This is from an old post I found on superbikeschool.com

'Street compound tires will wear much slower than race compound and are designed to wear off old rubber to be replaced by whats underneath. The compound is designed to last thru many heat cycles.


Bluing can be caused by more than one thing. Bottom line, it's rubber that has lost it's oil. The rubber is 'cooked'. Finding a narrow band of blue at the edge of the tire is not abnormal. It does not mean the whole tire is 'cooked'. The next time you roll it over to that point the old stuff will wear off and newer rubber exposed. Theoretically.

Regarding race compound tires...

Rubber will also react with the oxygen in the air to turn blue. Another rule of thumb: stick your thumb nail in the rubber. If it comes back, that's a good thing. If you can't dent the rubber with your fingernail...bad. Learn to read tire codes. Especially the "born on" date. Compound codes and country of origin might be nice to know as well...but, the rule is two years old. No matter what color it is. '
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NSR300R - Why did i ever have a 250...