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

[jakeumms]

I barely understand most of this and it kind of makes my head hurt but I am here for it.

[tzhangdox]

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

[givecigstosurfgroms]

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. 

[Firebert]

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."

[Roisto]

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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."

[close]

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.

[Chavo]

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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.

[close]

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.

[close]

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.

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.