Thursday, November 23, 2017 Detailed Auto Topics
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A hum or roar that increases with the speed of the vehicle is often a bad front wheel bearing. Like most things, the front-wheel bearings have undergone an extensive evolution. Many today are sealed and the only service is a replacement.

In the past, front wheel bearings had individually serviceable parts. The hub could be disassembled, cleaned, bearings packed with grease and adjusted. Today, the majority of front-wheel bearings are sealed. Some are pressed into the knuckle and require special tooling to replace. Others come as a hub bearing assembly and may be replaced with simple hand tools.

Parts of a GM sealed wheel bearing

Most domestic vehicles, including GM truck, SUV and car models use a hub bearing assembly on the front. The bearing, hub, mounting and wheel flange are built as an assembly. The ABS sensor, if so equipped, is also normally included. The advantage of the sealed bearing is ease of replacement. Unfortunately, they cost several times more than the press in or old style serviceable bearings.

A roar or hum from the wheel often means a bad wheel bearing 

Front wheel bearings fail for a number of reasons

An impact to the wheel may internally damage the bearing. Months may pass before the telltale hum noise starts. Once a sealed wheel bearing starts to make noise or develops slack, it will fail and has to be replaced. Driving through water may also cause wheel bearings to fail. The bearing is sealed, to keep lubricant in, but not to keep water out. When water enters a sealed wheel bearing the lubricant is destroyed, and the bearing will fail.

Out of round tires also cause damage to wheel bearings. A tire that vibrates will destroy a wheel bearing. Sealed front wheel bearings may be very expensive, many $250.00 to $500.00. Cheap tires, improperly mounted, are just too expensive, when the cost of wheel bearing damage is considered.

Replacing a GM sealed bolt-in bearing

rear view of bolt in sealed wheel bearing

Looking at the rear of the knuckle, two of the retaining bolts are visible. There are normally three or four, depending on the design. On front wheel and four-wheel drive vehicles, there is also an axle that passes through the bearing. The large nut on this axle needs to be removed. It is handy to have someone hold the brakes, to prevent the wheel from turning, while the nut is loosened. Once the axle nut has been loosened, never allow the weight of the vehicle to be applied to the wheel. Without support from the axle, the bearing will be damaged.

Support the brake caliper once remove to avoid damage to the hose

After the axle nut is loosened, the brake caliper has to be removed. There are two bolts on the rear of the knuckle. The caliper should be suspended from the body, using mechanic's wire. Allowing the brake caliper to hang from the brake hose will cause damage to the hose.

front flange of GM bolt in wheel bearing with brake rotor removed

Once the caliper is removed, the brake rotor will slip off. In some cases, the rotor may be rusted in place. Cleaning rust from the center hub and applying penetrating oil may help. You may also be able to use a block of wood to strike the back side of the rotor and free it from the hub. Clean any oil or finger prints from the rotor before reassembly to prevent brake problems.

Bearing removed from knuckle and ready for reassembly

After the retaining bolts and axle nut are removed, the hub bearing will slide out of the knuckle. If rusted in place, penetrating oil and a block of wood may help, as with the brake rotor.  Also be careful with the ABS sensor, if present.  Disconnect the sensor and any clips retaining the wire.

Clean all surfaces with a wire brush and reinstall the new bearing. The retaining bolts and drive-axle nut have a torque specification and should be properly tightened. Torque specifications are available through service data and are often supplied with the bearing. Never use an impact wrench on the large axle nut, without first tightly locking the brakes. The impact of the wrench may damage the axle shaft or transmission, if allowed to travel through.

Bolt-in sealed wheel bearings can last the life of the vehicle. Good tires, properly balanced and avoiding curbs and high water will prevent most problems.  If they do fail, they are not difficult to replace. With a little care and a few tools, the job can often be accomplished in about an hour.





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