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Why Front Wheel Bearings Fail
November 19, 2016
Wheel bearings come in a variety of designs. They don't require routine maintenance, but when they fail, learning the reason is important.
What is the function of a wheel bearing?
Wheel bearings allow wheels to rotate easily with minimized friction between the wheels and the axles to which the wheels are attached. This makes the motion more efficient, meaning less effort is required to rotate the wheels and less heat is generated.
Symptoms of a bad wheel bearing
When a wheel bearing fails, the most common symptom is a roaring noise that increases with vehicle speed. Most commonly we hear the roar from 15 through 50 MPH. The noise may not be noticeable at low speeds and may even go away at higher speeds. As the bearing gets worse, the roar will normally grow louder and more pronounced. If we sharply cut the steering wheel to one side while driving the pitch of the noise may change. Determining which of the wheel bearings are bad is not always easy.
Without specialized equipment, you may have to rely on your hearing. Having someone drive the vehicle, while listening, may increase the accuracy of the process. Another clue is checking for a bearing that feels loose. They design pressed-in bearings with no slack or free-play. We can remove the wheel and apply hand-pressure to the rotor at 12:00 and 6:00 O'clock. Pushing in and out should show no movement. When a pressed in bearing fails, slack will often develop and we can use this test to identify which bearing is bad.
Replacing pressed-in bearings is quite a bit more difficult than the bolt-in variety. Integral bearings are pressed into the knuckle and replacement requires a press and very specific service procedures.
Inside view of a pressed-in bearing
The balls in a wheel bearing are hardened steel. They roll in precision-machined grooves and held together by the axle shaft or a thread fastener. They do not design pressed-in bearings to be disassembled. Any force applied to the hub will dimple the bearing races, which results in premature failure. This type bearing is destroyed when removed, so no service is possible other than replacement. They are lubricated when assembled, and the lubricant lasts the life of the bearing.
Repeat failure, after replacement is often due to not diagnosing the original cause of failure, or improper installation.
Causes of wheel bearing failure
They lubricate and seal wheel bearings when they are manufactured. Petroleum based lubricants generate pressure when they are agitated. Engineers design wheel bearing seals to allow this pressure to escape. Unfortunately wheel bearing seals are not designed to be water tight. Driving on a flooded street, or through high water allows moisture to enter the bearing. This quickly emulsifies the lubricant and wheel bearing failure will follow. Water deep enough to reach the lower edge of the wheel can cause damage. Most modern bearings are not serviceable. Any water contamination means replacement of the bearing. An impact to the wheel, such as hitting a curb, can damage a wheel bearing also.
The tire and wheel bearing relationship
All force exerted on a wheel and tire will pass through the wheel bearing. A sharp impact from hitting a pothole or a curb will easily damage a wheel bearing. The balls in the bearing may be driven into the race by the force of hitting something. This creates small dimples or imperfections. The bearings rolling on this damaged surface will generate heat and wear. Debris from the wear contaminates the lubricant inside the bearing and hastens the failure. Sometimes this occurs very quickly but other times wheel bearing failure may take months to show symptoms, after an impact.
Out of round tires cause a continuous, hammering force, that can quickly cause damage. Not only bearings, but the entire suspension can be damaged. Premature bearing, tie rod, strut and ball joint failure often results from out of round tires. Considering the costs of repairing these problems illustrates why cheap tires are just too expensive. Buying a high quality tire will cost a lot less, when considered over time.
New tires that are improperly mounted and balanced are as bad as cheap tires. A new set of tires can quickly be ruined by improper service. Buying only high-quality, name-brand tires, and proper mounting and balancing, can save hundreds in preventable repair.
Diagnosing damaged components
An impact to the wheel, such as hitting a curb, can damage a wheel bearing. The hardened rollers can dimple the races and failure will follow. The steering knuckle may also be distorted, from an impact to the wheel. This is very difficult to diagnose and usually requires a specialist.
Another method, with the bearing removed, is to use an inside micrometer to check the bore for roundness. Any out of round, exceeding .002" will require knuckle replacement. Knuckle damage may also result from improper bearing installation. A hydraulic press can easily distort components, resulting in repeated bearing failure.Repeat bearing failure, following an accident is a tip-off, to have the knuckle verified.
The flange of a bearing hub can also bend from being struck. Run-out of the flange can be measured with a dial indicator. Variation of .004" means the flange is too bad to be used. A bent flange may also cause repeated brake shudder and brake noise. Run-out of the flange causes the rotor to wobble. The shank of the hub must also be checked for being round and having the proper diameter. A hub is likely bad if the bearing slips easily on or off. The bearing to hub is an interference fit. If the bearing slips on or off the hub, likely it is worn.