Explainer No. 5
Stop pedaling and the bike keeps rolling, with that tick-tick-tick. Inside the rear hub, little spring-loaded levers catch a ring of teeth when you push, and slide over them when you don't.
- Hub shell×1
The middle of the wheel. The spokes are laced to it, so it turns whenever the wheel does.
- Ratchet teeth×36
A ring of teeth around the inside of the shell. Each has a steep face for a pawl to push on and a sloped back for it to slide over.
- Pawls×3
Little hinged levers on the freehub body. Pushed forward, their tips jam against the teeth; overtaken, they get pressed down and ride over.
- Pawl springs×3
Keep each pawl tipped out against the teeth, ready to catch. They are what snaps the pawl back up after each tooth.
- Freehub body×1
Carries the sprockets (removed in the cut-away). The chain turns it, and it carries the pawls round.
- Chain, sprockets and cranks
Your legs turn the cranks, the chainring pulls the chain, and the chain turns the sprocket on the freehub body.
- Bearing balls×12
Let the freehub body spin smoothly on the axle. (Real hubs have them further along the axle; they're drawn in this slice for simplicity.)
- Axle
Clamped in the frame. Everything else turns around it.
Freewheel or freehub?
Until the mid-1980s most bikes had a thread-on freewheel: the sprockets and the ratchet in one unit that screws onto the hub. Most bikes now have a freehub instead, with the ratchet built into the hub and the sprockets slid onto it and held by a threaded lockring. Shimano made its first freehubs in 1978. Moving the bearings further out meant far fewer bent and broken axles, and room for more sprockets. ("Freewheel" is also the word for the mechanism itself, whichever kind, and that's how this page uses it.)
Pedaling keeps it tight
A thread-on freewheel screws onto the hub with an ordinary right-hand thread, and the chain pulls it round in the tightening direction. So it needs no tools to fit: pedaling screws it down.
Before the freewheel
On a penny-farthing the pedals turned with the front wheel, so going downhill riders took their feet off and rested their legs over the handlebars or on footrests. William Van Anden of Poughkeepsie, New York, patented a velocipede with a ratchet in its hub in 1869, and by 1899 freewheels were widely fitted to new bikes made in Britain.
Fixed on purpose
A track bike still has a fixed gear, and no brakes. The pedals turn whenever the wheel does, and the rider slows down by resisting them. Some hubs are threaded on both sides, so a fixed sprocket can go on one side and a freewheel on the other: flip the wheel round to switch.
The back-pedal brake
A coaster brake (a back-pedal brake) hub freewheels too, so you can coast. Pedal backward and, instead of clicking, it pushes brake shoes out against the inside of the hub shell. Fichtel & Sachs brought out its well-known Torpedo hub in 1903.
A quicker bite
Mountain bikers like hubs that catch almost instantly, which helps on rough, technical climbs. More teeth help, and so do pawls offset so they take turns to catch: Industry Nine's Hydra hub has six such pawls and 115 teeth, which together give 690 catches a turn, about half a degree apart. Not every freewheel uses pawls. DT Swiss presses two toothed rings face to face with springs, and sprag-clutch hubs like Onyx's, which wedge tilted pieces between two rings instead of using teeth, are nearly silent.