Wheels turning at 800 revolutions per mile can generate a lot of friction and heat. Roller-type or ball bearings packed in grease inside all four wheel hubs allow wheels to spin freely without generating excessive heat while supporting the weight of the vehicle. In addition, by reducing friction they improve fuel economy. Most wheel bearings are sealed and don’t need regular maintenance, but some need fresh grease periodically. They typically last more than 100,000 miles. Bearings can also be damaged by hitting potholes or curbs or in an accident. They can also fail, often because seals wear out and allow grease to escape and dirt and moisture to enter. When they do fail, they often make a roaring or rumbling noise. On vehicles with sealed bearings, the entire hub assembly must be replaced when the bearings fail.
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