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.
In a typical 3,400 lb. sedan, each pair of front wheel bearings as well as the rear wheel or axle bearings support 850 lbs. depending on the front-to-rear weight distribution of the car. That's a lot of weight to carry for tens of thousands of miles. The constant load eventually takes a toll on the bearings, even with proper lubrication and adjustment.
Tapered roller bearings are used in a number of automotive applications including inner and outer wheel bearings. Two things make tapered roller bearings different from other types of bearings.
Like a ball bearing which uses small steel balls between the inner and outer races to reduce friction, a tapered roller bearing uses rollers for the same purpose. The wider footprint provided by the rollers allows the bearing to support a much greater load.
Second, the rollers are angled so that they rotate in a cone-shaped path. In a straight roller bearing, the rollers are parallel to both races and all the rollers are perfectly cylindrical. But this design cannot handle side forces so a tapered design is needed for applications like wheel bearings, axle and differential bearings that must cope with horizontal thrust as well as vertical loading.
Quality bearings are made of high-grade steel, and the inner and outer races (called the cone and cup) as well as the rollers are case hardened (carburized and heat treated) to harden the surface of the metal. This improves wear resistance and the bearing's ability to withstand loads.
Tapered roller bearings are precision fit assemblies. So if the rollers, cup or cone are worn or damaged, the entire bearing must be replaced as a unit.
Most wheel bearings are capable of going well over 100,000 miles, but may fail if misadjusted, contaminated by dirt or water, or loss of lubricant.
Worn or leaking grease seals can allow grease to leak out of wheel bearings, and dirt and water to enter the bearing cavity.
Water is murder on wheel bearings because it causes rust and contaminates the grease. Any vehicle (especially boat trailers) that has been driven through hub deep water should have its wheel bearings cleaned, inspected and repacked with fresh grease.
NOISE. The first symptom of wheel bearing trouble is usually noise.
A rumbling, growling, chirping or cyclic noise of any kind from the vicinity of the wheels is a good indication that trouble is brewing and attention is needed.
If ignored long enough, a bad wheel bearing will eventually fail - sometimes with potentially deadly consequences if the vehicle loses a wheel! Maneuvering a three-wheeled vehicle is a feat best left to a trained stunt driver. So if a bearing is making noise, it should always be inspected.
Wheel bearing noise is usually proportional to vehicle speed, and does not change when accelerating, coasting or decelerating (which distinguishes it from differential, transmission or U-joint noise). The noise may change when turning, or become louder or even disappear at certain speeds.
But it shouldn't be confused with the clicks and pops produced by a worn outer CV joint on a FWD car. A bad outer CV joint usually only makes noise when turning, not when driving straight ahead.
Noisy or rough bearings need to be removed and inspected. If the bearing rollers, balls or races are worn, pitted, cracked or feel rough when rotated as an assembly, replacement is required. Heat discoloration indicates loss of lubrication or overtightening.
The bearing hub bore should also be inspected for damage and proper bearing fit. If the bore is damaged or does not hold the race snugly, your customer will need a new hub, rotor or drum.
Spindles on rear-wheel drive vehicles also need to be inspected for straightness, damage or cracks. Any problems here would call for replacement.
BEARING PLAY. Another problem that sometimes occurs with wheel bearings is play. 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 an out while watching for any looseness. As a rule, there should be no play on most FWD cars, but up to 0.010 in. of play in the front bearings may be acceptable on RWD applications with adjustable bearings.
If there's play in sealed bearings, replacement is necessary. But if the bearings are the adjustable variety, a simple adjustment might be all that's needed. In any event, "loose" wheel bearings should not be adjusted until the bearings have been removed and inspected for possible damage.
Bearings don't loosen up under normal use. So if the bearings are loose, the bearings may be worn, the hub nut may have backed off (a broken cotter pin or retainer), or the bearings were not adjusted properly in the first place.
Spinning the tires by hand is another way to check for bearing problems. Some brake drag is normal, but any noise from the bearings or roughness calls for further inspection.
PREVENTATIVE MAINTENANCE & SERVICE. Wheel bearings that are not factory sealed require periodic maintenance. But many are neglected and never receive any maintenance whatsoever except maybe when someone is doing a brake job. Even so, the adjustable variety should be cleaned, inspected and greased about every 30,000 miles or according to the vehicle manufacturer's recommendations.
In the case of trailer wheel bearings, even more frequent maintenance is required if the bearings have been in water. With a boat trailer, annual cleaning, inspection and regreasing is highly recommended (usually at the end of the season).
When wheel bearings are serviced, the old grease should first be removed and the bearings cleaned and inspected before being repacked with fresh grease. This will remove any contaminants from the hub that may cause problems later on, and eliminate any risk of incompatibility between the old and new grease.
Bearings must be dry before they're repacked with grease. Also, they should not be spun dry with an air gun because doing so may damage them. A lint-free paper towel should be used to dry the bearings as cotton rags can leave behind fibers that may cause trouble later on.
Recommend a quality high temperature wheel bearing grease (a #2 NLGI lithium-based grease, for example) or a synthetic wheel bearing grease. When the grease is added, about 3 heaping tablespoonfuls is usually enough for most passenger car and light truck hubs. The hub should not be packed solid with grease to allow room for expansion.
BEARING INSTALLATION. When wheel bearings are installed, they should always have new grease seals. Prying out the old seals usually bends and distorts them, even if they're brand new. So advise your customer not to reuse old seals.
Old seals can leak grease onto the brake, contaminating the linings causing uneven braking and may cause a brake pull to develop.
Bearing adjustment is very important! Overtightening adjustable tapered roller bearings is a common error that can lead to premature failure. Tapered roller bearings on the front of RWD vehicles are never preloaded.
They're snugged up with no more than 15 to 20 ft. lbs. of torque while rotating the wheel to make sure the bearings are seated. Then the adjustment nut is loosened 1/6 to 1/4 turn, and locked in place with a new cotter pin. As a rule, end play should be about 0.001 to 0.005 inches.
On FWD cars with adjustable tapered roller rear wheel bearings, the bearing adjustment procedure is usually the same as with RWD vehicles (zero preload) - but some do require a slight preload. Ford, for example, says the rear wheel bearings on a Taurus should be lightly preloaded to 24 to 28 in. lbs. (2 ft. lbs.). A new cotter pin should be used to lock the hub nut in place. If the hub nut is a locking nut with no cotter pin, recommend a new nut.