Freehub

A freehub is a component of a bicycle rear hub that both stores the ratcheting mechanism and provides a splined interface for a cassette. Unlike older freewheels, which threaded onto the hub, a freehub body attaches to the rear hub shell and carries the cassette. The freehub is driven by a set of pawls and a ratchet inside the body; when you pedal forward, the ratchet engages and the wheel turns. When you coast, the pawls disengage, allowing the wheel to spin while the crank remains stationary.

Construction and function involve a splined mounting surface on the freehub for sprockets and a lockring that secures the cassette to the freehub. The ratchet-and-pawl mechanism inside the freehub engages only in the forward direction, enabling freewheeling in the opposite direction. The freehub is powered directly by the hub shell, and its design determines how many gears can be mounted and how the gearing interfaces with the rest of the drivetrain.

Compatibility and standards vary. Freehubs are generally brand- and speed-specific, so not all cassettes fit all freehubs. Shimano, Campagnolo, and SRAM use different freehub interfaces, and some modern setups require different driver bodies (for example, 11- or 12-speed systems, or XD/XDR standards) to accept a given cassette. When upgrading or changing speeds, it may be necessary to replace or upgrade the freehub body.

Maintenance involves keeping the mechanism clean and lightly lubricated to ensure smooth engagement. Signs of wear or damage include grinding noises, slipping under load, or difficulty engaging the drivetrain. If the freehub or pawls wear out, replacement of the freehub body is typically required.