# Tyre width / grip (friction)

1. May 6, 2004

### KevinS

Hi!

In a non-related forum I'm trying to convince some people that the amount of dry friction has nothing to do with the size of the contact surface, just the normal force (Fn) and the friction coefficient. In this case it's about formula1, tyre width, wear and grip.

I say a wider tyre does not give more grip, but it allows you to use softer rubber without reducing the lifespan of the tyre. It's the softer rubber that gives you more grip.

Can someone point me to a website that explains this matter (normal force, friction coefficient) clearly? They don't seem to believe ME...

2. May 6, 2004

### Chi Meson

You are correct, theoretically.

It is easily explained, but read the full explanation:

the amount of frictional force is proportional to the surface area of contact, but it is also proportional to the pressure with which the two surfaces are "pressed" together.

If you increase the surface area however, you will by definition decrease the pressure since pressure is "weight divided by surface area." So a change in surface area will cause two counterbalancing effects that leave, as you said, only the weight of the object to be considered.

Back to the real world: as you said, larger tires are used in order that softer material can be utilized. The greatest force required from the tires occurs during cornering. If a certain force were required to make a turn at high speeds with thin tires, the material itself could fail (similar to the fact that a rubber band will snap). You can see that this happens in racing when you get a close look at the old tires at they are replaced: small chunks are often ripped out.

THis failure is not a failure of the frictional force; rather, it is a "stress" related failure of the material. Wider tires will reduce the stress per unit area of each tire (sticklers: yes, I know that's redundant!).