Wheel bearings come in a variety of designs. Many early designs, were made to be serviceable. Today most are either bolt-in or integral in design. Neither requires routine maintenance, but when they fail, learning the reason is important.
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. Technicians often use remote microphones or ultra-sound devices to examine each bearing while under load.
Without specialized equipment, the enthusiast will often have to rely on their 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.
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. Proper installation is covered in part two of this article.
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.
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.
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. Repeat bearing failure, following an accident is a tip-off, to have the knuckle verified.
Special tooling is available to check knuckles for straightness. Most shops and dealerships will NOT have such equipment, and a suspension specialty shop is a better bet. 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.
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.
Transient current flow
This topic is discussed in more detail in the article, Transient Current Flow, A Silent Killer. Insufficient grounds, can cause the electrical flow through the wheel bearings. Current drawn by the devices on the engine, has to return to the battery. The engine is electrically insulated by the engine mounts. If the engine grounds are poor, current will flow out through the drive-axles, bearings and knuckles to reach the body. This flow will quickly destroy the bearing and several other components, including the transmission. Technicians use a voltage-drop test to locate transient current flow.
Wheel bearing axle torque
End play on the bearing is controlled by the drive-axle retaining nut. If the nut is too loose or too tight, the bearing may fail. The specification for torque must be closely followed. Allowing a wheel to support the vehicle’s weight, before tightening the axle nut, may also damage the bearing. This sometimes occurs when the drive axle is replaced. Never allow the weight of the vehicle to rest on an untightened wheel bearing. Wheel bearing damage also results from using the weight of the vehicle, to hold the wheel, when loosening the axle nut.