Simple physics question

[peal]

A lot of partial answers above.

It is true that for the same tyre pressure and the same car weight, the contact patch area will be the same. So you would expect the amount of friction to be the same. But as there are not just frictional forces to consider, a wider tyre will usually be able to produce more grip, particularly when transversely loaded.

The soft rubber keys into the surface irregularities, so there is a mechanical grip component as well as simple surface friction.

What happens to the shape of the contact patch is critical in a couple of ways. Firstly, the wider the contact patch, then the shorter it is. What this means is that as the tyre revolves, the bit that gets flattened against the road undergoes less deformation (because a long contact patch has to be deformed more than a short one) and so the tyre will generate less heat from internal friction. This means that the tyre can be constructed with materials that don't have to operate at as high a temperature as one that sufferes more deflection. So in general, a wider tyre can use a stickier compound than a skinnier tyre and not overheat. There is more grip right there from the stickier compound. Taken to the extreme, stickier compounds get to the state of race slicks where the tyre is actually bonding itself to the road surface.

The other aspect of contact patch shape is that the force required to break a tyre free is somewhat dependant on the length of the contact patch in the direction that you are trying to break it free in. The wider tyre has more length in the transverse direction, so will tend to offer more grip for cornering at the expense of a loss of some grip for traction.

I remember when lower profile tyres first started coming out that the 65 series tyres were touted as having the most "square" contact patch and therefore offered the best mix of drive traction and cornering traction. Completely ignoring, of course, the fact that you might not want it that way on any car, let alone every car.

cheers

thanks very much gtsboy!

[Nick]

Anyone remember that guy that had the theory about the infinate contact patch and his go-kart?

[peal]

Anyone remember that guy that had the theory about the infinate contact patch and his go-kart?

Did you have to store the tyres and rims 10km apart ?

[260BC]

Co-efficient of friction does not increase in direct proportion to the amount of weight being applied, law of dimishing returns applies. Which is why for a circuit race car the more evenly weight is spread over the four wheels the more the total grip, everything else being equal.

[oldcorollas]

just to add to GTSboys post, and add a bit to 260DET...

tyre grip comes from:
1. pure friction (the non-messy kind), and this is relatively linear.
2. mechanical friction, where the rubber is pushed into defects in the road surface (this decreases with harder rubber or wider tyres.. to some extent)
and 3. chemical adhesion. this is hardest to quantify, but is why things like drag tyres (top fuel) can generate stupid amounts of traction. they kind of bond to the track, and that takes you out of "friction and normal force" territory...

with hot sticky chemically bonding tyres.. wider will always be better if you have a smooth enough track with the correct surface....

[pdawg]

the original post is wrong for the simple fact that you are referring to sliding friction and trying to compare it with rolling friction. there not the same thing at all. i think someone said something like that but too much to read. with rolling friction you need to have more surface area in contact with the road.

[out_in_front]

If you are walking on a drag strip and your laces are not done up properly you will leave your shoes behind - friction could not do that by itself. Also those suction cup things on your fridge / wall / glass hold themselves up using more than just friction. Both of these forces apply (to a much lesser extent) in tyres as well as friction.

[BigMuz]

Mainly because it just isnt that simple.

I recall arguing with 'the coach' about this...he swore that no matter how wide your tyre was the contact patch would be the same provided the weight of the car didnt change.

He didnt want to accept that a tyre isnt like a balloon.

He was right mate.

It might be a more efficient contact patch, and it may generate twice the lateral force, but it is the same size.

[BigMuz]

Co-efficient of friction does not increase in direct proportion to the amount of weight being applied, law of dimishing returns applies. Which is why for a circuit race car the more evenly weight is spread over the four wheels the more the total grip, everything else being equal.

True- at the right hand side of the graph.

At the left hand side it might be 2 kilos of lateral force for 1 kilo of normal force. (That agrees with your preposition but not exactly.)

So a kilo on the outside tyre that is generating a kilo of lateral force may generate 2 kilos of lateral force if applied to the lightly loaded inside wheel.