Wheel Bearings & Wheel Hubs: What They Are and How They Fail

The wheel bearing is a round metal part found in the center of the hub that connects the axles to the wheels and helps them turn smoothly. They usually have greased metal balls encased between two rings called races. Have you ever turned the steering wheel of your car and heard that unmistakable "whirring" noise from a bad wheel bearing? Worn wheel bearings have been making this noise for as long as they have existed. As they wear out, excess play develops in the bearing. This excess play, along with dust, dirt, and debris sneaking its way inside, will end up damaging the internal bearing surfaces. Once the wheel bearing surfaces are damaged, they have zero chance of survival. The wheel bearing's condition will worsen until it finally self destructs in spectacular fashion. As you can imagine, the ideal situation is to replace the wheel bearing long before it reaches the point of destruction.

What Is a Wheel Hub?

Once you pull the wheels off of a car, the first thing that you see is the wheel hub staring straight back into your eyes. That's because the hubs are the part that the wheels bolt on to. They are round, have wheel studs sticking out of them, and are designed to spin with heavy loads sitting on them at all times. Guess what else bolts onto the hub? Brake rotors of course! Wheel hubs can be driven by the axles or just freewheeling. Every wheel hub is, in some way, connected to a wheel bearing. They are either pressed or bolted together, and they frequently come as one "wheel hub and bearing" assembly. This makes installation significantly easier and cheaper. Just pull the old hub and bearing assembly off, and throw the new one on. No heavy duty pressing or special tools are needed.

How ABS Sensors Work

Wheel hubs have very close relationships with anti-lock braking system (ABS) sensors. That's because ABS sensors measure wheel speed. To do this accurately, they need to be close to the wheels without being in the way of all of the moving parts.

Car manufacturers have several ways of handling this task. The first is with a "tone ring." A tone ring attaches to the back of the wheel hub and it looks like a gear. With the ABS hub attached to a vehicle, there is an ABS sensor that sits next to the tone ring and measures wheel speed by watching how many teeth pass by the sensor within a certain length of time.

The other common method of measuring wheel speed is by adding the whole entire ABS sensor to the inside of the wheel hub itself. This can be a blessing and a curse. It simplifies the ABS system so that when an ABS sensor fails, the entire hub needs to be replaced with it. Pretty wasteful if you have a perfectly good working hub. Another ABS system design that skips the hub connection altogether and places the tone ring on the axle instead of the wheel hub. All these methods work great, you just need to make sure that you use the proper parts that are designed to work with ABS. Without that tone ring attached to the wheel hub or axle, the ABS light will turn on, and your vehicle won't stop as it was designed to.

Wheel Hub and Wheel Bearing Replacement Facts

Wheel hubs themselves rarely have issues because they are simple blocks of steel. The most common reason to replace a wheel hub is because the wheel bearing that is attached to it has failed. Wheel bearings can fail because of age, dirt and water contamination, or improper installation. Most hubs that are driven by axles require a very specific torque when installing. If that torque is too tight or too loose, the life of the wheel bearing can become shortened. That said, most wheel hub and bearing assemblies are very DIY friendly. If you can replace brakes yourself, you probably won't have any trouble replacing a wheel hub yourself either.