfacial region makes it almost impossible to derive quantitative estimates of the absolute magnitude of friction from physical properties of the rubber and surface. So assumptions must still be made.
https://www.brachengineering.com/content/publications/Wheel-Slip-Model-2006-Brach-Engineering.pdf
Page 4, mechanical adhesion
http://multiscaleconsulting.com/publications/Rubber_Friction_and_Tire_Dynamics_A_Comparison_of_Theory_with_Experimental_Data.pdf
Check out this experiment
http://www.dimnp.unipi.it/guiggiani-m/Michelin_Tire_Grip.pdf
Page 17, The mechanisms involved in the rubber-road interface friction
Yes, the plywood was very smooth. Unfortunately I haven't come up with a rough surface that I could do this test with. Also the rough surface would ideally have to be very homogenous and clean. That's gonna be tricky to find for any sort of friction test.
While the data you provided is interesting I don't really understand what your point with it is. The 50 page study I won't read now. Maybe later.
Expand Quote
Expand Quote
"According to many people here on Slap (& elsewhere) the Conical Fulls should therefore have 65% more grip."
- who says this? Are we just assuming that grip/static friction scales exactly linearly with the width of the contact patch? I was not under the impression that the claims being made were that grip/friction increases at the same rate as the increase in width of the contact patch.
If grip/static friction doesn't scale linearly with width of the contact patch, then your results (besides one outlier which can be easily attributed to human error) show that conical fulls consistently have less static friction than classics (of sometimes up to a difference of 0.1, which is not a trivial difference considering this value usually ranges from 0 to 1). Highlighting the major difference in coefficients when you calculate assuming that friction is directly proportional to the contact patch seems to me to be a red herring that tries to make the difference in your measurements appear less significant. Unless I missed the debate where the claim width scales directly with grip/friction was made? It would be good to link also the debates that you've had so people have full context.
Also, I suspect if you had the wheels on a set up on the board and put more weight on it (idk tape a bunch of weights to your board or something), it probably would have made the difference between the two wheels more pronounced. Seems to me that a board without the weight of a person standing on it probably isn't going to give results that differ by a very noticeable margin, making it easier to attribute the difference to experimental variance and human error.
I don't have a background in physics/material science so I'm definitely no expert. Intuitively you'd think a wider wheel results in more grip, even if not by much (this is also my anecdotal experience). I've heard that certain characteristics of the surfaces may not necessarily be caught by the formula defining the friction coefficient, though I'm definitely not willing to die on that hill so not taking any sides for now. Just pointing out some things that don't exactly convince me, yet.
Interesting though, looking forward to the next part.
That's what the common misconception is. Larger surface area meaning larger friction. So if a surface area is 1 unit of area doubling that area would also double the force needed to make the object slide, so the friction coefficient would have to double.
Linking the debates would be best, I agree but they are numerous and scattered all over the forum in several random topics. Unfortunately I can't be bothered to go find those.
Like in my reply to yourbreakfast I think the differences between the two wheels can be explained by various things. While they are systematic they are not very big. I'm quite sure you could not notice the difference, making it completely insignificant.
I am not trying to mask or hide anything. I put those calculated bars there to make the point that the surface area/contact patch width being directly proportional to friction simply isn't true.
I can't see putting more weight on the board changing anything but I will test that later on just not with this exact setup as the plywood would bend and manually tilting more weight repeatedly tens of times with 0.1° accuracy will be too difficult.
Yes, I was under the impression that the common understanding was that larger surface area means larger friction. However my only gripe is that you seem to be taking this claim one step further and asserting that everyone is saying that the relationship is exactly one to one: “if a surface area is 1 unit of area doubling that area would also double the force needed to make the object slide, so the friction coefficient would have to double.” It may well be the case that for 99a skateboard wheel urethane, a 70% wider contact patch only changes the friction coefficient by 5% (I just made these numbers up), the claim that larger surface area means larger friction would still hold true.
This also makes your experiment more difficult since its hard to detect such small differences with wheels that are, in the grand scheme of things, not too different. I never thought that the relationship (if there is one) is exactly linear or directly proportional, it may well be a logarithmically proportional or whatever. I think you get what I mean. What is your position on that?
If we are talking about a strictly one to one, linear proportionality then sure I definitely agree with you. But that claim even at face value seems a little silly (especially with no formula that says so), to me its somewhat like saying when a person doubles in height, they have to also exactly double in weight or something, thats obviously not true. Though the overall idea that ones weight increases with their height, regardless what the specifics of the proportion is, isn’t anywhere near as silly. All I’ve seen is people say that as you increase the width, the grip increases to some degree. I’ve never actually seen anyone claim that the relationship was one to one, or exactly doubling the area means exactly a doubling of the coefficient. Can you find an example of such a claim?
"Like in my reply to yourbreakfast I think the differences between the two wheels can be explained by various things. While they are systematic they are not very big. I'm quite sure you could not notice the difference, making it completely insignificant." - What are some of these things? Couldn't one argue that the same thing could potentially be said about the width: systematic differences but not very big? I mean we are picking at minor differences anyway aren't we?
I don't know what the exact argument of these people who disagree with me is because they never present it. They just circle around it and then end up insulting me because they can't prove their point.
As for wider wheels having 5% more grip being possible? I don't know. I guess anything is possible. I just haven't seen anything to back that up so that's why I have no reason to assume it'd be so.
I don't know if I'm gonna find anyone claiming that doubling the surface area doubles the friction. I mean I could go through all my old posts to try to find it but that doesn't sound very fun at all so I'd rather not do that. Also if I have misunderstood people on this, then so be it. I took that claim as the basis for my calculations because that's the only claim I know that has been argued. It's hard to make any calculations based on an argument like "hurr durr this idiot thinks wheel width has no effect on grip, what a fucking kook!"
The differences that could have made the wider wheel have a slightly lower coefficient of friction in my experiment could be that the wheels could be from different batches at the facility. I don't know how homogenous the batches are but at least in my experience sometimes at least 101A Formula Fours seem to vary noticeably in hardness. Wether this is true or not or just my skewed perception due to whatever I don't know. Another thing that could have affected it was fatigue in my arms and back lifting up the damn plank. Lifting it with 0.1° accuracy for tens of times in a row is surprisingly hard. I would like to minimize this by getting an electrical winch but it seems rather silly to buy one just to prove a point on the internet! :D Also I don't know if the surface could have become more saturated with tiny polyurethane dust to make the slide slightly better. There are tons of variables here. Overall though at least for me this test proved exactly what I suspected. Sure it would have been nicer to have the exact same number for every repetition from both wheels but that's not realistic when dealing with a thing like friction.
One could argue that considerably wider wheels have only slightly better grip but I haven't seen any data or theory to back that. Therefore I think that most likely that is not the case.
-
Roisto literally bringing back Physics Wheels 😜
-
Roisto, I agree that your experiment supports the conclusion that increasing the contact patch does not directly increase friction.
However, your experiment did not support a null hypothesis that friction is not dependent on contact patch. It actually found a statistically significant difference between the two sets of wheels.
Here are the tan(angle) shown in fig. 1 based on the numbers you provided. I rounded to the 3rd decimal.
0.649 0.629
0.649 0.629
0.667 0.652
0.642 0.618
0.64 0.654
0.672 0.608
0.664 0.632
0.654 0.644
0.667 0.641
0.669 0.584
I ran a t-test on these and here's what I found.
Group Group One Group Two
Mean 0.65730 0.62910
Standard Deviation 0.01187 0.02139
Standard Error of Mean 0.00375 0.00677
t = 3.6450
P=.0019
The P Value for statistical significance is .0019, which shows a very significant statistical difference between the data sets. Typically P<.05 is significant and P<.01 is highly significant.
So, based on the numbers you provided, your experiment would not support a hypothesis that friction is not dependent on contact patch.
-
Roisto, I agree that your experiment supports the conclusion that increasing the contact patch does not directly increase friction.
However, your experiment did not support a null hypothesis that friction is not dependent on contact patch. It actually found a statistically significant difference between the two sets of wheels.
Here are the tan(angle) shown in fig. 1 based on the numbers you provided. I rounded to the 3rd decimal.
0.649 0.629
0.649 0.629
0.667 0.652
0.642 0.618
0.64 0.654
0.672 0.608
0.664 0.632
0.654 0.644
0.667 0.641
0.669 0.584
I ran a t-test on these and here's what I found.
Group Group One Group Two
Mean 0.65730 0.62910
Standard Deviation 0.01187 0.02139
Standard Error of Mean 0.00375 0.00677
t = 3.6450
P=.0019
The P Value for statistical significance is .0019, which shows a very significant statistical difference between the data sets. Typically P<.05 is significant and P<.01 is highly significant.
So, based on the numbers you provided, your experiment would not support a hypothesis that friction is not dependent on contact patch.
Yes, the data shows a statistically significant difference however I would not be prepared to say that a wider contact patch has lower static friction based on the data due to all the possibilities for error in my measurements. Maybe if I get that electrical winch I could get more accurate data and could repeat the measurements a 100 times if I wanted. Getting different contact patch wheels from the same batch isn't gonna be easy though. I could cut up some wheels but that's a waste of wheels and I rather not waste my wheels. 😊
-
The main point of that race car tire article above seemed to be that without a change in weight (or tire pressure -- which maybe could be analogous to durometer) there is no change in contact patch given wheels of equal diameters but different widths. I know we don't think of a hard skate wheel as having any degree of deflection but it does, however minute. The change in the diameter of 54 to 58m of the wheel though does affect the contact patch size though so until you have two sets of equal wheel sizes I don't think these are directly comparable? Anyways, i don't know shit about physics so everything i said is probably wrong but i like this thread!
-
Expand Quote
Expand Quote
Expand Quote
"According to many people here on Slap (& elsewhere) the Conical Fulls should therefore have 65% more grip."
- who says this? Are we just assuming that grip/static friction scales exactly linearly with the width of the contact patch? I was not under the impression that the claims being made were that grip/friction increases at the same rate as the increase in width of the contact patch.
If grip/static friction doesn't scale linearly with width of the contact patch, then your results (besides one outlier which can be easily attributed to human error) show that conical fulls consistently have less static friction than classics (of sometimes up to a difference of 0.1, which is not a trivial difference considering this value usually ranges from 0 to 1). Highlighting the major difference in coefficients when you calculate assuming that friction is directly proportional to the contact patch seems to me to be a red herring that tries to make the difference in your measurements appear less significant. Unless I missed the debate where the claim width scales directly with grip/friction was made? It would be good to link also the debates that you've had so people have full context.
Also, I suspect if you had the wheels on a set up on the board and put more weight on it (idk tape a bunch of weights to your board or something), it probably would have made the difference between the two wheels more pronounced. Seems to me that a board without the weight of a person standing on it probably isn't going to give results that differ by a very noticeable margin, making it easier to attribute the difference to experimental variance and human error.
I don't have a background in physics/material science so I'm definitely no expert. Intuitively you'd think a wider wheel results in more grip, even if not by much (this is also my anecdotal experience). I've heard that certain characteristics of the surfaces may not necessarily be caught by the formula defining the friction coefficient, though I'm definitely not willing to die on that hill so not taking any sides for now. Just pointing out some things that don't exactly convince me, yet.
Interesting though, looking forward to the next part.
That's what the common misconception is. Larger surface area meaning larger friction. So if a surface area is 1 unit of area doubling that area would also double the force needed to make the object slide, so the friction coefficient would have to double.
Linking the debates would be best, I agree but they are numerous and scattered all over the forum in several random topics. Unfortunately I can't be bothered to go find those.
Like in my reply to yourbreakfast I think the differences between the two wheels can be explained by various things. While they are systematic they are not very big. I'm quite sure you could not notice the difference, making it completely insignificant.
I am not trying to mask or hide anything. I put those calculated bars there to make the point that the surface area/contact patch width being directly proportional to friction simply isn't true.
I can't see putting more weight on the board changing anything but I will test that later on just not with this exact setup as the plywood would bend and manually tilting more weight repeatedly tens of times with 0.1° accuracy will be too difficult.
Yes, I was under the impression that the common understanding was that larger surface area means larger friction. However my only gripe is that you seem to be taking this claim one step further and asserting that everyone is saying that the relationship is exactly one to one: “if a surface area is 1 unit of area doubling that area would also double the force needed to make the object slide, so the friction coefficient would have to double.” It may well be the case that for 99a skateboard wheel urethane, a 70% wider contact patch only changes the friction coefficient by 5% (I just made these numbers up), the claim that larger surface area means larger friction would still hold true.
This also makes your experiment more difficult since its hard to detect such small differences with wheels that are, in the grand scheme of things, not too different. I never thought that the relationship (if there is one) is exactly linear or directly proportional, it may well be a logarithmically proportional or whatever. I think you get what I mean. What is your position on that?
If we are talking about a strictly one to one, linear proportionality then sure I definitely agree with you. But that claim even at face value seems a little silly (especially with no formula that says so), to me its somewhat like saying when a person doubles in height, they have to also exactly double in weight or something, thats obviously not true. Though the overall idea that ones weight increases with their height, regardless what the specifics of the proportion is, isn’t anywhere near as silly. All I’ve seen is people say that as you increase the width, the grip increases to some degree. I’ve never actually seen anyone claim that the relationship was one to one, or exactly doubling the area means exactly a doubling of the coefficient. Can you find an example of such a claim?
"Like in my reply to yourbreakfast I think the differences between the two wheels can be explained by various things. While they are systematic they are not very big. I'm quite sure you could not notice the difference, making it completely insignificant." - What are some of these things? Couldn't one argue that the same thing could potentially be said about the width: systematic differences but not very big? I mean we are picking at minor differences anyway aren't we?
I don't know what the exact argument of these people who disagree with me is because they never present it. They just circle around it and then end up insulting me because they can't prove their point.
As for wider wheels having 5% more grip being possible? I don't know. I guess anything is possible. I just haven't seen anything to back that up so that's why I have no reason to assume it'd be so.
I don't know if I'm gonna find anyone claiming that doubling the surface area doubles the friction. I mean I could go through all my old posts to try to find it but that doesn't sound very fun at all so I'd rather not do that. Also if I have misunderstood people on this, then so be it. I took that claim as the basis for my calculations because that's the only claim I know that has been argued. It's hard to make any calculations based on an argument like "hurr durr this idiot thinks wheel width has no effect on grip, what a fucking kook!"
The differences that could have made the wider wheel have a slightly lower coefficient of friction in my experiment could be that the wheels could be from different batches at the facility. I don't know how homogenous the batches are but at least in my experience sometimes at least 101A Formula Fours seem to vary noticeably in hardness. Wether this is true or not or just my skewed perception due to whatever I don't know. Another thing that could have affected it was fatigue in my arms and back lifting up the damn plank. Lifting it with 0.1° accuracy for tens of times in a row is surprisingly hard. I would like to minimize this by getting an electrical winch but it seems rather silly to buy one just to prove a point on the internet! :D Also I don't know if the surface could have become more saturated with tiny polyurethane dust to make the slide slightly better. There are tons of variables here. Overall though at least for me this test proved exactly what I suspected. Sure it would have been nicer to have the exact same number for every repetition from both wheels but that's not realistic when dealing with a thing like friction.
One could argue that considerably wider wheels have only slightly better grip but I haven't seen any data or theory to back that. Therefore I think that most likely that is not the case.
I'm pretty sure the 'exact argument', as far as I understand, is just that contact patch has some (but not necessarily direct) effect on grip. I think Satan was making the point that the results of this experiment, whatever the conclusion is, may not be extrapolatable to real world scenarios on rougher ground. Much harder to replicate that for experiments though, so probably won’t be feasible.
I’m also curious as to why you attribute such a statistically significant result, that frothy pointed out, to things like batch homogeneity, arm fatigue and even tiny polyurethane dust left on the surface as opposed to even considering the fact that friction may be dependent, to some small degree, on the contact patch. Idk, contact patch sounds like it could be definitely be more of a factor than tiny polyurethane dust from brand new wheels left on the surface.
Nobody is disputing that this experiment supports your conclusion that increasing the contact patch does not directly increase friction. Then again, in all my years of lurking slap and other places on the internet, I’ve never actually seen a single person make that claim. I think the real question is whether friction is dependent on contact patch at all, and you results clearly have not disproven that claim. If you do want to prove/disprove this more general hypothesis, I’d be very curious about the results.
-
The main point of that race car tire article above seemed to be that without a change in weight (or tire pressure -- which maybe could be analogous to durometer) there is no change in contact patch given wheels of equal diameters but different widths. I know we don't think of a hard skate wheel as having any degree of deflection but it does, however minute. The change in the diameter of 54 to 58m of the wheel though does affect the contact patch size though so until you have two sets of equal wheel sizes I don't think these are directly comparable? Anyways, i don't know shit about physics so everything i said is probably wrong but i like this thread!
I can't find 54 mm Conical Fulls anywhere right now but as those are my favorite wheels, I will get a set once they are available here again. I could possibly repeat the experiments with those and the 54 mm Classics to clear doubts better.
Expand Quote
Expand Quote
Expand Quote
Expand Quote
"According to many people here on Slap (& elsewhere) the Conical Fulls should therefore have 65% more grip."
- who says this? Are we just assuming that grip/static friction scales exactly linearly with the width of the contact patch? I was not under the impression that the claims being made were that grip/friction increases at the same rate as the increase in width of the contact patch.
If grip/static friction doesn't scale linearly with width of the contact patch, then your results (besides one outlier which can be easily attributed to human error) show that conical fulls consistently have less static friction than classics (of sometimes up to a difference of 0.1, which is not a trivial difference considering this value usually ranges from 0 to 1). Highlighting the major difference in coefficients when you calculate assuming that friction is directly proportional to the contact patch seems to me to be a red herring that tries to make the difference in your measurements appear less significant. Unless I missed the debate where the claim width scales directly with grip/friction was made? It would be good to link also the debates that you've had so people have full context.
Also, I suspect if you had the wheels on a set up on the board and put more weight on it (idk tape a bunch of weights to your board or something), it probably would have made the difference between the two wheels more pronounced. Seems to me that a board without the weight of a person standing on it probably isn't going to give results that differ by a very noticeable margin, making it easier to attribute the difference to experimental variance and human error.
I don't have a background in physics/material science so I'm definitely no expert. Intuitively you'd think a wider wheel results in more grip, even if not by much (this is also my anecdotal experience). I've heard that certain characteristics of the surfaces may not necessarily be caught by the formula defining the friction coefficient, though I'm definitely not willing to die on that hill so not taking any sides for now. Just pointing out some things that don't exactly convince me, yet.
Interesting though, looking forward to the next part.
That's what the common misconception is. Larger surface area meaning larger friction. So if a surface area is 1 unit of area doubling that area would also double the force needed to make the object slide, so the friction coefficient would have to double.
Linking the debates would be best, I agree but they are numerous and scattered all over the forum in several random topics. Unfortunately I can't be bothered to go find those.
Like in my reply to yourbreakfast I think the differences between the two wheels can be explained by various things. While they are systematic they are not very big. I'm quite sure you could not notice the difference, making it completely insignificant.
I am not trying to mask or hide anything. I put those calculated bars there to make the point that the surface area/contact patch width being directly proportional to friction simply isn't true.
I can't see putting more weight on the board changing anything but I will test that later on just not with this exact setup as the plywood would bend and manually tilting more weight repeatedly tens of times with 0.1° accuracy will be too difficult.
Yes, I was under the impression that the common understanding was that larger surface area means larger friction. However my only gripe is that you seem to be taking this claim one step further and asserting that everyone is saying that the relationship is exactly one to one: “if a surface area is 1 unit of area doubling that area would also double the force needed to make the object slide, so the friction coefficient would have to double.” It may well be the case that for 99a skateboard wheel urethane, a 70% wider contact patch only changes the friction coefficient by 5% (I just made these numbers up), the claim that larger surface area means larger friction would still hold true.
This also makes your experiment more difficult since its hard to detect such small differences with wheels that are, in the grand scheme of things, not too different. I never thought that the relationship (if there is one) is exactly linear or directly proportional, it may well be a logarithmically proportional or whatever. I think you get what I mean. What is your position on that?
If we are talking about a strictly one to one, linear proportionality then sure I definitely agree with you. But that claim even at face value seems a little silly (especially with no formula that says so), to me its somewhat like saying when a person doubles in height, they have to also exactly double in weight or something, thats obviously not true. Though the overall idea that ones weight increases with their height, regardless what the specifics of the proportion is, isn’t anywhere near as silly. All I’ve seen is people say that as you increase the width, the grip increases to some degree. I’ve never actually seen anyone claim that the relationship was one to one, or exactly doubling the area means exactly a doubling of the coefficient. Can you find an example of such a claim?
"Like in my reply to yourbreakfast I think the differences between the two wheels can be explained by various things. While they are systematic they are not very big. I'm quite sure you could not notice the difference, making it completely insignificant." - What are some of these things? Couldn't one argue that the same thing could potentially be said about the width: systematic differences but not very big? I mean we are picking at minor differences anyway aren't we?
I don't know what the exact argument of these people who disagree with me is because they never present it. They just circle around it and then end up insulting me because they can't prove their point.
As for wider wheels having 5% more grip being possible? I don't know. I guess anything is possible. I just haven't seen anything to back that up so that's why I have no reason to assume it'd be so.
I don't know if I'm gonna find anyone claiming that doubling the surface area doubles the friction. I mean I could go through all my old posts to try to find it but that doesn't sound very fun at all so I'd rather not do that. Also if I have misunderstood people on this, then so be it. I took that claim as the basis for my calculations because that's the only claim I know that has been argued. It's hard to make any calculations based on an argument like "hurr durr this idiot thinks wheel width has no effect on grip, what a fucking kook!"
The differences that could have made the wider wheel have a slightly lower coefficient of friction in my experiment could be that the wheels could be from different batches at the facility. I don't know how homogenous the batches are but at least in my experience sometimes at least 101A Formula Fours seem to vary noticeably in hardness. Wether this is true or not or just my skewed perception due to whatever I don't know. Another thing that could have affected it was fatigue in my arms and back lifting up the damn plank. Lifting it with 0.1° accuracy for tens of times in a row is surprisingly hard. I would like to minimize this by getting an electrical winch but it seems rather silly to buy one just to prove a point on the internet! :D Also I don't know if the surface could have become more saturated with tiny polyurethane dust to make the slide slightly better. There are tons of variables here. Overall though at least for me this test proved exactly what I suspected. Sure it would have been nicer to have the exact same number for every repetition from both wheels but that's not realistic when dealing with a thing like friction.
One could argue that considerably wider wheels have only slightly better grip but I haven't seen any data or theory to back that. Therefore I think that most likely that is not the case.
I'm pretty sure the 'exact argument', as far as I understand, is just that contact patch has some (but not necessarily direct) effect on grip. I think Satan was making the point that the results of this experiment, whatever the conclusion is, may not be extrapolatable to real world scenarios on rougher ground. Much harder to replicate that for experiments though, so probably won’t be feasible.
I’m also curious as to why you attribute such a statistically significant result, that frothy pointed out, to things like batch homogeneity, arm fatigue and even tiny polyurethane dust left on the surface as opposed to even considering the fact that friction may be dependent, to some small degree, on the contact patch. Idk, contact patch sounds like it could be definitely be more of a factor than tiny polyurethane dust from brand new wheels left on the surface.
Nobody is disputing that this experiment supports your conclusion that increasing the contact patch does not directly increase friction. Then again, in all my years of lurking slap and other places on the internet, I’ve never actually seen a single person make that claim. I think the real question is whether friction is dependent on contact patch at all, and you results clearly have not disproven that claim. If you do want to prove/disprove this more general hypothesis, I’d be very curious about the results.
This experiment is part 1 of my skateboard wheel friction experiments. It is not supposed to be definite proof for all situations. From what I've understood is that people think that contact patch width is directly proportional to "grip" on skateboard wheels. What my tests prove is that this is not the case on an unloaded skateboard on birch plywood (using 99A 54 mm Classics and 99A 58 mm Conical Fulls). How far you can apply the results of this experiment is not something that can be factually determined. That is why I plan on doing more experiments. But according to this test it seems very likely that skateboard wheels also abide by the laws of physics. Further experiments might prove otherwise. Though I doubt that very much myself.
Why I attribute the small but statistically significant differences to experimental error and differences between batches is that i know of no scientific theory that supports the claim that increasing contact patch width on a smooth surface with relatively nonexistent deformation reduces static friction. So I am considering possible errors in the performance of the experiment and differences in the materials measured as would be logical.
Most claims here about wider contact patch increasing grip are very vague. I think that's just the nature of the discussion. No one starts out with a statement saying: "double contact patch width doubles grip" but instead just say "wider wheels are grippier" or something like that. I searched my post history for "grip" and "friction" and below are linked some posts that I have been replying to which are the main reason I started my experiments and this thread. I'm not trying to call anyone out here. I too have at some point in my life thought that doubling the contact area would double friction. It is a very common misconception and the actual facts are sorta counterintuitive.
https://www.slapmagazine.com/index.php?topic=82118.msg3055016#msg3055016
https://www.slapmagazine.com/index.php?topic=104815.msg3007974#msg3007974
https://www.slapmagazine.com/index.php?topic=82118.msg3000881#msg3000881
https://www.slapmagazine.com/index.php?topic=82118.msg3003548#msg3003548
https://www.slapmagazine.com/index.php?topic=68020.msg2690795#msg2690795
https://www.slapmagazine.com/index.php?topic=96289.msg2676091#msg2676091
https://www.slapmagazine.com/index.php?topic=96289.msg2675890#msg2675890
https://www.slapmagazine.com/index.php?topic=10804.msg3153349#msg3153349
https://www.slapmagazine.com/index.php?topic=68020.msg3141850#msg3141850
https://www.slapmagazine.com/index.php?topic=82118.msg2855192#msg2855192
https://www.slapmagazine.com/index.php?topic=82118.msg2854522#msg2854522
https://www.slapmagazine.com/index.php?topic=68020.msg2803894#msg2803894
https://www.slapmagazine.com/index.php?topic=68020.msg2803790#msg2803790
https://www.slapmagazine.com/index.php?topic=82118.msg2776274#msg2776274
https://www.slapmagazine.com/index.php?topic=82118.msg2776287#msg2776287
-
This experiment is part 1 of my skateboard wheel friction experiments. It is not supposed to be definite proof for all situations. From what I've understood is that people think that contact patch width is directly proportional to "grip" on skateboard wheels. What my tests prove is that this is not the case on an unloaded skateboard on birch plywood (using 99A 54 mm Classics and 99A 58 mm Conical Fulls). How far you can apply the results of this experiment is not something that can be factually determined. That is why I plan on doing more experiments. But according to this test it seems very likely that skateboard wheels also abide by the laws of physics. Further experiments might prove otherwise. Though I doubt that very much myself.
Why I attribute the small but statistically significant differences to experimental error and differences between batches is that i know of no scientific theory that supports the claim that increasing contact patch width on a smooth surface with relatively nonexistent deformation reduces static friction. So I am considering possible errors in the performance of the experiment and differences in the materials measured as would be logical.
Most claims here about wider contact patch increasing grip are very vague. I think that's just the nature of the discussion. No one starts out with a statement saying: "double contact patch width doubles grip" but instead just say "wider wheels are grippier" or something like that. I searched my post history for "grip" and "friction" and below are linked some posts that I have been replying to which are the main reason I started my experiments and this thread. I'm not trying to call anyone out here. I too have at some point in my life thought that doubling the contact area would double friction. It is a very common misconception and the actual facts are sorta counterintuitive.
Thanks for the links, funny to see a post by myself from a while back in there haha. Yeah, the claims are very vague as to be expected, which is why I’m hesitant to say that people think the double the contact patch => double the grip. I guess we are just interpreting these claims differently.
The reason why I’m suggest that there is a possibility that contact patch could be the cause of the differences, as opposed it be solely due to experimental error, is because in all except one, the classics have a higher coefficient than the conicals. Besides the small chance that there was a difference in formula between these two wheels, I can't think of any experimental variance that would constantly skew the results one way and not randomly. But sure, in such an error prone experiment, maybe there is something else we haven’t considered that may have lead to these results. I am just speculating.
I’m curious as to what your personal experience with wide/skinny wheels are. For me, a bluntslide variation on a ledge with classic slims goes so much better than when I try the same thing with conical fulls. Bombing a gnarlier hill on classic slims feels quite scary and uncontrollable whereas on conical fulls I definitely feel much more in control of things. Now this could all be in my head, but I definitely did notice these differences before I started paying attention to gear (I accidentally skated conical fulls and classic slims back to back lol).
Much of the difference when bombing hills I guess could be attributed to things other than friction. Though I can’t think of any other factors off the top of my head that would affect my experience with bluntslides. Would you say I’m imagining all of it? If not, what would you attribute this to if not friction?
-
I was wondering how to go about doing this kind of experiment, and my first vague idea was to use a scale like this:
(https://encrypted-tbn0.gstatic.com/shopping?q=tbn:ANd9GcQegBPDjYvoTYXPc-FT73IGn01FLvzVMybGeIZa2YUEWOjphvSR47naqWpONLeFo-eilQmI698U3kYKt5x90RUDdQeDXNMTio_PiBhtrrFuQo5QXMaGL4zg&usqp=CAE)
and put some weights on a skateboard and see if the required force to pull it sideways changed with different contact patch size.
But I ended up doing nothing, and your method was definitely more controllable. So I commend you Roisto.
-
I would hypothesize that if you took a spring scale and hooked it up to a skateboard wheel, dragging it horizontally at a constant velocity without rolling, you would not see a difference in friction correlating to the width. There are countless videos of experiments showing this property and it is described in nearly every physics textbook ever published. You’ve attempted to recreate this here and are finding similar results as one would expect.
The problem is it’s not the only force on a wheel when skating. Even the skaters mass applying pressure is not constant as you lighten your feet for a split second almost like a jump to press horizontally into a slide and quickly redistribute the load unevenly over the wheels. There are many other forces at play when the wheel is rolling and the effort to break into a slide changes depending on the angle it is applied. I’m not arguing a counterpoint, just simply pointing out that it is far more complicated than a simple textbook example can describe. Consider it on a microscopic level where friction occurs. With an uneven surface of the wheel and the ground, a wider wheel has a higher probability of making contact. In practice, the forces on a wheel when it skips and slides change rapidly on that microscopic level. They are far too unpredictable to simplify into an experiment such as this.
I truly commend you for taking the time to prove a point, but I’m afraid you are doing so in vain. The beauty of skateboarding lies in the fact that there is an extreme amount of science and physics at play that can be transcended into a “feeling”. Many men have dedicated their entire lives to observing and formulating much simpler equations. I respect your experiment, but it does not prove that thinner wheels provide as much grip as wider wheels when sliding while skateboarding. Once you are sliding and hold everything perfectly constant, sure, but that’s not what I’m arguing against. It’s actually the exact opposite as I am suggesting there is no way to control the experiment to that degree in practice and still call it skateboarding. You can’t isolate one property of friction and apply it throughout a series of actions that are subject to many other physical properties.
-
I would hypothesize that if you took a spring scale and hooked it up to a skateboard wheel, dragging it horizontally at a constant velocity without rolling, you would not see a difference in friction correlating to the width. There are countless videos of experiments showing this property and it is described in nearly every physics textbook ever published. You’ve attempted to recreate this here and are finding similar results as one would expect.
The problem is it’s not the only force on a wheel when skating. Even the skaters mass applying pressure is not constant as you lighten your feet for a split second almost like a jump to press horizontally into a slide and quickly redistribute the load unevenly over the wheels. There are many other forces at play when the wheel is rolling and the effort to break into a slide changes depending on the angle it is applied. I’m not arguing a counterpoint, just simply pointing out that it is far more complicated than a simple textbook example can describe. Consider it on a microscopic level where friction occurs. With an uneven surface of the wheel and the ground, a wider wheel has a higher probability of making contact. In practice, the forces on a wheel when it skips and slides change rapidly on that microscopic level. They are far too unpredictable to simplify into an experiment such as this.
I truly commend you for taking the time to prove a point, but I’m afraid you are doing so in vain. The beauty of skateboarding lies in the fact that there is an extreme amount of science and physics at play that can be transcended into a “feeling”. Many men have dedicated their entire lives to observing and formulating much simpler equations. I respect your experiment, but it does not prove that thinner wheels provide as much grip as wider wheels when sliding while skateboarding. Once you are sliding and hold everything perfectly constant, sure, but that’s not what I’m arguing against. It’s actually the exact opposite as I am suggesting there is no way to control the experiment to that degree in practice and still call it skateboarding. You can’t isolate one property of friction and apply it throughout a series of actions that are subject to many other physical properties.
Well put. This excitement was very cool and I’m glad you did it but there are too many variables at play during even the most controlled skating
-
Maybe those wheels are too relatively similar to make a difference. My experience with early '90s bearing covers was that they offered little grip. Maybe you can do another experiment with early '80s Powell Cubic IIIs vs. Toxic 39ers.
-
Expand Quote
This experiment is part 1 of my skateboard wheel friction experiments. It is not supposed to be definite proof for all situations. From what I've understood is that people think that contact patch width is directly proportional to "grip" on skateboard wheels. What my tests prove is that this is not the case on an unloaded skateboard on birch plywood (using 99A 54 mm Classics and 99A 58 mm Conical Fulls). How far you can apply the results of this experiment is not something that can be factually determined. That is why I plan on doing more experiments. But according to this test it seems very likely that skateboard wheels also abide by the laws of physics. Further experiments might prove otherwise. Though I doubt that very much myself.
Why I attribute the small but statistically significant differences to experimental error and differences between batches is that i know of no scientific theory that supports the claim that increasing contact patch width on a smooth surface with relatively nonexistent deformation reduces static friction. So I am considering possible errors in the performance of the experiment and differences in the materials measured as would be logical.
Most claims here about wider contact patch increasing grip are very vague. I think that's just the nature of the discussion. No one starts out with a statement saying: "double contact patch width doubles grip" but instead just say "wider wheels are grippier" or something like that. I searched my post history for "grip" and "friction" and below are linked some posts that I have been replying to which are the main reason I started my experiments and this thread. I'm not trying to call anyone out here. I too have at some point in my life thought that doubling the contact area would double friction. It is a very common misconception and the actual facts are sorta counterintuitive.
Thanks for the links, funny to see a post by myself from a while back in there haha. Yeah, the claims are very vague as to be expected, which is why I’m hesitant to say that people think the double the contact patch => double the grip. I guess we are just interpreting these claims differently.
The reason why I’m suggest that there is a possibility that contact patch could be the cause of the differences, as opposed it be solely due to experimental error, is because in all except one, the classics have a higher coefficient than the conicals. Besides the small chance that there was a difference in formula between these two wheels, I can't think of any experimental variance that would constantly skew the results one way and not randomly. But sure, in such an error prone experiment, maybe there is something else we haven’t considered that may have lead to these results. I am just speculating.
I’m curious as to what your personal experience with wide/skinny wheels are. For me, a bluntslide variation on a ledge with classic slims goes so much better than when I try the same thing with conical fulls. Bombing a gnarlier hill on classic slims feels quite scary and uncontrollable whereas on conical fulls I definitely feel much more in control of things. Now this could all be in my head, but I definitely did notice these differences before I started paying attention to gear (I accidentally skated conical fulls and classic slims back to back lol).
Much of the difference when bombing hills I guess could be attributed to things other than friction. Though I can’t think of any other factors off the top of my head that would affect my experience with bluntslides. Would you say I’m imagining all of it? If not, what would you attribute this to if not friction?
I can't do blunts so can't say anything about those but powerslides, reverts and just general grip carving around I have noticed absolutely no difference between wheels of different width. I just recently changed from worn out 54 mm Conical Fulls to 54 mm Radial Slims and the grip is the same as far as I can tell. 99A and 101A it feels like the static friction is largely the same but the kinetic friction is noticeably less on 101A. Changed from 99A Conicals to 101A Conical Fulls this summer and getting them to slide was just as easy IMO but I was slipping out like mad on the 101As while with the 99As I could keep it much better in control. Wether this is true or not, I do not know but it was my experience this summer when I paid really close attention to it.
I was wondering how to go about doing this kind of experiment, and my first vague idea was to use a scale like this:
(https://encrypted-tbn0.gstatic.com/shopping?q=tbn:ANd9GcQegBPDjYvoTYXPc-FT73IGn01FLvzVMybGeIZa2YUEWOjphvSR47naqWpONLeFo-eilQmI698U3kYKt5x90RUDdQeDXNMTio_PiBhtrrFuQo5QXMaGL4zg&usqp=CAE)
and put some weights on a skateboard and see if the required force to pull it sideways changed with different contact patch size.
But I ended up doing nothing, and your method was definitely more controllable. So I commend you Roisto.
Pic doesn't show up but I'm guessing you're talking about a hang scale? I've thought about that too. The problem with that is that getting an accurate reading might be really tricky. Also using a steady pulling force & speed would be essential, so getting an electrical hoist to pull it would be best, especially if you plan on putting some weight on the board. I feel that otherwise the experimental error would be quite large.
I would hypothesize that if you took a spring scale and hooked it up to a skateboard wheel, dragging it horizontally at a constant velocity without rolling, you would not see a difference in friction correlating to the width. There are countless videos of experiments showing this property and it is described in nearly every physics textbook ever published. You’ve attempted to recreate this here and are finding similar results as one would expect.
While that is a classic experiment, so is this. The one with the spring scale is much better for finding out the kinetic friction though, which I haven't even attempted here. So it's not really the same.
The problem is it’s not the only force on a wheel when skating. Even the skaters mass applying pressure is not constant as you lighten your feet for a split second almost like a jump to press horizontally into a slide and quickly redistribute the load unevenly over the wheels. There are many other forces at play when the wheel is rolling and the effort to break into a slide changes depending on the angle it is applied. I’m not arguing a counterpoint, just simply pointing out that it is far more complicated than a simple textbook example can describe. Consider it on a microscopic level where friction occurs. With an uneven surface of the wheel and the ground, a wider wheel has a higher probability of making contact. In practice, the forces on a wheel when it skips and slides change rapidly on that microscopic level. They are far too unpredictable to simplify into an experiment such as this.
That is exactly why friction is something you have to measure and can't be calculated based on material properties. It's too fucking difficult to look at both materials in depth and then come up with a formula for the friction. And as for the forces changing while skating, yes of course they do. But they'd change just the same for both wheels, so I don't see what difference it would make. Generally static friction is higher than kinetic friction. That's why you lighten the load on the board for a powerslide cuz if you wouldn't, you'd slip out by applying the constant force needed to break over the static friction. It's essentially quite simple.
I truly commend you for taking the time to prove a point, but I’m afraid you are doing so in vain. The beauty of skateboarding lies in the fact that there is an extreme amount of science and physics at play that can be transcended into a “feeling”. Many men have dedicated their entire lives to observing and formulating much simpler equations. I respect your experiment, but it does not prove that thinner wheels provide as much grip as wider wheels when sliding while skateboarding. Once you are sliding and hold everything perfectly constant, sure, but that’s not what I’m arguing against. It’s actually the exact opposite as I am suggesting there is no way to control the experiment to that degree in practice and still call it skateboarding. You can’t isolate one property of friction and apply it throughout a series of actions that are subject to many other physical properties.
To me a statement like this is quite insane. "No matter what you can't prove me wrong because I refuse to believe anything, so I'm right!" I mean, you do you, it just doesn't make any sense to me. Being a skateboarder I do know that skateboarding is 98% in the mind. Minor changes can have a huge impact. How much of that is placebo and how much of it is actual I can't say. But if it's not quantifiable in any way then I'd say that it's likely a placebo effect. Nothing wrong with that, it's a scientifically proven thing but knowing that you're essentially deluding yourself can be quite useful IMO. I seem to notice a difference in skating a board with a 14.5" and a 14.375" wheelbase for example. That's a 0.125" difference, 3.175 mm. That's crazy little. There's no doubt that such a change won't impact the force required to pop but wether that change is significant enough to notice is a whole another thing. Also for that there are many other things that impact it, so I'm not entirely convinced that I could notice that difference in wheelbase on two exactly same shape boards. Would be interesting to test that out but finding such boards doesn't seem very likely.
Maybe those wheels are too relatively similar to make a difference. My experience with early '90s bearing covers was that they offered little grip. Maybe you can do another experiment with early '80s Powell Cubic IIIs vs. Toxic 39ers.
I wasn't around for either of those wheels but weren't the 80s wheels relatively soft? No idea how hard the bearing covers were but if they were harder, that alone would explain the differences. Even though there's no direct correlation between wheel durometer and grip generally softer wheels have much more grip.
-
I can't do blunts so can't say anything about those but powerslides, reverts and just general grip carving around I have noticed absolutely no difference between wheels of different width. I just recently changed from worn out 54 mm Conical Fulls to 54 mm Radial Slims and the grip is the same as far as I can tell. 99A and 101A it feels like the static friction is largely the same but the kinetic friction is noticeably less on 101A. Changed from 99A Conicals to 101A Conical Fulls this summer and getting them to slide was just as easy IMO but I was slipping out like mad on the 101As while with the 99As I could keep it much better in control. Wether this is true or not, I do not know but it was my experience this summer when I paid really close attention to it.
I see, thats quite interesting. Obviously the difference between different durometers is to be expected. So what do you think the functional difference between wide and skinny wheels are? Besides weight and how much grinding room they leave on your truck.
-
Expand Quote
I can't do blunts so can't say anything about those but powerslides, reverts and just general grip carving around I have noticed absolutely no difference between wheels of different width. I just recently changed from worn out 54 mm Conical Fulls to 54 mm Radial Slims and the grip is the same as far as I can tell. 99A and 101A it feels like the static friction is largely the same but the kinetic friction is noticeably less on 101A. Changed from 99A Conicals to 101A Conical Fulls this summer and getting them to slide was just as easy IMO but I was slipping out like mad on the 101As while with the 99As I could keep it much better in control. Wether this is true or not, I do not know but it was my experience this summer when I paid really close attention to it.
I see, thats quite interesting. Obviously the difference between different durometers is to be expected. So what do you think the functional difference between wide and skinny wheels are? Besides weight and how much grinding room they leave on your truck.
That's a really good question. One I have never really considered before. And I can't think of any other benefit that narrower wheels would have apart from having less air resistance and thus being able to roll faster. I don't think the air resistance differences of skateboard wheels really have any significant impact though seeing as they're such a small part of the total air resistance of the board and the rider. And other benefits of wider wheels probably negate that benefit very easily. Narrower wheels also don't hydroplane as easily as wider wheels but that's hardly an issue with skateboard wheels. This is something I need to think more about as it really piqued my interest. 🤔
-
Expand Quote
Expand Quote
I can't do blunts so can't say anything about those but powerslides, reverts and just general grip carving around I have noticed absolutely no difference between wheels of different width. I just recently changed from worn out 54 mm Conical Fulls to 54 mm Radial Slims and the grip is the same as far as I can tell. 99A and 101A it feels like the static friction is largely the same but the kinetic friction is noticeably less on 101A. Changed from 99A Conicals to 101A Conical Fulls this summer and getting them to slide was just as easy IMO but I was slipping out like mad on the 101As while with the 99As I could keep it much better in control. Wether this is true or not, I do not know but it was my experience this summer when I paid really close attention to it.
I see, thats quite interesting. Obviously the difference between different durometers is to be expected. So what do you think the functional difference between wide and skinny wheels are? Besides weight and how much grinding room they leave on your truck.
That's a really good question. One I have never really considered before. And I can't think of any other benefit that narrower wheels would have apart from having less air resistance and thus being able to roll faster. I don't think the air resistance differences of skateboard wheels really have any significant impact though seeing as they're such a small part of the total air resistance of the board and the rider. And other benefits of wider wheels probably negate that benefit very easily. Narrower wheels also don't hydroplane as easily as wider wheels but that's hardly an issue with skateboard wheels. This is something I need to think more about as it really piqued my interest. 🤔
My personal experience is that narrow wheels are faster on smooth ground whereas really wide wheels feel a bit more sluggish, but narrow wheels are shittier and slower on rougher terrain. I don't know the physics behind this but maybe its because wider wheels distribute your weight over a larger surface area? I've definitely skated some spots that aren't really skateable on skinny wheels but do ok on conical fulls.
Besides having more room to grind and being lighter, I think another benefit of narrow wheels is that they make your board feel more nimble and easier to flip. Intuitively this could to be related to why narrow trucks seem to make boards easier to flip (according to some) because it brings the weight of the wheels closer to the centre of the axis of rotation as opposed to right on the very edge like with wider trucks? What do you think
-
Don’t throw quotes around words I didn’t say and attribute them to me. You’re complaining people attack you personally over this and you’re doing exactly that to others. I’m not here making a claim. In fact, I’ve agreed your experiment proved what you wanted it to. It’s just too narrow to be meaningful. There’s a ton of other forces at play when you’re rolling along and break into a slide that you’re ignoring. I’m not claiming I know them all, I’m just saying I know that it’s not all accounted for and your experiment here doesn’t really say anything about skateboarding in action.
You’re chasing the dragon.
-
Expand Quote
Expand Quote
Expand Quote
I can't do blunts so can't say anything about those but powerslides, reverts and just general grip carving around I have noticed absolutely no difference between wheels of different width. I just recently changed from worn out 54 mm Conical Fulls to 54 mm Radial Slims and the grip is the same as far as I can tell. 99A and 101A it feels like the static friction is largely the same but the kinetic friction is noticeably less on 101A. Changed from 99A Conicals to 101A Conical Fulls this summer and getting them to slide was just as easy IMO but I was slipping out like mad on the 101As while with the 99As I could keep it much better in control. Wether this is true or not, I do not know but it was my experience this summer when I paid really close attention to it.
I see, thats quite interesting. Obviously the difference between different durometers is to be expected. So what do you think the functional difference between wide and skinny wheels are? Besides weight and how much grinding room they leave on your truck.
That's a really good question. One I have never really considered before. And I can't think of any other benefit that narrower wheels would have apart from having less air resistance and thus being able to roll faster. I don't think the air resistance differences of skateboard wheels really have any significant impact though seeing as they're such a small part of the total air resistance of the board and the rider. And other benefits of wider wheels probably negate that benefit very easily. Narrower wheels also don't hydroplane as easily as wider wheels but that's hardly an issue with skateboard wheels. This is something I need to think more about as it really piqued my interest. 🤔
My personal experience is that narrow wheels are faster on smooth ground whereas really wide wheels feel a bit more sluggish, but narrow wheels are shittier and slower on rougher terrain. I don't know the physics behind this but maybe its because wider wheels distribute your weight over a larger surface area? I've definitely skated some spots that aren't really skateable on skinny wheels but do ok on conical fulls.
Besides having more room to grind and being lighter, I think another benefit of narrow wheels is that they make your board feel more nimble and easier to flip. Intuitively this could to be related to why narrow trucks seem to make boards easier to flip (according to some) because it brings the weight of the wheels closer to the centre of the axis of rotation as opposed to right on the very edge like with wider trucks? What do you think
Narrower wheels would accelerate faster but not keep their speed as well as wider wheels due to moment of inertia differences. Smaller effect than on wheel diameter though. A wheel being "faster" could mean a number of things. But I can't think any other reason why a narrower wheel would be faster. Mind you, I haven't looked into this very much so it may be that I am neglecting something here. 🤔 Wider wheels definitely go over rough ground much better.
For flips, yes this is exactly how it goes. I haven't successfully flipped my board in ages so I don't have any sense about how significant the difference might be. Same board, same trucks, different width wheels probably less so than same board, same wheels and wider trucks. I guess this could all be calculated if one wanted.
Don’t throw quotes around words I didn’t say and attribute them to me. You’re complaining people attack you personally over this and you’re doing exactly that to others. I’m not here making a claim. In fact, I’ve agreed your experiment proved what you wanted it to. It’s just too narrow to be meaningful. There’s a ton of other forces at play when you’re rolling along and break into a slide that you’re ignoring. I’m not claiming I know them all, I’m just saying I know that it’s not all accounted for and your experiment here doesn’t really say anything about skateboarding in action.
You’re chasing the dragon.
I'm sorry sharkin. My message came off as quite hostile. I got annoyed. I should take a breath and think about how I deliver my message. This has been an ongoing problem in my life since forever. I should know better by now. 😬
But isn't what you said essentially that nothing can be done to prove you wrong in this? Maybe I misunderstood but that is how I read it. I would like to know what these forces are that you're talking about. Also mind you that this experiment was only about static friction, not kinetic friction. Kinetic friction experiments I will do later on.
-
Dude he literally said he's not making a claim, so theres nothing for you to prove wrong. No need to assume that everyone is having a crack at you.
As far as the stuff we talked about, when I said that narrower wheels feel faster on really smooth ground, the feeling is probably due to the acceleration. Idk wider wheels sometimes just feel sluggish and 'glued' do the ground compared to narrow wheels which feel more nimble to me on really nice ground.
The flippability thing I'd agree with you. Going from ace 44s to 55s made a board take way more effort to flip than going from radial slims to conical fulls on the same board. Though holding all else constant, especially for flip tricks I struggle with a bit more, getting wheels with noticeably different width will still make a difference in how it feels to flip.
What do you think of this? Especially the bolded part, seems like it would still be relevant to skateboard wheels. Found it on google:
"Friction is proportional to the normal force of the asphalt acting upon the car tires. This force is simply equal to the weight which is distributed to each tire when the car is on level ground. Force can be stated as Pressure X Area. For a wide tire, the area is large but the force per unit area is small and vice versa. The force of friction is therefore the same whether the tire is wide or not. However, asphalt is not a uniform surface. Even with steamrollers to flatten the asphalt, the surface is still somewhat irregular, especially over the with of a tire. Drag racers can therefore increase the probability or likelihood of making contact with the road by using a wider tire. In addition a secondary benefit is that the wider tire increased the support base and makes it hard to turn the car over in a turn or in a mishap."
-
Dude he literally said he's not making a claim, so theres nothing for you to prove wrong. No need to assume that everyone is having a crack at you.
As far as the stuff we talked about, when I said that narrower wheels feel faster on really smooth ground, the feeling is probably due to the acceleration. Idk wider wheels sometimes just feel sluggish and 'glued' do the ground compared to narrow wheels which feel more nimble to me on really nice ground.
The flippability thing I'd agree with you. Going from ace 44s to 55s made a board take way more effort to flip than going from radial slims to conical fulls on the same board. Though holding all else constant, especially for flip tricks I struggle with a bit more, getting wheels with noticeably different width will still make a difference in how it feels to flip.
What do you think of this? Especially the bolded part, seems like it would still be relevant to skateboard wheels. Found it on google:
"Friction is proportional to the normal force of the asphalt acting upon the car tires. This force is simply equal to the weight which is distributed to each tire when the car is on level ground. Force can be stated as Pressure X Area. For a wide tire, the area is large but the force per unit area is small and vice versa. The force of friction is therefore the same whether the tire is wide or not. However, asphalt is not a uniform surface. Even with steamrollers to flatten the asphalt, the surface is still somewhat irregular, especially over the with of a tire. Drag racers can therefore increase the probability or likelihood of making contact with the road by using a wider tire. In addition a secondary benefit is that the wider tire increased the support base and makes it hard to turn the car over in a turn or in a mishap."
The part i highlighted in red leads me to believe the writer doesn't really know what he's talking about. I think it is well established that a wider car tire does not change the contact area but merely changes the shape of it. Also car tires are way softer than skateboard wheels and are also filled with air and they have much bigger mass weighing down on them. So the deformation of a car tire compared to the deformation of a skateboard wheel are two very different things.
Now if the contact area would be smaller, that would lead to increased pressure and more deformation. Depending on the surface roughness, this could fill the voids better. If we assume that the voids are perfectly filled at all times then having a larger contact area could possibly provide more grip due to catching. However the pressure is less and I don't know exactly how that affects the materials clinging to each other but I am assuming it all cancels out as that's essentially what happens with friction.
-
No worries Rois. Respect.
I honestly don’t know what forces are at play. It’s too complicated and well above my understanding. I took calculus based physics in college 13 years ago but barely remember what I learned. I know that every example we looked at was so simplified it has hardly practical. Every problem on the tests held the entire world constant while we solved for a few variables. Maybe this is something I can’t get over.
To add some of my experience in this particular area, I’ve ridden conical fulls and classic slims back to back. The classic slims you barely had to push to get them to slide. The conical fulls required a much smaller angle of horizontal force to break. Both were easy to slide (they’re F4s, duh!) but it was definitely the case that the slims had less traction and were more prone to slipping out on high speed carves and rolling around on rough ground.
I think there’s a post, perhaps you’ve linked to it on the previous page, where someone said it takes more force to break a wider wheel into a slide but once sliding the difference between wide and slim are negligible. I think that’s worth something looking into with anything. The log shaped “curve of slideability” on a graph would look initially steeper on one than the other if that makes sense.
-
such a small difference in fig 1 could possibly be accounted for by the combined differences in weight and height between setups—conicals are heavier, with a higher center of gravity—so thinking of controls could be good for that. adding a standard weight load to both setups could help for the former, but no idea of what the effect size for the latter might be.
surface type controls would be good too. i think that, unlike in an assumption of perfectly smooth ground, in uneven surfaces the probability distribution of % contact area might have more to do with what we think of as "grip".
more methods for applying force would also be good to test out, applied evenly and unevenly from different directions.
i guess there are lots of assumptions to be made and fleshed out. this is a cool idea tho. will check on your next experiment.
-
Expand Quote
Dude he literally said he's not making a claim, so theres nothing for you to prove wrong. No need to assume that everyone is having a crack at you.
As far as the stuff we talked about, when I said that narrower wheels feel faster on really smooth ground, the feeling is probably due to the acceleration. Idk wider wheels sometimes just feel sluggish and 'glued' do the ground compared to narrow wheels which feel more nimble to me on really nice ground.
The flippability thing I'd agree with you. Going from ace 44s to 55s made a board take way more effort to flip than going from radial slims to conical fulls on the same board. Though holding all else constant, especially for flip tricks I struggle with a bit more, getting wheels with noticeably different width will still make a difference in how it feels to flip.
What do you think of this? Especially the bolded part, seems like it would still be relevant to skateboard wheels. Found it on google:
"Friction is proportional to the normal force of the asphalt acting upon the car tires. This force is simply equal to the weight which is distributed to each tire when the car is on level ground. Force can be stated as Pressure X Area. For a wide tire, the area is large but the force per unit area is small and vice versa. The force of friction is therefore the same whether the tire is wide or not. However, asphalt is not a uniform surface. Even with steamrollers to flatten the asphalt, the surface is still somewhat irregular, especially over the with of a tire. Drag racers can therefore increase the probability or likelihood of making contact with the road by using a wider tire. In addition a secondary benefit is that the wider tire increased the support base and makes it hard to turn the car over in a turn or in a mishap."
The part i highlighted in red leads me to believe the writer doesn't really know what he's talking about. I think it is well established that a wider car tire does not change the contact area but merely changes the shape of it. Also car tires are way softer than skateboard wheels and are also filled with air and they have much bigger mass weighing down on them. So the deformation of a car tire compared to the deformation of a skateboard wheel are two very different things.
Now if the contact area would be smaller, that would lead to increased pressure and more deformation. Depending on the surface roughness, this could fill the voids better. If we assume that the voids are perfectly filled at all times then having a larger contact area could possibly provide more grip due to catching. However the pressure is less and I don't know exactly how that affects the materials clinging to each other but I am assuming it all cancels out as that's essentially what happens with friction.
Do you have a source for this? Even if this was not true, in the context of skateboard wheels haven't we all agreed that a wider wheel leads to more contact area? Also, how does a smaller contact area lead to more deformation? I think the more grip due to likelihood of catching you're referring to is what the whole point of this paragraph is about even if you do take issue with some of the details.
My experience with slim and wide wheels when breaking into slides is very similar to Sharkins. I don't know if there's many hills where you are from. But if you find a gnarlier hill (for your ability whatever it is) where you'll have to powerslide frequently and somewhat hold on for dear life, try do it with classics then try the same thing with conical fulls. I will be seriously surprised if you still insist they feel exactly the same afterwards.
-
While wide wheels allow you to comfortably roll over crusty ground it absolutely will cause more friction when power sliding on crusty ground or sliding an imperfect ledge.
There’s more wheel to snag. I feel like nobody brings this up.
-
Expand Quote
Expand Quote
Dude he literally said he's not making a claim, so theres nothing for you to prove wrong. No need to assume that everyone is having a crack at you.
As far as the stuff we talked about, when I said that narrower wheels feel faster on really smooth ground, the feeling is probably due to the acceleration. Idk wider wheels sometimes just feel sluggish and 'glued' do the ground compared to narrow wheels which feel more nimble to me on really nice ground.
The flippability thing I'd agree with you. Going from ace 44s to 55s made a board take way more effort to flip than going from radial slims to conical fulls on the same board. Though holding all else constant, especially for flip tricks I struggle with a bit more, getting wheels with noticeably different width will still make a difference in how it feels to flip.
What do you think of this? Especially the bolded part, seems like it would still be relevant to skateboard wheels. Found it on google:
"Friction is proportional to the normal force of the asphalt acting upon the car tires. This force is simply equal to the weight which is distributed to each tire when the car is on level ground. Force can be stated as Pressure X Area. For a wide tire, the area is large but the force per unit area is small and vice versa. The force of friction is therefore the same whether the tire is wide or not. However, asphalt is not a uniform surface. Even with steamrollers to flatten the asphalt, the surface is still somewhat irregular, especially over the with of a tire. Drag racers can therefore increase the probability or likelihood of making contact with the road by using a wider tire. In addition a secondary benefit is that the wider tire increased the support base and makes it hard to turn the car over in a turn or in a mishap."
The part i highlighted in red leads me to believe the writer doesn't really know what he's talking about. I think it is well established that a wider car tire does not change the contact area but merely changes the shape of it. Also car tires are way softer than skateboard wheels and are also filled with air and they have much bigger mass weighing down on them. So the deformation of a car tire compared to the deformation of a skateboard wheel are two very different things.
Now if the contact area would be smaller, that would lead to increased pressure and more deformation. Depending on the surface roughness, this could fill the voids better. If we assume that the voids are perfectly filled at all times then having a larger contact area could possibly provide more grip due to catching. However the pressure is less and I don't know exactly how that affects the materials clinging to each other but I am assuming it all cancels out as that's essentially what happens with friction.
Do you have a source for this? Even if this was not true, in the context of skateboard wheels haven't we all agreed that a wider wheel leads to more contact area? Also, how does a smaller contact area lead to more deformation? I think the more grip due to likelihood of catching you're referring to is what the whole point of this paragraph is about even if you do take issue with some of the details.
My experience with slim and wide wheels when breaking into slides is very similar to Sharkins. I don't know if there's many hills where you are from. But if you find a gnarlier hill (for your ability whatever it is) where you'll have to powerslide frequently and somewhat hold on for dear life, try do it with classics then try the same thing with conical fulls. I will be seriously surprised if you still insist they feel exactly the same afterwards.
Had this great link discussing a lot of things about car tires but can't find it now. I can try to dig it up later. Here's one on bike tires which perform pretty much the same as car tires:
https://www.schwalbetires.com/tech_info/rolling_resistance
At the same inflation pressure, a wide and a narrow tire have the same contact area. A wide tire is flattened over its width whereas a narrow tire has a slimmer but longer contact area.
Smaller contact area leads to more deformation due to the pressure being higher as the force stays the same but the area is smaller.
Wether skateboard wheels have a larger contact area with wider wheels I don't know. I haven't looked into it enough to say much about it. I guess it all depends on how skateboard wheels deform. But that is besides the point as friction does not depend on contact area.
We've got hills around here. Got one that's quite nice for powersliding too. Freshly paved so you actually get some speed after powersliding instead of just going slowly along on the rough as shit asphalt. I could go do A/B tests with Classics and Conical fulls on it once spring is around although I think I'd have to get 54 mm Conical Fulls for it to be a fair comparison. Hopefully those will be available around here soon enough.
-
I think what the quote I linked is trying to say isn't that the feeling of increased 'grip' or 'traction' is due to the increased surface area resulting in more friction per say, but rather that your wheel (if we agree that the contact surface is larger), is more likely to 'catch' onto uneven bits and pieces on a non uniform surface if that makes sense.
-
so you're the smartest person on slap? haha cool stuff
-
I think what the quote I linked is trying to say isn't that the feeling of increased 'grip' or 'traction' is due to the increased surface area resulting in more friction per say, but rather that your wheel (if we agree that the contact surface is larger), is more likely to 'catch' onto uneven bits and pieces on a non uniform surface if that makes sense.
Yes, I understand that but a smaller contact patch would dig in deeper and catch more strongly due to increased pressure. I think this is the basis why friction does not depend on surface area. You have to keep in mind that no matter how smooth the two contacting surfaces are, there’s still gonna be catching at some level, however minuscule and this is one part that forms friction.
-
Expand Quote
I think what the quote I linked is trying to say isn't that the feeling of increased 'grip' or 'traction' is due to the increased surface area resulting in more friction per say, but rather that your wheel (if we agree that the contact surface is larger), is more likely to 'catch' onto uneven bits and pieces on a non uniform surface if that makes sense.
Yes, I understand that but a smaller contact patch would dig in deeper and catch more strongly due to increased pressure. I think this is the basis why friction does not depend on surface area. You have to keep in mind that no matter how smooth the two contacting surfaces are, there’s still gonna be catching at some level, however minuscule and this is one part that forms friction.
Do you have a source for this? Imagining real world scenarios in my head, the differences in pressure seem like it would be a very minuscule difference whereas the actual motion of 'catching' is what makes the difference, rather than how strongly it 'catches'. Ultimately a larger surface area is much more likely to catch onto more surface when it comes to uneven terrain. At this point I'm pulling this shit out my ass, but its still fun to think about.
Yes, no matter how smooth the surfaces are, there is catching and unevenness. However that doesn't necessarily mean that in real world applications, slim wheels and wide wheels are going to behave the same relative to each other on both glass and rough asphalt.
I recently graduated and still live right next to the uni campus, maybe I should find the appropriate faculty and show up to their office hours with some of these questions lol.
-
Expand Quote
Expand Quote
I think what the quote I linked is trying to say isn't that the feeling of increased 'grip' or 'traction' is due to the increased surface area resulting in more friction per say, but rather that your wheel (if we agree that the contact surface is larger), is more likely to 'catch' onto uneven bits and pieces on a non uniform surface if that makes sense.
Yes, I understand that but a smaller contact patch would dig in deeper and catch more strongly due to increased pressure. I think this is the basis why friction does not depend on surface area. You have to keep in mind that no matter how smooth the two contacting surfaces are, there’s still gonna be catching at some level, however minuscule and this is one part that forms friction.
Do you have a source for this? Imagining real world scenarios in my head, the differences in pressure seem like it would be a very minuscule difference whereas the actual motion of 'catching' is what makes the difference, rather than how strongly it 'catches'. Ultimately a larger surface area is much more likely to catch onto more surface when it comes to uneven terrain. At this point I'm pulling this shit out my ass, but its still fun to think about.
Yes, no matter how smooth the surfaces are, there is catching and unevenness. However that doesn't necessarily mean that in real world applications, slim wheels and wide wheels are going to behave the same relative to each other on both glass and rough asphalt.
I recently graduated and still live right next to the uni campus, maybe I should find the appropriate faculty and show up to their office hours with some of these questions lol.
https://www2.virginia.edu/ep/SurfaceScience/friction.html
That friction is independent of contact area can be found in any educational material about friction basics.
-
Why would you need an experiment to prove this? Wtf?
-
Why would you need an experiment to prove this? Wtf?
Because people claim that basic physics do not apply to skateboard wheels. I've been getting shit for stating this for quite a while and I'm sick of it so I decided to make experiments to hopefully prove that basic physics do apply to skateboard wheels also. So far it doesn't seem to have done any good though. :D
-
Expand Quote
Expand Quote
Expand Quote
I think what the quote I linked is trying to say isn't that the feeling of increased 'grip' or 'traction' is due to the increased surface area resulting in more friction per say, but rather that your wheel (if we agree that the contact surface is larger), is more likely to 'catch' onto uneven bits and pieces on a non uniform surface if that makes sense.
Yes, I understand that but a smaller contact patch would dig in deeper and catch more strongly due to increased pressure. I think this is the basis why friction does not depend on surface area. You have to keep in mind that no matter how smooth the two contacting surfaces are, there’s still gonna be catching at some level, however minuscule and this is one part that forms friction.
Do you have a source for this? Imagining real world scenarios in my head, the differences in pressure seem like it would be a very minuscule difference whereas the actual motion of 'catching' is what makes the difference, rather than how strongly it 'catches'. Ultimately a larger surface area is much more likely to catch onto more surface when it comes to uneven terrain. At this point I'm pulling this shit out my ass, but its still fun to think about.
Yes, no matter how smooth the surfaces are, there is catching and unevenness. However that doesn't necessarily mean that in real world applications, slim wheels and wide wheels are going to behave the same relative to each other on both glass and rough asphalt.
I recently graduated and still live right next to the uni campus, maybe I should find the appropriate faculty and show up to their office hours with some of these questions lol.
https://www2.virginia.edu/ep/SurfaceScience/friction.html
That friction is independent of contact area can be found in any educational material about friction basics.
Yes I know that all basic physics books say that, but I've never seen anyone say that it is because of the smaller contact patch digging deeper due to increased pressure. (then again I haven't looked very hard either)
In that link, it does note some potential exceptions, saying that the coefficient of friction m is "independent of load L and macroscopic area A if P is constant or much larger than t0," with t0 being the initial shear yield stress I believe. "This fails when adhesion is strong (large t0)," though I'm not sure if these exceptions apply to this situation in skating.
Thanks for the link though, nice and concise but very clear. I see what you were trying to say, "2) the friction force is proportional to the applied load." Though it seems that its not necessarily as black and white as you make it out to be.
-
it seems that its not necessarily as black and white as you make it out to be.
We aren't making anything out to be different than what scientists have been stating, which is that friction is independent of surface area. Nobody stating otherwise has any evidence other than, "it seems."
-
Sure, quote the bit where I say “it seems” but ignore the above paragraph where the article being referred to states that there are conditions in which the model fails, showing that this is quite literally not a black and white issue.
I’ve never actually disagreed with the premise that friction is completely independent of surface area, just mentioned in my own experiences whilst noting that they are likely biased. Also thought out loud about how other factors could result in the feeling of increased ‘grip’ or ‘traction’ such as how smooth a wheel feels on rough ground as opposed to friction being the sole factor that affects ‘traction’ in the colloquial sense.
Also, this experiment is literally insufficient in supporting the hypothesis that friction is independent of contact patch (refer to t-test on first page), more comprehensive testing is required.
-
Rite so some super skinny wheel that flexes will be grippier. Thats more then just contact width tho. Thats overall width as well. A better example would be the bones fatty which is structurly rigid from its width but has a comparativly smaller c. Patch.
-
Let's investigate the relationship between load, surface area, and angle of force.
Let's say I weigh X kg and I'm standing perfectly over my wheels so all that X kg of my weight is putting pressure on the contact patch. To slide, I shift my weight diagonally on my wheels, transferring some of my X kg of weight into a lateral force.
On a wheel with a smaller contact patch vs a larger contact patch, there is more of a chance (assuming the same angle) that my weight transfers to a lateral force on the side of the wheel. The wider wheel I would have more pressure on top of the wheel. In this case, the pressure on the top of the wheel and the side of the wheel are different depending on the contact patch.
This is is my best way to describe the phenomenon of "thinner wheels seem to break into a slide easier than wide wheels."
-
I barely understand most of this and it kind of makes my head hurt but I am here for it.
-
Let's investigate the relationship between load, surface area, and angle of force.
Let's say I weigh X kg and I'm standing perfectly over my wheels so all that X kg of my weight is putting pressure on the contact patch. To slide, I shift my weight diagonally on my wheels, transferring some of my X kg of weight into a lateral force.
On a wheel with a smaller contact patch vs a larger contact patch, there is more of a chance (assuming the same angle) that my weight transfers to a lateral force on the side of the wheel. The wider wheel I would have more pressure on top of the wheel. In this case, the pressure on the top of the wheel and the side of the wheel are different depending on the contact patch.
This is is my best way to describe the phenomenon of "thinner wheels seem to break into a slide easier than wide wheels."
Interesting, this is plausible. If this is valid, could this also mean that it would feel easier to break into a slide on skinny trucks as opposed to wide trucks since the wheels would be set further in? Hmmm
-
The shape of the wheel outside the contact patch is a major factor in the wheel's structural integrity and make no mistake the wheels structural integrity affects how it slides. For example if you cut the sides off a wheel and left only the contact patch (but still beveled the corners a tiny amount to get rid of that grippy lip) the wheel would skip more when sliding you'd likely notice. Because taking the sides off would increase lateral flex. We can't really be having this discussion and be omitting these huge factors.
-
Let's investigate the relationship between load, surface area, and angle of force.
Let's say I weigh X kg and I'm standing perfectly over my wheels so all that X kg of my weight is putting pressure on the contact patch. To slide, I shift my weight diagonally on my wheels, transferring some of my X kg of weight into a lateral force.
On a wheel with a smaller contact patch vs a larger contact patch, there is more of a chance (assuming the same angle) that my weight transfers to a lateral force on the side of the wheel. The wider wheel I would have more pressure on top of the wheel. In this case, the pressure on the top of the wheel and the side of the wheel are different depending on the contact patch.
This is is my best way to describe the phenomenon of "thinner wheels seem to break into a slide easier than wide wheels."
My classics seem to grip better than my conical fulls since there is more weight applied per area: the weight is distributed across the surface, so therefore there is more load per square inch on a thinner wheel. This is is my best way to describe the phenomenon of "wider wheels seem to break into a slide easier than thin wheels."
-
Expand Quote
Let's investigate the relationship between load, surface area, and angle of force.
Let's say I weigh X kg and I'm standing perfectly over my wheels so all that X kg of my weight is putting pressure on the contact patch. To slide, I shift my weight diagonally on my wheels, transferring some of my X kg of weight into a lateral force.
On a wheel with a smaller contact patch vs a larger contact patch, there is more of a chance (assuming the same angle) that my weight transfers to a lateral force on the side of the wheel. The wider wheel I would have more pressure on top of the wheel. In this case, the pressure on the top of the wheel and the side of the wheel are different depending on the contact patch.
This is is my best way to describe the phenomenon of "thinner wheels seem to break into a slide easier than wide wheels."
Interesting, this is plausible. If this is valid, could this also mean that it would feel easier to break into a slide on skinny trucks as opposed to wide trucks since the wheels would be set further in? Hmmm
There might be something to this. I can't refute it straight off the bat at least. I've been hella busy this week so I haven't had time to think about this properly but I'll get to it later on when I have more time and am not completely exhausted.
-
Expand Quote
Expand Quote
I can't do blunts so can't say anything about those but powerslides, reverts and just general grip carving around I have noticed absolutely no difference between wheels of different width. I just recently changed from worn out 54 mm Conical Fulls to 54 mm Radial Slims and the grip is the same as far as I can tell. 99A and 101A it feels like the static friction is largely the same but the kinetic friction is noticeably less on 101A. Changed from 99A Conicals to 101A Conical Fulls this summer and getting them to slide was just as easy IMO but I was slipping out like mad on the 101As while with the 99As I could keep it much better in control. Wether this is true or not, I do not know but it was my experience this summer when I paid really close attention to it.
I see, thats quite interesting. Obviously the difference between different durometers is to be expected. So what do you think the functional difference between wide and skinny wheels are? Besides weight and how much grinding room they leave on your truck.
That's a really good question. One I have never really considered before. And I can't think of any other benefit that narrower wheels would have apart from having less air resistance and thus being able to roll faster. I don't think the air resistance differences of skateboard wheels really have any significant impact though seeing as they're such a small part of the total air resistance of the board and the rider. And other benefits of wider wheels probably negate that benefit very easily. Narrower wheels also don't hydroplane as easily as wider wheels but that's hardly an issue with skateboard wheels. This is something I need to think more about as it really piqued my interest. 🤔
Width of the wheel and/or its contact patch is largely the byproduct of shape. With the advent of bowl riding (and tricks like edgers), manufacturers wanted to stiffen up the lip while rounding the inside edge to roll into/out of pool coping. On one extreme, the G&S Rollerball went right over protrusive coping.
(https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQOxbB1zlrvbzjGBvfBsFit0VsEX5ep1uU5frkI3ggqYxQQGlyDIg&s)
I would think that these would also "tip" the board more easily since the axis is pushed in closer and there's no flat surface. They were said to wear prematurely and would probably flatspot incredible easily. Something more modern like a Spitfire classic full or SFM are more round and thus wider (for clearing coping) in contrast to the more square and thin Tablets (to keep you locked in). I would conjecture that taller wheels are also scaled in all dimensions for aesthetic reasons.