The Causes of Cupping in Tiresby Richard Rowe
A good mechanic, like a good doctor, is first and foremost a diagnostician -- a person who knows where to look for problem signs, and how to recognize them for what they are. Tire wear patterns can offer an amazing degree of insight into suspension and chassis problems, provided you know how to read the signs.
What It Is
Tire cupping -- aka "scalloping" -- is a kind of tire wear consisting of regular divots or scoops carved out of the tire's tread. Depending on the location of the cupping, you may feel it as a vibration that increases with vehicle speed before you start seeing scoops of flat spots appear on the tire. The sensation is not unlike that of having an unbalanced wheel or tire -- which is ironic, since unbalanced wheels and tires are one possible cause.
Middle of the Tread
Scalloping can happen along the center of the tire's tread, or along one side. If it occurs down the middle of the tread, the cause usually goes back to the tire or suspension on that wheel regularly bouncing as it goes down the road. Under-inflated tires are prone to center-tread scalloping, particularly if they're very tall or low-quality tires. Underinflation tends to cause lots of small scallops. The same is true of unbalanced wheels and bad wheel bearings, but these scallops tend to be deeper, sharper and less tapered on either side. Larger, broader scallops down the center are more often the result of bad shock absorbers.
Along the Edge
Scalloping along one edge, typically the inner edge, tends to happen as a result of problems deeper in the suspension. Bad wheel bearings can cause edge scalloping, but so can bad ball joints, bad steering or sway-bar end links, damage to the steering linkage or control arm, and severely damaged suspension bushings. This kind of wear happens because something in the suspension is bouncing or vibrating when that wheel is compressed during cornering. Edge scalloping could be a sign of impending disaster, so have it checked out immediately.
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