I had the same isse with other brands of tires too. It is not a structual cracks. They are cosmetic only, and are caused with flex in tires, and mostly cause buy underinflation or lack of use on the tires. That was how it was explained to me. BTW, I put plenty of hard miles on them and auto-x's too, and never had any trouble.

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1980 Euro 380 SLC (project), 1991 VW GTi (Project), 2003 VW Jetta Wagon 1.8t (wifes car), 1992 Isuzu Rodeo Sport (Parts Hauler)