Technical Reference
Wheel Bearing Glossary
Plain-English definitions of the technical terms your mechanic and repair manual will use.
Hub Assembly
A sealed, pre-greased bearing unit that combines the bearing races, rolling elements, seal, ABS sensor ring, and wheel flange into a single bolt-on component. Replaced as a complete unit when worn. Standard on most post-2000 passenger vehicles.
Tapered Roller Bearing
A bearing design using tapered rollers between inner and outer races. The tapered geometry allows the bearing to handle both radial (vertical) and axial (side) loads simultaneously. Used in older vehicles, heavy trucks, trailers, and applications requiring adjustable preload.
Bearing Preload
A slight compressive force applied to a bearing assembly during installation. Correct preload eliminates play without causing excess friction. Tapered bearings require manual preload adjustment during installation (typically 5-15 in-lb drag). Too much preload causes overheating; too little causes play and accelerated wear.
Spalling
Fatigue failure of bearing race surfaces where small flakes of metal break away from the surface. Spalling occurs after millions of stress cycles when the race surface's fatigue limit is exceeded. The spalled surface causes rough running, noise, and metal contamination of the bearing grease.
AllData Flat-Rate
A labor time guide published by AllData (owned by AutoAnything/Cox Automotive) that specifies the industry-standard labor time for each repair procedure on each vehicle. Shops bill these times rather than actual clock time. The guide is derived from factory service manual procedures.
ABS Tone Ring
A toothed ring pressed into the hub assembly that the ABS wheel speed sensor reads magnetically. As the tone ring passes the sensor, it generates a pulsed signal proportional to wheel speed. A failing bearing with play corrupts this signal, triggering the ABS warning light.
Knuckle Bore
The precision-machined hole in the steering knuckle into which the hub assembly is pressed or bolted. The bore diameter is held to tight tolerances (H7 or H8 fit). Damage to the knuckle bore from a catastrophically failed bearing can require knuckle replacement ($150-$400).
Inner Race / Outer Race
The two precision-ground rings between which the rolling elements (balls or rollers) run. The inner race is pressed onto the shaft or spindle and rotates with it. The outer race is pressed into the housing or knuckle bore and remains stationary. Spalling typically begins on whichever race carries the highest load.
NLGI Grade
National Lubricating Grease Institute grade for grease consistency. Wheel bearing grease is typically NLGI #2 (the most common multipurpose grade) or #3 (stiffer, used in high-temperature applications). High-temperature rating of at least 350°F is required for wheel bearings near brake rotors.
Axle Nut Torque
The specification for tightening the large nut that clamps the hub assembly onto the axle shaft. Typically 150-230 ft-lb depending on vehicle and axle size. Under-torque allows hub play; over-torque causes overheating. Always use a torque wrench; impact wrenches alone are insufficient for accurate torque.
Brinelling
Dent-like indentations in a bearing race caused by a single overload impact (pothole strike, kerb hit). Unlike spalling (fatigue), brinelling happens instantly. A brinelled race creates a notch that causes vibration at a specific speed corresponding to the roller passing frequency.
Press Fit
A mechanical interference fit where the hub assembly is pressed into the knuckle bore (or onto the shaft) with force because the hub is very slightly larger than the bore. Press fits require a hydraulic press or hub press adapter to install and remove correctly. Hammering the component directly can damage races and bearings.
CV Joint (Constant Velocity Joint)
A flexible coupling in front-wheel-drive and some AWD driveshafts that transmits torque at a constant speed regardless of the joint angle. The outer CV joint stub shaft passes through the center of the wheel bearing. A failed wheel bearing with play can accelerate CV joint wear on driven axles.
Flat Rate
A shop billing system where the customer is charged for a fixed number of hours (per AllData or Mitchell1 guides) regardless of actual labor time. A skilled technician who completes a 1.5-hour job in 45 minutes profits; one who takes 3 hours absorbs the difference. All repair bills reflect flat-rate hours, not clock time.
Wheel Speed Sensor
The magnetic or Hall-effect sensor that reads the ABS tone ring to determine individual wheel speed. Signals feed the ABS, traction control, and stability control systems. On modern hub assemblies the sensor is a separate bolt-on component; on some designs it is integrated into the hub and replaced with it.
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