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Q&A: Why Wheel Bearings Fail

The leading cause of a wheel bearing failure is its seal. The inside of a bearing can be a hot place. When a bearing is cooling off, the contracting metal, air and lubricant can create a vacuum that is usually held by the seals. If the seals are worn and can’t hold the vacuum, the bearing or sealed hub unit will suck in outside air, debris and water. In some parts of the country that use salt on the roads, it can almost be as corrosive as ocean water on the wheel bearings.

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What is the leading cause of wheel bearing failure?

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The leading cause of a wheel bearing failure is its seal. The inside of a bearing can be a hot place. When a bearing is cooling off, the contracting metal, air and lubricant can create a vacuum that is usually held by the seals. If the seals are worn and can’t hold the vacuum, the bearing or sealed hub unit will suck in outside air, debris and water. In some parts of the country that use salt on the roads, it can almost be as corrosive as ocean water on the wheel bearings.

As these contaminants circulate through the grease and between the races and bearings, the components wear. Once a bearing is worn, the wear rate is accelerated by seals that no longer keep out contaminants, and increased heat may break down and eventually expel the lubricants. This is a slippery slope that could quickly lead to catastrophic failure.

Do I need to uncover why the previous bearing failed?

Yes. When a bearing wears out, it is usually a case of inadequate lubrication, faulty installation or improper adjustment. For the repair to be successful, you must first determine why the previous bearing failed. For sealed hub units, examining the internal bearings and races is impossible.

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Interview the customer to find out what kind of roads they drive on. Also, ask what types of loads they carry. If the customer overloads the vehicle, bearing damage could be inevitable. The most common failure pattern for bearings is for those on the passenger side of the vehicle to fail first since the passenger-side bearings are exposed to the most standing water in the gutter. If the bearings on the driver’s side of the vehicle fail first, take a close look at the passenger side bearings — failure may not be far behind.

Besides reducing friction, what else does wheel bearing grease/lube do?

Lubricants aid in carrying away heat, protecting bearing surfaces from corrosion and reducing friction.

A cheaper hub unit has the same appearance as a high-quality, brand-name unit. What is the difference?

Bearings are precision products that require complex manufacturing processes. Inferior bearings that use low-quality steel and have poor heat-treating can wear and spall prematurely. The poor-quality steel may have inclusions of hard or soft metal that can cause a premature failure. In summary, an inexpensive bearing may look the same as a high-quality bearing, but sometimes it is what you can’t see that makes a difference between a comeback and a satisfied customer.

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Also, some cheaper hub units use smaller bearing sizes than the OEM intends. This can lead to premature failure. Unfortunately, the only way to tell is to destroy the new bearing.

Can wheel bearing grease wear out?

Grease is a precise combination of oil, thickener and additives. Grease acts like a sponge to retain and release the oil. But as a result of time and temperature conditions, the oil-release properties can become depleted. When this occurs, the grease becomes worn-out.

I cannot get a consistent toe reading on a car I am aligning, and the tie rods and steering gear are tight. What could be the problem?

A problem that may occur with older, serviceable wheel bearings is incorrect adjustment. Too much play can allow steering wander (which may be mistaken for worn steering components or the need for an alignment). One way to check wheel-bearing play is to raise the wheels off the ground and rock the tires in and out while watching for any looseness. As a rule, there should be no play on most FWD cars, but up to .010 inch of play in the front bearings may be acceptable on RWD cars and trucks with adjustable bearings.

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On FWD cars with adjustable tapered roller rear wheel bearings, the bearing adjustment procedure is usually the same as with RWD vehicles (zero pre-load), but some do require a slight pre-load.

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