Subject: Why does friction increse when warming the tyres of a racing car?

I have carried out preliminary experiments into why the tyres of 
a racing car are warmed prior to a race and have discovered that it is 
due to friction, that is the frictional force between tyre and road 
surface increses with temperature. However I have been unable to find 
sufficient information to explain on a molecular level why the 
frctional force increses with temperature.

I would be very gratefull if you would be able to send me any 
information about this, or guide me in the right direction.

Re: Why does friction increse when warming the tyres of a racing car?