# Tyre width and grip.

1. Sep 9, 2006

### David Laz

This may be a bit of a silly question and I'm not too sure if I've posted it in the right place, But I'm a bit confused and need something cleared up.

In physics/maths classes I've always read/been told that friction is not a function of contact area and only that of force.

Now I'm going to go ahead and make an assumption and say more friction between tyre and road = more grip when accelerating or cornering. Surely this makes sense as there will be less loss of traction if there is more friction.

I love my exotic/performance cars and one thing you'll notice is that they all have very very wide tyres. The New Bugatti Veyron (makes 1000hp, does 0-100km/h in 2.5s, top speed, 407kmh+ etc etc) Has something like 345mm width tyres. The Enzo Ferrari, Porsche Carrera GT have like 335mm or something. Compare this to the average sedan which has 245mm, or small cars which have around 220. Now I guess the ultimate question is why do the performance cars have wider tyres if in fact, they do not provide extra grip?

Say if I were to slap on a set of 245mm performance tyres on the Veyron of Enzo, surely I wouldn't expect it to accelerate or corner as fast.

Does the extra grip perhaps provide greater stability or heat distribution?

Thanks.

2. Sep 10, 2006

### Clausius2

I have asked to myself that question several times, and I always arrive to the conclusion that the independence of the coeficient of friction on surface is only valid under some assumptions made about the contact. That contact must be the contact of two infinitely rigid surface, with no deformation, as we are said in the elementary physics class. Later, in tribology of machines one realises that friction is made up by means of adhesion, and the theory of adhesion talks about microwelding and deformation of both surfaces. Let's say that at first order the friction coefficient is independent of the surface, but at second order it is not. Definitely, wider tyres do larger grip and larger traction capacity.

3. Sep 10, 2006

### Danger

You'll notice, too, that the tires on race cars are not only wider than on passenger vehicles, but also generally smooth (slicks). The rubber compound is also much softer than with commercial units. This is all to maximimize surface contact and adhesion.

4. Sep 10, 2006

### David Laz

Thats true. Generally the softer it is the stickier they become when they reach optimal temperature... Or so I'm lead to believe.

Clausius2, I've never studied any adhesion theory but I think I understand what you're saying.

5. Sep 11, 2006

### brewnog

Coefficient of friction is nondimensionalised to allow you not to have to consider contact areas. As Clausius hinted, this model for friction is pretty poor in any case.

There are occasions when narrow tyres will provide superior traction to wide tyres. These are generally in adverse conditions (rally cars will sometimes be fitted with super skinny tyres for use on some types of snow), wide tyres can be worse in wet conditions due to a greater risk of aquaplaning, and I can't drive my kit car in the rain because it's so light that its 225 tyres practically float on any surface water.

I posted some stuff about water displacement on tyres a bit ago on here, run a search if you fancy it. Bed time.

6. Sep 11, 2006

### turbo

My good friend was the national champion in his class in drag racing for several years. One of his secrets was to shade his slicks to prevent temperature-driven increases in inflation pressure on sunny days. He had BIG slicks fastened to the rims with what appeared to be sheet metal screws, and IIR, his rear tire pressure was typically ~4 psi off the line. He was running a 340 Duster that would show you the oil pan on every launch. Killer car!

7. Nov 28, 2009

### shaun_o_kane

This is what I've picked up as a kart racer and on the web - and I believe it.

1) The size of the contact patch is irrelevant - the main factor in determining the grip is the tyre temperature. The colder the day - the higher the trye temp needs to be to get grip.

2) Tyres works best in a very narrow temperature band - usually tryes get too hot on a dry track.

2) The contact patch size is the same for fat and thin tyres for the same psi. (simple physics)

3) Fat tyres are better on a dry track because they contact patch is wide rather than long, so the tyre carcass destorts less when the trye rolls. That keeps the tyre temperatures down.

4) There is no grip without slip - and the coefficient of friction model is a good one for a given temperature.

5) All this goes out the window if a trye aquaplanes - that's why street tyres have nobbly bits - for driving in the wet.

6) For most cars, braking distance is determined by the heat dissapation ability of the brakes. That's why a MacLaren Mercedes can stop s quickly even though its a heavy car.