Axle BearingsAxle bearings are what your vehicle rolls on. Whether your vehicle is front wheel drive (fwd), rear wheel drive (rwd), or all wheel drive (4wd), it has an axle bearing at each wheel. In this article we will explain what they are and what they do, and what to look and listen for if you suspect you have a bad axle bearing. All vehicles ride on axle bearings, there are different applications depending on the vehicle, but the same principle applies. Rear wheel drive vehicles use an axle that is mounted in the rear end housing and utilizes an axle bearing at the outer (wheel) end to support the axle. Front wheel drive vehicles have an outer axle bearing as well sometimes referred to as a “hub bearing”. Both axle bearings do the same job; they are just a little different in appearance. Rear wheel drive axle bearings are usually pressed into either the rear end housing or are pressed onto the axle itself whereas front wheel drive axle bearings are mounted in the front spindle assembly. Fwd applications usually bolt into the spindle assembly, but on older models they are pressed in and will require special tools and equipment to replace them.Axle bearings can last over 100k miles depending on the application. A truck used to carry heavy loads will most likely need them before a small fwd vehicle that is used for commuting. When wheel bearings begin to go bad, you will usually hear a slight grinding noise, this noise will usually change when turning the vehicle as the load changes on the bearing when the car transfers weight. In most instances if you turn the vehicle right and the noise goes away or diminishes, you most likely have a bad right side axle hub bearing, same theory goes for left turns. On rear wheel drive applications, a gear oil leak is usually the first indicator of a bad axle bearing. When the bearing begins to go bad, it will let the axle move more than the axle seal can compensate for, causing a gear oil leak. This should be repaired as soon as possible, if the leak is bad enough it can actually catch fire from the heat generated by the brakes. When replacing your axle bearings, always use a high quality OEM (Original Equipment Manufacturer) replacement part. Less expensive bearings are that for a reason, they usually use a lower grade of steel and can fail much sooner than an OEM part.Bearings consist of an inner race, outer race and the bearings themselves that are between the two races. The races are hardened steel pieces that isolate the bearing from the piece it is pressed into (axle tube) or mounted onto (spindle). Whether the inner or outer race is the stationary one will depend on the application. All bearings are constructed of hardened steel.Axle bearings are made in much the same way, with some slight differences depending on application. Severe duty applications will use a roller style of bearing, which means it consists of several cylindrical hardened bearings riding between two races, this style of bearing helps dissipate the load over a wider area. Vehicles that don’t see severe duty usually use a ball style bearing, which is the same inner and outer race, but with round bearings instead of cylindrical. Both of theses style bearings are usually used in rwd applications and are lubricated by the rear end oil. Most fwd applications use what is called a ”sealed” bearing assembly. These bearings require no external lubrication and contain the seal as part of the bearing assembly. On older rwd vehicles the front axle bearings, or “wheel bearings” as they are commonly called consist of 2 separate bearings, inner and outer and a grease seal. These are roller style bearings, usually cone shaped so that the axle nut can set the preload (tension). Wheel bearings of this style require the proper high temp bearing grease to keep them lubed, and will fail if not properly lubed.Replacing axle bearings can be a difficult task for the do-it-yourselfer. On fwd applications, you will need to separate the lower ball joint from the control arm, remove the axle from the outer hub assembly, and then bolt in the new one, reverse procedure to assemble. If the bearing hub assembly doesn’t bolt in, you may need to remove the entire spindle assembly and have it pressed apart and together by an automotive machine shop. Rear wheel drive vehicles require the axle to be removed from the rear end housing. On Some applications the axle is held in with bolts at the end of the axle tube. After removing the brake caliper and rotor, the axle should come out with the removal of these bolts. Other models will require the differential cover to be removed to gain access to the cross pin and retaining bolt. Once removed the axle can be pushed in a little allowing the “C” clip to be removed, once the clip is removed the axle will pull out of the rear end housing. Then the axle bearing can be removed with a slide hammer and the proper attachment. The new bearing should be installed with the proper tool to keep it from being damaged. Always replace the axle seals when doing the bearings and inspect the rear brakes for oil contamination, if this has happened the brake pads should be replaced at the same time to prevent any kind of stopping problems. As with any vehicle repair, proper safety attire must be worn and the vehicle needs to be properly supported before any work begins.