The ticking bike wheel
Why does it tick when you stop pedalling?
The question
Stop pedalling on a bike and the back wheel starts to tick. What's making the noise?
Have a guess, then show the answer
Little spring-loaded catches called pawls. Coasting, the wheel's teeth slide past them, and every tooth pushes each pawl in until its spring snaps it back. Click.
Play
Pedal, then coast
This is the inside of the back hub. Pedal and watch the pawls lock in. Coast and watch them get pushed in and snap back. Change the speed and the number of teeth.
Coasting: 24 clicks every turn of the wheel. At 15 km/h for real, that's about 47 a second: a buzz.
What's happening
- 1
Inside the hub, spring-loaded pawls press outwards on a ring of slanted teeth that turns with the wheel.
- 2
Pedal, and the pawls jam against the steep side of each tooth. The wheel has to follow, so your pedalling drives the bike.
- 3
Stop pedalling and the wheel keeps rolling, faster than the pawls are turning. Now the teeth come at them from the other side.
- 4
A gentle slope pushes each pawl in, and its spring snaps it back into the next gap. Click.
- 5
Most hubs catch at 18 to 36 points a turn, so that's how many clicks you get each time the wheel goes round. One way it grips; the other way it lets go, one click at a time.
The maths
Clicks a second while coasting, for a ring with N teeth on a wheel of radius r rolling at speed v:
v ÷ 2πr is how many times the wheel turns each second. A road wheel is about 0.34 m in radius, so at 3 m/s (about 11 km/h) with 24 teeth that's roughly 34 clicks a second: a buzz, not separate ticks.
Simplified, and sources
This shows one common design, pawls on the hub and teeth on a ring, with simplified shapes and three pawls that click together; the drawing turns at 4% of real speed. Some hubs use two toothed rings instead (a star ratchet), and some are nearly silent.