Bearings
Hubs Bearing Guide
The hub is the central part of your bikes wheels (front and rear), which connects to the wheels rim via the spokes and through which the axle is fitted, enabling the wheel to freely spin on two sets of bearings.
As bikes have front and rear wheels, so too have they front- and rear-specific hubs. The front hub is simply designed to enable the wheel to spin, while the rear hub is a little more complex as it also forms part of the bikes transmission ? the cassette or sprocket which drives the rear wheel is attached to the rear hub, which on most bikes also features mechanism to allow you to coast or freewheel (exceptions being fixed-gear or fixie bikes and track bikes).
Your wheels spin more than any other part of your bike. Therefore, having the best bearings in this application is crucial to producing the best results.