Does Friction Depend on Contact Patch Width? An Experiment - Part 1

[Roisto]

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.


Picture 1 - The wheels


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.


Picture 3 - Conical Full 58 mm width


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.


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).


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 , 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.


Picutre 7 - Board and trucks and such


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.


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.


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.

[chappers]

so ur one of them brainy types huh

[Stephens Lawyer]

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.

[jay_nev]

Thanks for doing this, I'll be following along and look forward to future tests as well.

[Skart]

Tldr buy full conicals rite

[Roger__Kook]

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/
and here:
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/
and here:
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!

[satan]

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.

[spanyard]

What was your control?

[satan]

http://racingcardynamics.com/racing-tires-lateral-force/
Hysteresis

[tzhangdox]

"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.

[Firebert]

I thought it was common knowledge here that weight not surface area was the primary factor in grip/friction in wheels/tires

[somethingmustbreaknow]

i appreciate this. more pls.

[Xen]

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....?

[bbk]

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?

[FROTHY]

Roisto, can you please post the numbers for each of the 20 measurements in figure 1?

[Kevve]

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.

[yourbreakfsat]

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.

[backinaction]

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...

[Eds_gallerist]

Love the paper outline and the low-key nerdery.  Curious to see what's coming next. Would gnar if I could.

[weedgod94]

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.

[Roisto]

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.

[satan]

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

[tzhangdox]

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.

[close]

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?

[Roisto]

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.

[close]

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.

[close]

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.

[close]

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!  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.

[Gray Imp Sausage Metal]

Roisto literally bringing back Physics Wheels 😜

[FROTHY]

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]

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. 😊

[Stephens Lawyer]

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!

[tzhangdox]

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"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.

[close]

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.

[close]

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?

[close]

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!  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.

[Roisto]

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

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"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.

[close]

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.

[close]

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?

[close]

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!  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.

[close]

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