How do bicycle gears change the ride?

A hill can make bicycle pedals feel heavy. Then a click of the gears changes the ride: the pedals become easier to turn, but each turn carries the bicycle a shorter distance. Nothing about the hill has changed. The bicycle has changed the trade between turning force and travel.

The pedals turn the front toothed ring, called the chainring. Its teeth pull a chain. The moving chain turns another toothed ring at the back, called a sprocket. While the rider drives the bicycle forward, that sprocket turns the rear wheel. Changing gear moves the chain onto a different-sized ring.

Consider a front ring with 36 teeth. One complete pedal turn pulls 36 chain links past a fixed point. A rear sprocket with 12 teeth must turn three times to pass those links: 36 divided by 12 is 3. With a 24-tooth rear sprocket, the same moving chain produces only one and a half turns. The tire stays the same size in both cases.

A bigger rear sprocket also gives the chain more turning leverage, rather like pushing farther from the hinge of a door. For the same turning force needed at the back wheel, the rider can supply less turning force at the pedals. The price is more pedal turns to cover the same road distance. The larger rear sprocket therefore gives a lower gear.

Gears do not supply the energy for the climb; the rider does. They let a rider exchange a few hard pedal turns for more, easier ones. How fast the bicycle travels still depends on how fast the pedals turn, as well as wheel size and road conditions. A small change in where the chain sits can make human legs and a steep road work together more comfortably.