How to Read Your Tires and What They're Trying to Tell You
Improper tire inflation is the single most important factor that causes uneven tire wear. Tire wear will happen naturally thanks to friction that happens during driving, and it's this friction that makes it possible to drive at all. In the absence of friction (e.g., when attempting to drive on an icy surface) your car will not move. Tires are designed to create friction, which produces the force necessary to propel your car, and over time this friction contributes to tire wear.
There are three easy steps to inspecting your tires: Look, Feel, and Test. First, visually inspect all of your tires. Second, feel the tread by hand to detect wear such as cupping, scalloped dips on the edge of the tire, and feathering, treads with a rounded edge on one side and a sharp edge on the other. Finally, test all four tires with a tire pressure gauge for proper inflation. You can purchase a tire pressure gauge for a few bucks at any auto parts store.
So what exactly are you looking for while inspecting your tires? Here are a few examples of tread wear patterns that may indicate a problem:
Middle Tread Wear
Excessive wear in the middle of your tire may indicate that there is constantly too much air pressure in the tire. This high level of air pressure causes the tire to ride on the center of the tread and wears it prematurely. Sometimes this middle tread wear pattern can result from extremely wide tires on narrow rims.
Outer Tread Wear
Extreme wear on the outer edges of your tire is most often the result of consistent under inflation. Under inflation causes the tire to flatten out and puts too much contact with the road on the outer treads, which then wear rapidly. However, if you've been diligent in frequently assuring that your tires are properly inflated, this type of wear can be the result of a curved or damaged steering element or may indicate the need for a wheel alignment.
One-Side Tread Wear
Another indication of a poor wheel alignment is if you notice that an inner or outer rib wears faster than the rest of the tire. Generally, excessive curvature in the front suspension will cause too much load on one side of the tire, but it's also very possible that the misalignment could be caused by worn ball joints or worn control arm bushings.
"Feathering" and "Cupping" Tread Wear
Feathering is a condition when each tread rib develops a slightly rounded edge on one side and a sharp edge on the other, while cupping is scalloped dips that appear around the edge of the tread on one side or the other. By running your hand over the tire, you can feel the difference between the sharper edges created by feathering and the smoother dips from cupping.
Feathering is most commonly caused by an incorrect toe-in setting, but weakened bushings in the front suspension can also cause the wheel alignment to shift as the car moves down the road.
Cupping is usually an indication of damaged or bent suspension parts. A wheel alignment is a temporary cure at best. Any worn component that connects the wheel to the car (ball joint, wheel bearing, shock absorber, springs, bushings) can cause cupping. Worn components should always be replaced and the worn tire should be balanced and moved to a different location on the car. Occasionally, wheels that are out of balance will wear in this manner, but often wheel imbalance usually shows up as bald spots between the outside edges and center of the tread.
Second-rib Tread Wear
Second-rib wear is typically found only in radial tires and appears where the steel belts end in relation to the tread. If you pay close attention to tire pressure and you consistently rotate the tires, this type of wear can be kept to a minimum. A small amount of wear at the second rib of a radial tire can be considered normal, but excessive amounts of wear point toward tires that are too wide for the wheels.
Remember, the "eyeball" method of tire inflation is almost never a good idea. Simply adding air to your tires until there is no bulge at the bottom is an inaccurate guess at best. Always take a few moments to Look, Feel and Test with a reliable tire gauge and your car's tires will enjoy a long, healthy life.