Adapted from an article in Brake & Front End.
All of the weight on some vehicles may rest on an area no larger than the size of a business card. When a vehicle hits a curb, pothole or other object in its path, the force is transferred to the small surface area of the bearing. The impact may cause damage to the races and the rollers/balls. This damage is called “Brinelling.”
Brinelling is a material surface failure/defect caused by contact stress/impact that exceeds the material’s hardness limit. The result is a permanent dent or “Brinell” mark. Brinell marks may or may not cause the bearing to make noise immediately. But, as the marks continue rotating, it could be damaging the entire bearing. If the impact is great enough, the pre-load on the bearing can change, leading to more damage and noise. Brinelling damage can also be caused by incorrect wheel bearing torque.
Diagnostics
The classic symptom of a bad wheel bearing is typically a cyclic chirping, squealing or growling noise that changes in proportion to vehicle speed. The sound may disappear at some speeds or only occur at certain speeds. The noise may get worse when turning, or it may disappear momentarily. So, it’s difficult to make a diagnosis based on noise alone.
The noise can also be misleading. A caliper that’s sticking or a brake pad that’s loose and dragging may make a metallic scraping noise as it rubs against the brake rotor. The frequency of the noise will also change in proportion to vehicle speed, but will often go away or change when the brakes are lightly applied. Noise that only occurs when braking is likely a brake problem, such as worn pads, not a bad wheel bearing.
Tires can cause cyclical noise if the bands have shifted or if there is a road force imbalance. Uneven wear due to alignment issues is another factor for noise. Worn or failed bushings can also cause cyclical noise to be transferred to the body and frame.
Wheel bearing noises tend to travel through the drivetrain and exit through a transmission or engine mount that’s “grounding” the noise from the drivetrain to the chassis. This grounding effect causes a wheel bearing noise to be transferred far from its point of origin. For these and other reasons, it’s often very difficult to detect the early stages of wheel bearing failure.
If the growling noise were more noticeable in a specific gear range, a worn bearing in the transmission would generally cause the bearing noise. If the bearing noise is constant regardless of acceleration or deceleration and is approximately three times wheel speed in frequency, the faulty bearing would generally be found on the transmission output shaft or the driveshaft intermediate support bearing.
Stiff Bearings
When you pull the new bearing from the box, you will notice the bearing is difficult to turn. This is normal. The grease in the new bearing is designed to be thicker during transport to prevent balls or rollers from rubbing on the races and causing surface damage. As the bearing heats up after installation, the solids in the grease melt and change.
Removing the axle nut should be performed while the brake caliper bracket is still on the vehicle. Lock the rotor into place with a punch in the fins of the rotor. On vehicles with solid rotors, remove the nut while the vehicle is on the ground with the wheel on the vehicle. Do not use an impact wrench. Damage can occur to the CV joint or differential.
Putting the proper torque on the center nut sets the preload for the bearing and keeps the bearing from separating while in operation. The torque specification for this center nut is critical to the performance and longevity of the hub.
Some hubs come with a new nut in the box. This is typically when a one-time-use self-staking nut secures the hub. In these applications, a new nut must always be used when installing a hub. Reuse of the old nut could potentially cause the nut to loosen during vehicle operation.
Many vehicles require the wheels to be on the ground for final torquing to OEM specifications. This ensures the proper mating of the split inner rings of the bearing needed to achieve the proper internal clearance. For specific mounting instructions, refer to the vehicle manufacturer’s service manual for that model.
Pressing Bearings
When pressing in the new bearing, make sure the adapter does not come in contact with the seal, and make sure the seal with the tone ring faces inward.
When pressing the bearing into the bore, make sure it is not cocked or misaligned.
The improper installation of an axle nut is one of the most common causes of wheel bearing comebacks. Most axle nuts should not be reused. If an old nut is used, it could work loose and destroy the preload of the bearing. When the preload is lost, the bearing will make a low frequency grumble. You can retighten the axle nut to see if the noise remains. Sometimes the noise will go away, but in some cases, the permanent bearing damage has already occurred.
In the late 1970s, the hub bearing began to appear on FWD vehicles. It was a sealed, lubricated for life, pre-adjusted bearing with a mounting flange attached to the strut knuckle, or rear axle flange and a hub for the rotor, wheel and CV joint. The bearing could be either a ball or roller type. Passenger car and light truck hub bearings are not adjustable.
Flange Care
The hub is directly affected by the condition of the bearing. The driver may first notice a noise coming from the wheel of the vehicle when the steering wheel is turned. There will be noticeable end play when the wheel is unloaded. Checking with a dial indicator will show an end play greater than 0.004” (0.100 mm).
Bearing end play can also affect a wheel speed sensor and cause an intermittent ABS trouble code. If the bearing flange has runout, that runout will be magnified at the rotor friction surface. A runout of 0.0005” (0.00254 mm) at the bearing flange could result in 0.001” (0.0025 mm) runout at the rotor friction surface.
As electronic stability control (ESC) braking systems become more complex, the wheel bearing will still be the central component in the system’s operation. With the introduction of the electronic wedge brake (EWB) just around the corner, the caliper, wheel speed sensor and chassis controller will become the ABS system. These changes will require greater care in the servicing of the total suspension system.
One of the leading causes of premature hub failure is improper torquing of the axle nut. Most drive axle hubs have a center nut that must be torqued down onto the axle shaft to a specific torque value.
