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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:
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.
Topic: Does Friction Depend on Contact Patch Width? An Experiment - Part 1 (Read 5302 times)