How a bicycle's gears work · OakCrest Academy

🚲 How a bicycle's gears work

Your feet push the pedals, and the crank arms, which act as levers, turn the chainring at the front.
Each tooth of the chainring pulls one link of the roller chain; the links are exactly half an inch (12.7 mm) apart, pin to pin.
The chain turns a smaller sprocket on the rear wheel, so the wheel turns more times than the pedals: the gear ratio is chainring teeth ÷ sprocket teeth. With 50 and 18 teeth, the ratio is 50 ÷ 18 ≈ 2.78 wheel turns for each pedal turn.
Each wheel turn rolls the bicycle about 2.1 m, so one pedal turn carries you ratio × 2.1 m: about 5.8 m in that gear.
A bigger rear sprocket gives a smaller ratio. Each pedal stroke then needs less force but carries you a shorter distance. The ratio of the force driving the bicycle forward to the force on the pedal is called mechanical advantage, and a bigger sprocket increases it. The derailleur moves the chain from one sprocket to another.
Inside the rear hub, spring-loaded pawls catch a ratchet ring when you pedal. When you stop, they click over its teeth, so the wheel can coast (freewheel).

New words

chainring: the large toothed ring turned by the pedals
sprocket: a toothed wheel that a chain fits onto; the rear ones together are called the cassette
ratio: how many times one number contains another; here, wheel turns per pedal turn
derailleur: French for 'de-railer': the mechanism that moves the chain from one sprocket to another
pawl: a small hinged catch that lets a ratchet turn one way but not the other
circumference: the distance once round a circle; for this wheel, about 2.1 m

🏠 Try it for real

With a grown-up, count the teeth on a real bicycle's front chainring and on one rear sprocket, then divide to find the ratio. Mark the tire with chalk, turn the pedals exactly once, and count how many times the mark comes round. Does the evidence agree with your calculation? If it does not, what could explain the difference?

💭 Talk about it

Explain it: in 2–4 sentences, tell a grown-up why a 32-tooth rear sprocket makes climbing a hill easier but slower than an 11-tooth one. Use the words ratio, force and distance. History link: J. K. Starley's Rover safety bicycle (1885) was the first commercially successful bicycle with a chain to the rear wheel; earlier designs had tried the idea. Before it, the penny-farthing's front wheel had to be enormous, because each pedal turn could only turn that wheel once. Why would a chain make that huge wheel unnecessary?

Now explain it back: point at each part and say what it does.