Road surface temperature and grip

[carver]

hot tires=grip

cold tires=most bins wins

[Blackbird]

Thoreau said "simplify, simplify."

Thoreau clearly wasn't a physicist. Road temperatures are largely irrelevant in themselves other than their ability to keep your tyres hot and elastic, which is, as the good people have said; bloody important. Of course, when the road gets too hot, you lose grip, tar goes everywhere and then you have to take to your bike with a rag and WD40 to get the stuff off.

Nothing's ever simple

[Mikkel]

So to complicate the simplification: all I want to know is how the coefficient of friction of a road surface varies with temperature in the range already stated. Nothing more. Not looking at any practical applications here. Just what the road itself does.
I told you it was a slow day...

The coefficient of friction describes how two surfaces interact - the road, in itself, does not have a coefficient of friction.
What Jantar and Blackbird said is spot on - but I would like to add to it:

Tyres warm up in two way:
1. Through friction with the surface during acceleration, braking and cornering.
2. Through deformation - i.e. the flexing of the tyre when travelling along the road.

Mechanism 1. happens only at the surface of the tyre whereas 2. happens throughout the entire cross-section of the tyre. On a cold road mechanism 1. will be less effective and if you are riding sedately on a cold day you may never gain any heat gain from 1. at all.

2. on the other hand takes place all the time that the wheel turns and since it happens throughout the tyre - not just at the surface - the heat will be retained for longer.
The larger the deformation of the tyre the greater the heat gain. Tyre deformation, generally speaking, depends upon the total weight of the vehicle, tyre volume and tyre pressure. The heavier the vehicle - the larger the deformation. The larger the tyre - the larger the deformation. Higher tyre pressure - smaller deformation.

So, on a cold surface it might be worth while dropping your pressures a little bit to allow more heat build-up in your tyres.

[cowpatz]

I would imagine the tyre pressure would have to be reduced significantly in order to get sufficient deformation to incease tyre temps by any noticeable margin. Any gains would be hard to measure as they would be offset by the effects of a consequential larger footprint area and the change to the handling characteristics due to tyre profile change and the increased sidewall flex.

[Blackbird]

I would imagine the tyre pressure would have to be reduced significantly in order to get sufficient deformation to incease tyre temps by any noticeable margin. Any gains would be hard to measure as they would be offset by the effects of a consequential larger footprint area and the change to the handling characteristics due to tyre profile change and the increased sidewall flex.

Yep. With a lardy bike like the 'bird, the bike is really sensitive to tyre pressures because of carcass flex. The standard pressure of 42psi front and rear is higher than a true sports bike largely because the 'bird is 230 kg dry as opposed to (say) 170 kg for a sports bike. Even an incompetent like me can feel a deterioration in handling if the tyre pressure drops below 38-40 psi! It's also why I can't use a lot of tyres which are suitable for sports bikes because the carcass construction simply isn't strong enough. The attached photo shows a Dunlop D220 tyre I had on the 'bird about 6 years go. The weight of the 'bird and its tendency to "push" the front in standard trim, caused sidewall flex when cornering and the "triangling/cupping" wear pattern is really noticeable. When Avon released their "B" spec Azaro and later the Storm which both had additional sidewall reinforcing, the problem went away.

It's a complicated old world, innit?

[captain_andrey]

Of course the temperature of both surfaces is important. Road, believe it or not, also flexes under the load of the tyre. On a microscopic level, there is always a difference.
Google came up with this one study on static friction co-efficient of materials relative to temperature of the material.
http://www.idspackaging.com/Common/P...oefficient.htm
This will also be incredibly easy to test if you have some spare time.