How to Diagnose Tire Wear Cuppingby Jody L. Campbell
Tire wear is to your car's suspension what blood pressure and pulse rate are to your heart, and diagnosing tire wear patterns can tell you as much as an EKG readout would to a doctor. A good diagnostician is much like a doctor, examining data that might mean very little to anyone else, and reading into it to uncover the the problem it indicates. To do that, you're going to have to understand a bit about how your suspension does what it does, and what happens when it does the wrong things.
Determine whether or not you've got cupping -- aka "scalloping." Scalloping is the result of a cyclic loading and unloading of the tire, either because the tire is bouncing or because something is wobbling in the suspension. If the tire is safe enough to drive on, then take it for a test drive and roll the windows down. A scalloped tire will generally emit a rhythmic thrumming, possibly accompanied by shake or vibration that gets worse with higher speed. If the scalloped tire is on the front, you may be able to feel vibration, or a shake or shimmy through the wheel. This alone can indicate a lot of problems, so the next step is to check the tire itself for scalloping.
Lift the corner of the car with the suspect tire with a floor jack, and secure it on a jack stand. You need unrestricted access to the wheel, and it has to be far enough off the ground that you can spin it. With the tire up in the air, look for signs of obvious dips in the tire tread; they may be across the entire width of the tire or along one edge. A small flashlight with a tight beam, held at a right angle to the tread, will allow you to spot these dips easier because the high spots on the tire will block the light and cast shadows into the dips. A laser pointer is even better if you have a steady hand and good eyes; the tiny laser beam can expose shallower and more subtle dips in the tread.
Take note of the position of the dips in the tread, whether they're in the middle of the tire, cross the width of the tread, or occur only along one edge. If the scalloping occurs in spots down the middle of the tread, or along the entire width of the tread, then the scalloping is the result of "wheel hop." Wheel hop is exactly what it sounds like -- the wheel and tire are bouncing as you drive. Most of the time, mild "wheel hop" scalloping with shallow, subtle dips in the tread, is the result of an unbalanced tire, and it will even out after you balance the tire and drive on it for some time. However, deeper scallop marks with more defined edges can indicate that the wheel is bouncing because of a broken or worn-out shock absorber.
Note scalloping that occurs only along one edge of the tire. Edge scalloping will almost always indicate a more serious problem deeper in the suspension, or with the wheel bearing. As the suspension deflects in unintended ways, the tire will lean either in or out, putting more stress on the edge than on the center. This, combined with the regular dips of scalloping, indicates that something is wobbling in your suspension. Note first whether the scalloping is on the inner or outer edge, and whether the peaks between the scallops are jagged and sharp, or subtle and rounded off. Pay attention also to the size, depth and regularity of the edge scalloping.
If the peaks are completely rounded off, then odds are good that the tire is moving in and out while operating; these "changes in toe angle" cause the tire to scrape slightly sideways down the road, which rounds the high points off the peaks. Very rounded peaks, with a bit of "feathering" wear going into the scallop indicates a change in toe angle, which in turn indicates a bad steering end-link, bad sway bar end-link or bad control arm bushings. Depending on the suspension design, toe-related edge scalloping can occur along the inner or outer edge of the tire, though it will most often occur along the outer edge. Rounded scalloping along the inner edge might indicate strut bushing or isolator issues, though upper ball joint and lower control arm bushing failure can also cause it.
If the peaks are sharper and more sudden, the wheel is probably changing "camber" quickly, rapidly leaning in and out while pointed fairly straight. This is strongly indicative of failure of the ball joints, the strut, control arm bushing or even wheel bearings. As a general rule, wider and less frequent dips mean that the movement cycles are slower, which in turn means that heavier things are moving around. The wider the dips, the further into your suspension you can expect the problem to be. Narrower and more frequent edge dips, and peaks with sharper edges, are more indicative of a bad wheel bearing or bearing assembly than anything further in. Jagged dips occurring with psychotic irregularity mean something bad is broken or loose in the suspension, requiring immediate attention.
- There's an unrelated kind of tire problem that some also refer to as "cupping." This "cupping" happens because of low air pressure. The tire requires sufficient air pressure to push the center of the tread down; without it, the edges of the tire will push down and into contact with the road, while the middle of the tread arches -- or "cups" -- upward. Chronic low air pressure is easy to diagnose, because both edges of the tire tread will be extremely and uniformly worn, while the center of the tread looks brand new. Conversely, an over-inflated tire will lift the edges up and put more pressure on the center of the tread. This will rapidly wear the center of the tire tread, while the inner and outer edges remain pristine.
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