Wheel bearing sales usually occur when a vehicle owner hears a clicking or grinding sound coming from one of his vehicle’s front or rear wheels. As with any automotive part, sales can be increased and warranty returns reduced if the parts professional has a basic understanding of how wheel bearings work and how they should be serviced. Any wheel bearing not only bears the weight of the vehicle, it must also contain the forward thrust of the driving axles. If the vehicle happens to be hauling a load or towing a trailer, the work of the wheel bearings is drastically increased. If the vehicle is driven at highway speeds, the wheel bearing also must support the lateral thrust loads of driving around curves and sharp corners. Consequently, if a wheel bearing is worn, poorly lubricated, or incorrectly installed, it will fail.
Conventional Wheel Bearings
Tapered roller front wheel bearings are sold as two separate parts: a bearing “cone” and bearing race or “cup.” The smaller front wheel bearing is designed to simply hold the front wheel on the spindle. Thanks to the positive wheel alignment camber angle used in most tapered-roller bearing applications, the larger cone is designed to bear the weight of the vehicle. When tapered roller bearings are used on some four-wheel front axles and some rear-wheel drive axles, the bearings are identical. In either case, a tapered roller bearing must be adjusted to the specified end play.
It’s always important to sell tapered roller bearings and races as sets because excessive end play causes the tapered rollers to wear in a barrel-shaped configuration. Ball bearings, which are used on some antique vehicles, will experience pitting due to their small contact areas. Like tapered roller bearings, ball bearings should be sold as sets to prevent warranty returns.
A conventional front wheel bearing can be a tapered roller bearing or, on antique vehicles, a ball bearing. The tapered roller bearings require a specified preload while bearings generally last longer with a slight amount of end play. The conventional front wheel bearing also requires a tight dust cover and a new inside oil seal to keep the dirt and moisture out of the bearing. The wheel bearings on most older vehicles also must be cleaned and “repacked” at the auto manufacturer’s recommended intervals. If the vehicle has been driven in deep water, the wheel bearings must be serviced shortly after. Above all, the bearing must be lubricated with a water-resistant wheel bearing grease that won’t squeeze out of the bearing races under heavy loads and high operating temperatures.
Sealed Hub Assemblies
Modern sealed bearing hub assemblies follow a completely different design criteria because modern front suspensions are generally designed with zero or negative camber angle. This means that the weight of the vehicle is distributed more equally between the inner and outer bearings because the wheel tilts inward at the top. So the sealed hub is generally designed with identical-diameter ball bearings that are slightly preloaded to prevent end-play from developing in the bearing assembly, which reduces rolling friction by keeping the brake pads from dragging agains the brake rotor. A preloaded hub assembly also carries higher vehicle loads while withstanding the forward thrust generated by the drive axles. As long as the hub assembly maintains its preload, the bearing seals will keep out dirt and moisture. Because spindle nut torque can affect bearing preload, it’s important to torque the spindle nut to the auto or part supplier’s recommendations.
To prevent damage, tapered wheel bearing races or “cups” should be driven into the bearing hub with a soft aluminum bearing driver available from your specialty tool source. Some front-wheel drive vehicles equipped with removable wheel bearings require special tooling available from your under-car tool supplier. While unitized bearing hub assemblies require no special tooling, they can be very difficult to remove if they’re rusted into the steering knuckle assembly. In these cases, bearing installation is best left to the service professional.