Understanding wider tires and grip

[theogonzalez]

I'm in grade 11 and i was talking to my pphysics teacher about friction. You see race cars with wider tires than road cars, some people will say "oh, this car could benefit from wider tires" but i haven't encountered an equation that takes into account surface area for friction (either static or kinetic). My teacher said there is a certain coefficient of friction for let's say rubber on asphalt so i guess surface area won't change that. Could someone please explain why wider tires give better grip. 

[Halc]

9 hours ago, theogonzalez said:

Could someone please explain why wider tires give better grip.

Wider tires do give somewhat better grip. It is not a linear function, so a wider tire has somewhat less load per area, but not in proportion as is suggested by the linear coefficient of friction equation.

Wider tires increase weight and drag, so you don't want to go overboard. Less pressure per area decreases wear, and wear is an incredibly important factor in a long race. Wide tires maintain better contact on imperfect tracks, which contain debris and wet/oid spots.

The suspension dynamics is more important in the turns where turning grip is more important than thrust, so the suspension is designed to transfer more weight to the front outside wheel in a turn. If turning performance wasn't an issue, the front tires could be far smaller and lighter as they are in drag races. They are not used for acceleration in either case.

[Moontanman]

I once had a formula firebird, 1974, and I put 60 series tires on it. It gripped the curves better than it did with regular tires on it. I figured it was due to a larger footprint on the tires. BTW, it looked great too! 

[OceanBreeze]

On 6/5/2024 at 10:18 AM, theogonzalez said:

I'm in grade 11 and i was talking to my pphysics teacher about friction. You see race cars with wider tires than road cars, some people will say "oh, this car could benefit from wider tires" but i haven't encountered an equation that takes into account surface area for friction (either static or kinetic). My teacher said there is a certain coefficient of friction for let's say rubber on asphalt so i guess surface area won't change that. Could someone please explain why wider tires give better grip. 

As your teacher said, rubber has a fixed coefficient of friction, just like every other substance. Therefore, both static and kinetic friction are independent of the area in contact. This is also somewhat true for rolling friction caused by deformations. But, in contrast to classical laws of friction, the part of rolling friction that is caused by adhesive forces is very dependent on the area of the surfaces in contact. Molecular adhesion is an important factor in elastic materials, and it is proportional to the surface area; the more surface area, the more adhesion the rubber has to the road surface irregularities at the molecular level.

Rubber, and rubber-like materials exhibit a viscoelastic effect that essentially makes the rubber change its effective coefficient of friction based on size of contact patch. So in effect, for high powered racing cars, the more tire on the ground the better. Street tires usually have a coefficient of friction around 0.5- 0.7, depending on size and tire pressure. For R compound tires, which get more “sticky” as the tire heats up, and full width race slicks, the coefficient can reach 1-1.3, and in some cases as much as 1.5 with the right amount of wheel spin. (When the coefficient of friction is greater than 1, it is easier to pick the object up than to slide it)

The equations for adhesive friction are highly complex. Often the coefficient of adhesive friction is determined empirically, through experimental measurements.

However, if you do wish to see the equations, this paper should fulfill that wish.