The wheel bearings in your car take a lot of abuse. Not only do they have to support the entire weight of your vehicle while it is traveling over rough roads, potholes and maybe the occasional curb, they also have to take the lateral forces of corners you take and have to do all that while allowing your wheel to spin at thousands of revolutions per minute. Unlike your engine bearings, your wheel bearings don’t have a constant supply of oil lubricating and cooling them, they have to be self-sufficient and also be sealed tight enough to keep road dust and water out of them.
Wheel bearings are impressive machines that accomplish all these tasks for hundreds of thousands of miles. Wheel bearings can support both horizontal loading of the lateral loading of corners by being tapered. This means the bearing surfaces, in most case tiny drums that ride in a bearing race, are situated at an angle. Mechanics call these taper roller bearings. The roller bearing is the tiny drum that supports the load while allowing the wheel to turn and the taper is what supports both horizontal and lateral loading. These types of bearings work in conjunction with bearing grease that is added just before the bearing is installed and a seal that is installed outside the bearing to keep the grease in and dust and water out. If your vehicle has a solid axle like a truck or SUV’s rear axle you may have a variety of bearing types depending on the manufacturer of the axle. Also, many new vehicles today come with sealed bearings which are entire units that cannot be serviced but must be replaced in their entirety. Sealed bearings are very convenient and easy to replace but sometimes cost more than old tapered style roller bearings. Sealed bearings are mostly found on the front wheels of trucks and SUVs with independent front suspension and on most newer cars.
No matter what type of bearing you have, the symptoms of a failed bearing are the same. A wheel bearing will usually fail due to pitting or small damage on the surface of the rollers or the bearing race. Both the surface of the rollers and the race is precision machined to tight tolerances and highly polished to allow the rollers to pass easily over the race with the addition of bearing grease for lubrication and cooling. Over time the bearing will wear slightly, allowing microscopic pieces of metal into the grease. These microscopic pieces of metal, as well as any contamination that gets into the bearing, will wear away at the highly polished surfaces causing pitting and deformation. Since so much weight is riding on your wheel bearings, and so much load is put on them in corners, even the smallest amount of surface damage to your bearing can cause a lot of noise.
Bearing noise can sound a lot like a brake pad dragging or grinding. It can also sound like whirring, whining or humming depending on how much sound deadening material your vehicle has in it. Bearing noise will always be dependent on vehicle speed meaning as you speed up or slow down the noise should change frequency or loudness. Cornering may affect the noise from a damaged front wheel bearing but you may not notice it if it is a rear wheel bearing issue. Also, the longer you drive, the often the louder the humming or whining will be as the bearing grease heats up. Bearing damage usually happens slowly over the course of days or weeks or maybe even months. You may notice the noise getting louder and louder over 50-100 miles. Make sure you replace a worn bearing quickly, but it may be ok to drive for a few days to verify you have a bearing problem. A worn bearing will generate more heat than a good bearing so you can sometimes find a worn bearing by feel your vehicle’s wheels near the lug nuts. If one wheel is significantly hotter than the others, it may have a bad bearing.
The best way to check for bearing damage is raise both front wheels off the ground if it is a front wheel bearing issue, or both back wheels if the noise is coming from the rear. Anytime you have multiple wheels of your vehicle off the ground, be careful to have it properly supported and blocked. Make sure your vehicle is in neutral and the emergency brake is off. Next, spin both of the wheels. They should both spin freely and easily and continue to spin after you stop pushing them. If one wheel turns significantly harder than the other then it has a bad wheel bearing. If your wheel feels loose on your car and all the lug nuts are tight then you have an extremely damaged wheel bearing and should consider replacing it before driving the vehicle again.
Always replace your wheel bearings with the manufacture recommended bearings. Wheel bearings can effect fuel mileage, performance and ride quality so make sure you get the right replacement parts.
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