bigger tyres and more grip

[Mini_Me]

How do you describe, in a physics point of view, why a wider tyre has more grip? This question sounds really stupid, and probably is, but i can't see it.

The simplified formula for friction being: friction = friction co-eff x support force.

The support force for all four tyres combined in a simplified situation doesn't change? (equals weight of the car) and assuming the same type of rubber, neither should the co-efficient of friction.

Or to think of it another way, a larger contact patch would just mean less pressure per unit area of the contact patch therefore generating less friction force per unit area.

If someone can explain it for both cornering and acceleration it would be great.

In another thread GTSboy said something like that the increase in grip with wider tyres can somewhat be attributed to a stiffer sidewall (often larger wheels for wider tyres, hence lower sidewall). am i talking rubbish?

[Passion-Fingers]

with a wider tyre, more er.. contact patch i guess, you get less localised heating of the tyre aswell, allowing it to continue having the same (better) friction co-efficient for longer.

....I think.

[AlexinPerth]

Basically, wider tyres will have a short, wide contact patch, and there are more of these patches which will fit on the circumference of the tyre.

A narrow tyre has a long, narrow contact patch, and fewer of these patches fit around the circumference of the tyre.

(assuming the same circumference in both cases)

Loss of grip is usually attributed with the surface of the tyre liquifying, hence too much heat. Having more contact patches around the circumference means that it takes longer to heat the tyre up to boiling point.

Additionally, have a look here:

http://www.chris-longhurst.com/carbibles/index.html?menu.html&tyre_bible.html

under the section called "Fat or thin?"

Hope this helps.

Cheers,
Alex.

[Mini_Me]

wow, a very confusing, but good read. thanks!

[GTSBoy]

The physics is really quite simple.

If you ignore the reality of tyre flexing as they rotate past the contact patch and take two tyres with the same compound rubber and the same tyre pressure, one wide and one skinny, then they will have a contact patch that is the same size and the frictional forces will be the same.

But in reality, as the tyre rotates, the bit that goes nto contact with the ground must deform from round to locally flat. This deformation cause internal frictional heating (this is the main reason why tyres get warm when driven). A skinny tyre has a longer contact patch. Therefore the amount of tyre that deforms and more importantly the amount of deformation to flatten off that longer patch makes more heat. The rubber compound must be kept within it's temperature limits or it will go off and start slipping. So you need a harder rubber compound on a narrow tyre to avoid overheating the tyre.

Expressed another way, the wider tyre can have a softer compound for the same work expected and not die. The softer compound gives the increased grip.

There are also factors which are separate but hard to isolate when testing wide vs skinny tyres, especially on the same car. These relate to sidewall height and stiffness that changes with width.

cheers

[bigmuz]

Agree^

A simple way of putting it is that if the compound is working in its correct heat range, it forms a billion little hooks that grab the road. The more the merrier! This is why we can get tyres that deliver more than 1g of lateral acceleration with no downforce.

If it was simple friction that would be unpossible!

Muz

[thehelix112]

A skinny tyre has a longer contact patch

I'm not following this point. What if you are talking about two tyres, one fat, one skinny, that have identical ODs? Then the length of the contact patch will be the same. Same deformation, same heat. The difference here may well be the thermal capacity of the rubber? More rubber means it can absorb more heat?

And, I think the wider the tyre/contact patch the more tyre undergoes forwards/backwards deformation (assuming a useful slip-ratio), hence more heat is generated. So this is a negative of wider tyres?


The rest of your points are excellent, thanks!

[260DET]

My understanding is that for the same OD, a wider tyre can use a softer compound because it will not heat up as much as its skinny brother. Softer compound = more grip of course, plus the wider one is less likely to overheat and lose grip that way.

[thehelix112]

Richard,

Yes I got that, but WHY won't it heat up as much?

Dave

[smellytofu]

Tyres heat up as they go thru the deformed and natural state as it rolls. That's why race cars warm up their tyres by inducing more deformation in the tyres when going straight by swerving left and right.

By having a smaller sidewall, the tyres heat up slower so you can run a softer compound.

[260DET]

Richard,

Yes I got that, but WHY won't it heat up as much?

Dave

More total area to heat up plus the footprint does not cover as bigger area of the total tyre tread?

[GTSBoy]

thehelix,

The answer is very very simple. For any tyre at all....any tyre, holding up the same vehicle (ie weight loading on the tyre), at the same tyre pressure, the area of the contact patch will be the same. Doesn't matter if it is a 165/80 or a 265/35. (Of course, if you were to extend your comparison out as far as pushbike tyres and mine truck tyres the rule breaks down, but that is only because they are not in scale with each other). So if the contact patch is the exact same area, and one tyre (the skinny one) is only half as wide as the other tyre, then that skinny tyre's contact patch must be twice as long. ie, the tyre must deform more as it goes around and touches the ground.

cheers