It is usually a series of flat or slightly concave spots around the outer edges of a tire tread. Often caused by underinflation. Be aware that the suggested tire pressure given in the owner's manual is often less than it needs to be to prevent cupping. I had been running the suggested psi of 36 lbs and had cupping in both tires. After replacing both tires with the same OEM's I raised the psi to 40 and have had no sign of cupping after several thousand miles.
Most of the time it is caused by improper tire pressure. It usually is on the front. The front tire is not as solidly mounted to the bike as the rear. Once it starts there is no stopping it. Before you holler that you keep it at the factory pressure be advised that 2 lbs. of pressure can make a difference as well as tread design. As the tire moves over uneven surfaces, it "crawls" over small imperfections in the road and flexes. As it repeats itself it "cups". If you watch an extended front end chopper you can see this happen. Tire imbalance is another way to cause it. The tire is hitting the pavement unevenly meaning it hits harder at certain spots on the tire.
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