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Google: By eliminating any grooves cut into the tread, such tires provide the largest possible contact patch to the road, and maximize traction for any given tire dimension.

My understanding is that the friction force between any two object does not depend on the area in contact. So there should be the same friction force between the tire and the ground whether the tire is smooth or not. How does this relate to traction and tractive forces?

Does it mean that for two identical cars A and B. If A has slicks and B has treaded tires. The engines are of equal power. They will have the same friction with the road but because A has better traction it will accelerate at a higher rate?

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    $\begingroup$ Maximum (limiting) friction force between any two object does not depend on the area in contact. $\endgroup$
    – lucas
    Commented Jun 29, 2016 at 19:05
  • $\begingroup$ So the slicks are less likely to skid than the treaded tires but if they do skid then the slicks are no better (the same as) than the treaded tires? $\endgroup$
    – Kantura
    Commented Jun 29, 2016 at 19:09
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    $\begingroup$ Most times, required friction force for movement, is less than maximum friction force. $\endgroup$
    – lucas
    Commented Jun 29, 2016 at 19:15
  • $\begingroup$ I agree that the required friction force for movement of the slick tires is less than maximum friction force of the slick tires, and the required friction force for movement of the treaded tires is less than maximum friction force of the treaded tires. But my question is how both limiting friction and dynamic friction compare between the slicks and the treaded tires? $\endgroup$
    – Kantura
    Commented Jun 29, 2016 at 19:21
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    $\begingroup$ Limiting friction and dynamic friction aren't compared between the slicks and the treaded tires. Friction force is compared. $\endgroup$
    – lucas
    Commented Jun 29, 2016 at 19:34

2 Answers 2

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The claim that frictional force does not depend on area of contact is not, in general, true.

It is a good enough approximation for many cases that it can be used for many real world calculation and even more back-of-the-envelope estimates, but it fails under several circumstances.

Conditions for failure of the not-area-dependent approximation include (but are not limited to):

  • Either surface suffers permanent deformation
  • Either surface deforms through a linear distance comparable to the size of the contact patch.
  • Either material is stressed beyond it's linear deformation limit.

The most common day-to-day situation where the Physics 101 model for friction breaks down is tire-on-road friction, and especially when softer rubbers are used in the tire surface.

To get a sense of the complexity of tire-on-road friction, find some paper on the matter and just read the abstract. It's a live problem despite a lot of money pushing to understand it.

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One thing everyone seems to have forgotten is the "tar" in tarmac. There is more than just friction on a race track there is also adhesion. drag strips will typically have fresh tar at the beginning and be very tacky. think of it like the tires being glued to the track both by the tar and the melted "rubber" material of the tires. the more surface area for the glue, the stronger the adhesion.

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