Freehubs

A freehub is a bicycle hub component that contains the ratcheting mechanism and provides a mounting surface for a cassette. It allows the wheel to rotate freely when the rider stops pedaling, while still driving the wheel when power is applied. The freehub is the part of modern rear hubs that engages with the rider’s crank during forward pedaling and disengages during coasting.

Historically, the ratcheting mechanism could be built into a screw-on freewheel that threaded onto the hub. In a freewheel system, the whole freewheel unit carries the pawls and ratchet. In contrast, a freehub places the ratchet and pawl mechanism inside the hub body and uses a separate cassette that attaches to the freehub. This arrangement enables larger gear ranges, easier cassette changes, and generally more robust construction.

Construction and operation: The freehub body is attached to the hub and provides a splined surface for the cassette. Inside the body, pawls engage a ratchet ring mounted on the hub shell when pedaling, driving the wheel. When the rider stops applying pedal force, the pawls disengage and the hub free-rotates, allowing coasting. Modern systems typically use sealed bearings and standardized spline patterns to accept multiple sprockets.

Compatibility and maintenance: Freehubs are not universally interchangeable; different brands and speeds use distinct spline interfaces and widths. The most widespread are Shimano/SRAM-compatible freehubs, with Campagnolo and some other brands using different standards. Some 12‑speed systems require specific freehub bodies. Maintenance generally involves keeping the mechanism clean and lightly lubricated, and replacing worn pawls, ratchet rings, or the freehub body if wear is excessive.