The Complete Handbook of Bicycle Headset Bearings

Chapter 1: Understanding Bicycle Headset Bearings

A bicycle headset bearing is the component that allows the fork to rotate smoothly within the frame, transferring the rider's steering input to the front wheel. It sits at the top and bottom of the head tube, where the fork's steerer tube passes through the frame. The headset bearing is the interface between a moving part (the fork/steerer) and a stationary part (the frame). Without it, the fork would be rigidly locked in place, the front wheel would not turn, and the bicycle would be unrideable.

Calling it a "bearing" is a simplification, because a headset bearing assembly is a system of components that work together. At its core, the bearing is a set of rolling elements (balls or rollers) contained between two hardened steel races. But the complete headset system also includes the bearing cups that press into the frame's head tube, the crown race that sits on the fork's crown, and the compression ring and preload bolt that remove play from the system. Each of these parts must work correctly and in precise alignment, or the bearing, no matter how high its quality, will fail prematurely.

The loads on a headset bearing are substantially different from those on, say, a wheel bearing. A wheel bearing concerns itself with radial loads - forces perpendicular to the axle. A headset bearing deals primarily with axial loads - forces along the axis of the steerer tube. When you brake hard, the force tries to push