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Tyre companies boast of their wider tires for better grip on road. Also, the F1 cars have broad tires for better grip. But as far as I know Friction does not depend on the surface area of contact between the materials. Even the formula says so.. $F=\mu mg$ (where $F$ = Force of friction, $\mu$ = coefficient of friction, $m$ = mass, $g$ = gravity)

Can anyone please tell me the relation between broad tires and road grip ?

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"But as far as I know Friction does not depend on the surface area of contact between the materials." This is true in physics 101 and is how we teach it, but it breaks down when there is non-trivial deformation of either material. Tires are well known as a "everyday situation" where this rule is not a good guide. I believe we have some users who know this matter in detail and they may give you a good answer. –  dmckee Jun 11 '12 at 13:38
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A simplistic answer is to consider the finite material strength of the rubber/tarmac. The larger the contact area, the greater the amount of bonds available to absorb the force before tearing. –  Nic Jun 11 '12 at 13:43
    
reeeeally wish high school physics would at least make reference to spatial components like these even if the mathematics is not introduced. –  Trevor Alexander Jan 17 at 8:32
    
Wider tires, not "bigger". –  Danny Varod yesterday

2 Answers 2

up vote 16 down vote accepted

It's a surprisingly complicated question. Given your mention of friction, probably the main point is that for a car tyre the friction is not linearly dependant on load. Wikipedia has some information about this here.

If you had perfectly smooth surfaces the friction is actually proportional to the area of contact and independant of the load. This is because friction is an adhesive effect between atoms/molecules on the surfaces that are in contact. However in the real world surfaces are not smooth. If you touch two metal surfaces together the contact is between high spots on the two surfaces so the area that is in contact is much less than than the apparent area of contact. If you increase the load you deform these high spots and broaden them, so the effect of load is to increase the real area of contact. The real area of contact is approximately proportional to the load, and the friction is proportional to the area of contact, so the friction ends up being approximately proportional to the load.

However a rubber type is a lot softer than metal, and a road is a lot rougher than a metal plate. Even at low loads the tyre deforms to key into the irregularities in the road, so increasing the load has a lesser effect. That's why you get the sub-linear dependance described in the Wikipedia article.

But this is only the start of the complexity. If you use a wider tyre the contact patch area isn't necessarily bigger. A wider tyre has a wider shorter contact patch while a narrow tyre has a narrower longer contact patch. The contact patch area depends on the tyre pressure, the deformation of the sidewalls and probably lots of other things I can't think of at the moment.

And anyway, if by "grip" you mean grip when cornering, the grip isn't just controlled by the contact patch area. When a car is cornering the contact patch is being twisted. This is known as the slip angle. The wider shorter contact patch on a wide tyre has a smaller slip angle and as a result grips better.

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Thanks a lot. It has cleared a large part of my confusions. –  Luftwaffe Jun 12 '12 at 7:23
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I didn't see a mention of temperature. Tires grip also varies with the temperature of the tire; a wider tire might not produce more grip than a skinnier one if there isn't sufficient friction (generated typically by driving fast enough to cause a significant slip angle) to heat the tire to its target heat range. This is particularly true of tires intended to work at higher temperatures such as race tires. –  james creasy Apr 13 '13 at 0:21

The reason why wider tires are chosen over thiner ones is not because of greater friction. And the reason is not complicated, or does not "start a complexity".. the reason why wider tires are chosen is because they last longer than thin ones! Nothing about friction at all. Racers would always have to replace thiner ones, so they figured, why not get larger ones!

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protected by Qmechanic Mar 24 at 8:18

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