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Wheel bearings perform two very important jobs in a vehicle’s Suspension. They allow the wheels to rotate with minimal friction, and they support the vehicle’s weight. To do both jobs, the bearings must be in near-perfect condition. The seals must also be leak-free, too, to keep the lubricant inside the bearings and contaminants out.
In a typical 3,400-lb. sedan, each pair of front-wheel bearings, as well as the rear-wheel or axle bearings, support 850 pounds depending on the front-to-rear weight distribution of the car. In a large 6,000-lb. SUV, each bearing carries about 1,500 pounds. It’s a lot of weight to support for tens of thousands of miles.
Though most wheel bearings are engineered to last 150,000 miles or more, the constant load takes a toll on the bearings, the grease and the seals. A wheel bearing may fail prematurely as a result of misadjustment (in the case of serviceable bearings), contamination or loss of grease. The seals are the most vulnerable part of the wheel bearing assembly, and once a seal starts to leak, the bearings are in trouble. A worn or damaged grease seal can allow grease to leak out of the bearings, and dirt and water to enter the bearing cavity. Unless somebody discovers and corrects the problem almost immediately, the bearings are doomed.
With sealed bearing assemblies, the seals cannot be replaced separately. If a seal has failed, the entire hub assembly must be replaced. But on older vehicles that do have serviceable wheel bearings, a bad seal can be replaced with a new one to extend the life of the bearings.
Water is bad on wheel bearings because it causes rust and contaminates the grease. Most passenger car and light truck wheel bearing seals are not designed to keep out water which is under pressure against the seal (as is the case when the hub is submerged). Consequently, any vehicle (especially boat trailers) that has been driven through hub-deep water or caught in a flood should have its wheel bearings cleaned, inspected and repacked with fresh grease. This obviously is not possible with sealed wheel bearing assemblies, so all you can do is check for play or noise — and replace the unit if it has suffered damage.