Like many other components on your car, tires usually wear out before they fail. Tire wear is a serious issue for those concerned about vehicle safety, since bald tires offer far less grip in the rain and are more likely to blow out at random.
Tires have three basic parts, the tread -- which touches the ground -- the sidewall that connects the tread to the rim and an inner layer of rubber that seals the air inside. When the tread wears down, it exposes the reinforcing bands underneath.
Many drivers assume that if the tire still has visible tread depth and the reinforcing bands aren't visible, then the tire is still good. However, the Federal Motor Carriers Safety Administration says that 1/8-inch of tread is the safe limit for steering tires and 1/16-inch is the minimum allowable tread for non-steering tires. While that applies specifically to tractor-trailer tires, it's a safe rule for cars as well.
A bald tire may be one without enough tread to function properly, but don't assume the tire is safe just because the tire still has some tread. Bad suspension alignment and improper tire pressure can cause the tire to wear more in one place than in another, eventually exposing the bands and causing the tire to fail. Catch these failures before they happen by periodically inspecting your tread to ensure it's wearing evenly.
