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tire traction


sluggo
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This would have been a response to closed thread 'Galilean relativity.

When an auto tire has traction on a hard surface road, the road surface does not conform to the tire, the tire being a flexible material, conforms to the road surface.

My more important question, why does anyone revisit an obsolete theory with all the verification for the new theory?

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Probably the majority of material combinations (typically, rubber tire on pavement) goes the way you say: the tire conforms to the road surface. But there are also combos like hard plastic wheel on rubber road surface, like a model cart on a treadmill, where it's the other way around since the wheel is harder than the road surface. In the end, whichever one deforms, it results in whatever contact patch area it results in*, (and, if it's the road surface, the raised areas contribute whatever force they contribute) and the frictional force is what it is. 

But what does not effect this frictional force, or the acceleration on the vehicle riding on the wheel, is whether the road is considered to be stationary and the vehicle moving, or the vehicle considered stationary and road moving. Not since Galileo, and especially not since Michelson and Morley, does this distinction hold any physical meaning.

* addendum: Thinking about this got me to Amontons' laws of friction, which say that the contact patch area does not matter. Only the cofficient of friction (mu, a property of the materials only) and the normal force, i.e., weight, matter. This took me a step back as it's extremely counterintuitive and counter to everyday experience. A basic walking through the reasoning seems to check out, though. Say you have a box on the floor, and you're pulling it sideways on the floor with a rope. It's got some weight, a certain mu, and those 2 things result in a certain force on the rope before it starts sliding. If you keep the density and overall weight the same but then change the dimensions so the floor contact area is greater, then every point of contact has less weight on it, (and therefore contributes less frictional force) but there are more of them (so the overall frictional force stays the same.)

But then why are dragster tires so giant, and underinflated, to provide the big contact patch that seems so obviously necessary? https://www.f1technical.net/forum/viewtopic.php?t=9333 A post, about #3 down in in this thread, seems to provide a good explanation, that can be summarized as that rubber on asphalt provides frictional force by several different mechanisms, some of which are separate from the idealized Amontons' laws, and depend on contact area independent of normal force. In the end whatever theoretical explanations are floating around, the real-life effect is tested through decades of competition where, uhm, the rubber meets the road, and the optimal solution (more contact area = more better) has been firmly established.

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25 minutes ago, ArthurSmith said:

Experimental evidence beats intuition.

Well in this case the experimental evidence matches the intuition, which is that contact area does effect frictional force. Both of which beat a simplistic idealization, which has too many (of too much relative magnitude) other mechanisms stacked on top.

I'm seeing it like this (and open to being corrected, I literally learned about this today): There's Kepler's equations for elliptical orbits, which are mathematically perfect. Since then we've identified dozens of perturbations that modify orbits from the Keplerian idealizations, but to such a small relative extent that most backyard astronomers, to the level of precision required, can not tell tell the difference (at least not over the span of a few revolutions).

On the other hand Amontons' friction laws seem kinda like that, except that the stacked perturbations are of a much higher relative size, such that they overwhelm the idealization so much that even the casual (intuitive) observer knows that you need an underinflated balloon tire with big contact patch.

Edited by EfisCompMon
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8 hours ago, EfisCompMon said:

Well in this case the experimental evidence matches the intuition, which is that contact area does effect frictional force. Both of which beat a simplistic idealization, which has too many (of too much relative magnitude) other mechanisms stacked on top.

I'm seeing it like this (and open to being corrected, I literally learned about this today): There's Kepler's equations for elliptical orbits, which are mathematically perfect. Since then we've identified dozens of perturbations that modify orbits from the Keplerian idealizations, but to such a small relative extent that most backyard astronomers, to the level of precision required, can not tell tell the difference (at least not over the span of a few revolutions).

On the other hand Amontons' friction laws seem kinda like that, except that the stacked perturbations are of a much higher relative size, such that they overwhelm the idealization so much that even the casual (intuitive) observer knows that you need an underinflated balloon tire with big contact patch.

All I meant was if your model is an accurate predictor of reality (Kepler's ellipses) keep the model. If not, discard the model rather than reality.

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