A good example of the importance of Tg is the difference in the behavior of PET and the nylons when used for tire cord. As the tires roll at high speed, they heat up to temperatures above the Tg of nylon but below that of PET. When the vehicle stops for a period long enough to allow the tires to cool, the flat spot that occurs where the tire rests on the road surface is set in to the nylon. Then, when the vehicle begins to move again, the nylon cord retains the flat spot \until the tire heats back up. Until that happens the result is a bumpy ride.
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