That relates to the heating up and cooling down of tyres as they are used.
Depending upon the compound of the tyre, a tyre is designed to work at an optimal (optimum!!) temperature. The softer the tyre, the higher temperature it reaches and as a result, the stickier it gets.
Certain soft tyres can only cope with getting hot and then cold again a certain few times. And generally, the softer the tyre, the fewer times it will cope with these heat cycles. )If they are harder tyres, they won't heat up a lot anyway (and provide less grip), so this is less of a problem.
If you look at slicks, these are less likely to cope with heating up and down and hence need tyre warmers to keep them from cooling down and going through this cycle.
The more durable the tyre - read harder - the less it will heat up and hence the less sticky it will get.
On Moto GP the tyres are only designed for one race anyway so as long as they heat up once, they don;t care once they're cooled down - they just bin 'em.
Did that make sense?
Tim
:frog :