A wheel bearing is a set of steel balls held together by a metal ring called a race. They help wheels spin fast with as little friction as possible. They are used on all kinds of vehicles, from bicycles to aircraft and cars. On a car, a wheel bearing rides on a metal axle shaft and fits tightly inside the hub, which is a hollow chunk of metal at the center of the wheel. The hub holds the lug bolts that you use to bolt the tire onto the wheel.