I worked in a tire recap shop when I was young. It takes extreme heat to affect the hardness or softness of rubber. I can't imagine we are generating that kind of heat even with the heavy and fast 1/24 scratchbuilt cars. If the tires were too hot to handle after a heat race I'd say maybe, but I just don't see it in our application.
As far as urethanes go, the molding is a cold pour process so again I think you would be talking about extreme heat to charge the tire's shore rating. With that said I do know they will "come apart" if in the truing process you get too aggressive and create too much heat. So if they were reaching a heat cycle close enough to change the shore rating I think they would just come apart. But then again that is a LOT of heat, way more than what is experienced by the car itself on the track. I did a fair amount of experimentation when I was molding urethane tires, and I do know that having them under a very hot lamp does nothing to affect the shore rating (as measured by a shore meter). The heat from the lamp was probably in excessive of any tire temperatures the cars themselves could generate.














