The Purpose of Wheel Bearings
Bearings have been around since the ancient Egyptians were building the pyramids. The concept behind a wheel bearing is simple: Things roll better than they slide. When things slide, the friction between them slows them down. If two surfaces can roll over one another, friction is greatly reduced. The ancient Egyptians placed round logs under heavy stones so they could roll them to the building site, thus reducing the friction caused by dragging the stones over the ground.
Although bearings reduce friction a great deal, automotive wheel bearings still take a lot of abuse. Not only do they have to support the weight of your vehicle while traveling over potholes, different types of roads, and the occasional curb, they must also withstand the lateral forces of corners you take and must do all of this while allowing your wheels to spin with minimal friction at thousands of revolutions per minute. They must also be self-sufficient and sealed tightly to prevent dust and water contamination. Modern wheel bearings are durable enough to accomplish all of this. Now that’s impressive!
Most vehicles sold today are equipped with wheel bearings that are sealed inside a hub assembly and require no maintenance. Sealed bearings are found on most newer cars, and on the front wheels of trucks and SUVs with independent front suspension. Sealed wheel bearings are engineered for a service life of more than 100,000 miles, and many are capable of going twice that distance. Even so, average bearing life can range from 80,000 to 120,000 miles depending on how a vehicle is driven and what the bearings are exposed to.
A typical hub contains an inner and an outer wheel bearing. Bearings are either roller or ball style. Tapered roller bearings are the best alternative since they more efficiently support both horizontal and lateral loads and can stand extreme shock such as hitting potholes. Tapered bearings have bearing surfaces situated at an angle. Tapered roller bearings are usually mounted in pairs with the angle facing opposite directions so they can handle thrust in both directions. Steel roller bearings are tiny drums that support the load. The taper or angle supports horizontal and lateral loading.
Wheel bearings are made using high quality and high spec steel. The inner and outer races, rings with a groove where the balls or rollers rest, and the rolling elements, rollers or balls, are all heat-treated. The hardened surface adds considerably to the wear resistance of the bearing.
An average vehicle weighs around 4,000 lbs. That is a lot of weight that must be supported over thousands of miles. To perform as required, wheel bearings must be in almost perfect condition, have adequate lubrication, and be sealed to keep lubricant in and contamination out. Although wheel bearings are engineered to last a long time, constant load and turning takes a toll on the bearings, grease, and seals. Premature wheel bearing failure results from damage due to impact, contamination, loss of grease, or a combination of these.
Once a wheel bearing seal starts to leak, the bearing has begun the process of failure. A damaged grease seal will allow grease to leak out of the bearings, and dirt and water can then enter the bearing cavity. Water is the worst thing for bearings as it causes rust and contaminates the grease. Since so much weight is riding on the wheel bearings during driving and cornering, even the smallest amount of race and bearing damage will create noise.
If the seals on a sealed bearing assembly fail, the seals cannot be replaced separately. The entire hub assembly needs to be replaced. Wheel bearings that are not factory sealed, which are rare today, require periodic maintenance. They should be cleaned, inspected, repacked with new grease, and have new seals installed approximately every 30,000 miles or according to the manufacturer’s recommendations.
The first symptom of a wheel bearing problem is noise coming from the vicinity of the wheels. It usually starts with a barely audible growling, whirring, humming, or some kind of cyclic noise. The noise will generally increase in severity as the vehicle is driven. Another symptom is steering wandering resulting from excessive wheel bearing play.
Wheel bearing noise does not change when accelerating or decelerating but may change when turning. It may become louder or even disappear at certain speeds. It is important to not confuse a wheel bearing noise with tire noise, or with the noise a bad constant velocity (CV) joint makes. Faulty CV joints usually make a clicking noise when turning.
Diagnosing a wheel bearing noise is not always easy. Deciding which one of your vehicle’s wheel bearings is making the noise can also be difficult, even for a seasoned technician. Therefore, many mechanics often recommend replacing multiple wheel bearings at the same time as they may not be sure which one has failed.
A common way to inspect wheel bearings is to raise the wheels off the ground and rotate each wheel by hand while listening and feeling for any roughness or play in the hub. On vehicles with sealed wheel bearings, there should almost no play (less than .004 inches at most) or no play, and absolutely no roughness or noise. Inspecting for play can be accomplished by holding the tire at the 12 o’clock and 6 o’clock positions and rocking the tire back and forth. If there is any noticeable play, the wheel bearings are loose and need to be replaced or serviced.
Faulty wheel bearings may also impact your vehicle’s anti-lock brake system (ABS). Excessive play, wear, or looseness in the hub will often cause the sensor ring to wobble as it rotates. Wheel speed sensors are very sensitive to changes in the air gap between the tip of the sensor and the sensor ring. Consequently, a worn wheel bearing may cause an erratic signal which will set a wheel speed sensor trouble code and result in the ABS warning light coming on.
A wheel bearing failure can have serious consequences, especially if it occurs while driving at highway speeds and the vehicle loses a wheel. That’s why you should have an ASE certified technician inspect your wheel bearings at least annually, and test drive your vehicle to listen for any troublesome noises.