[bobcowan]

Yeah, over time they do get hard. Street tires do the same thing. Check the manufacturing date stamped into the sidewall. They shouldn't be more than 4-5 years old.

They also get hard with repeated heat cycles. You're right - with autocross they don't get that hot, so they do last longer. They only way to know is to check the durometer; but you would have needed to do that when they were new.