I would say without a doubt it is the cornering forces that the tire is subjected to. A race tire will last 2000 miles on the street but only 200 miles on the track. The sides always wear out first. Just look at the tire bits on the tire after a good session. When you put seriuos G forces on that tire it is going to wear.
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Ride Fast - Take Chances
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Ride Fast - Take Chances