What is a Wheel Bearings?
Wheel bearings are designed to support and guide the rotating wheel and hub assembly over a fixed or steered axle. There are two main types of bearings used, which are classified by the shape of the rolling parts. Ball race bearings which have ball shaped rollers and taper roller bearings, which have cone shaped rollers.
The ball race bearing cross section shows the balls running in groves supported by a carrier. This type of bearing can carry both radial and axial loads in both directions. This type of bearing is typically used for the front axle of front wheel drive vehicles, although many modern vehicles have ball race type bearing fitted to both axles. Adding an extra row of bearings can increase the load capacity of the ball race bearing.
Taper roller bearings are always fitted in pairs facing opposite. This configuration is capable of carrying very high radial and axial loads and is usually found on the rear axle. These bearings are usually adjustable and can be inspected during a service. They are either lubricated by the rear axle oil, or can be removed, cleaned and repacked with grease, in both cases the lubricant is prevented from leaking by an oil seal. Roller bearings are sealed and no maintenance can be carried out.
Wheel bearings are precision made from high quality hardened steel with the cases made from bronze, aluminium or mild steel and generally last for may thousands of road miles. Wheel bearings can fail due to hard impact such as striking the curb, continued use on rough terrain or poor maintenance, but in general, the longevity of a bearing is determined by lubrication. If a seal fails then oil or grease will escape and water, brake and road debris will be able to enter the bearing case. This will massively reduce the life of the bearing. Bearing failure can be detected by a whining noise from a road wheel area, or if excess wear has taken place, a rumble. Once a bearing noise has been detected it is important to have the bearing replaced as the noise is due to wear and lateral movement. This movement will generate excess heat and further wear which can damage other components including the axle, hub and brake callipers, and in extreme circumstances the wheel and hub can fall off the vehicle.
Replacing wheel bearings is a specialist job where cleanliness is essential and typically a press is needed. When a bearing is to be pressed in, care must be taken not to apply any pressure to the inner race, as this will damage the bearing. In the case of taper roller bearings, adjusting the bearing to remove end float with the correct preload is a job for an experienced technician. With many vehicles having ABS braking, some bearings have magnetic pickups fitted which will not stand any impact and must be fitted the correct way round, or the ABS system will not operate.