Gear trains

Gears are wheels with teeth around the outside. When several wheels are interlocked, they can transfer motion from one place to another, eg in some hand whisks or on bikes.

A close-up image of the rear of a bicycle - including the chain, wheel and brakes.

Gears can change the direction or the speed of movement. As there are teeth around the edge of the gears they grip together and so can withstand a greater force, enabling them to move large items such as cars or bicycles.

Gear trains

Gear trains are when two or more gears are joined together. In a simple gear train, the drive gear causes the driven gear to turn in the opposite direction.

A simple gear train showing drive gear with sixty teeth and the driven gear with 15 teeth.

Smaller gears with fewer teeth turn faster than larger gears with more teeth. This difference in speed is called the gear ratio.

Gear ratio = number of teeth on driven gear ÷ number of teeth on the drive gear


The drive gear has 60 teeth and the driven gear has 15 teeth.

Gear ratio = 15 ÷ 60

= 0.25

For each rotation of the drive gear, the driven gear would rotate four times.

Gear ratio = 1:4

This is known as gearing up. If the drive gear had 15 teeth and the driven gear had 60 teeth, the gear ratio would be 4:1 which is known as gearing down.


If a cyclist is pedalling with a drive gear of 50 teeth and a driven gear of 25 teeth, what is the gear ratio?

Gear ratio = 25 ÷ 50

= 0.5

Gear ratio = 1:2

If the drive gear and the driven gear are separated by another gear, called the idler, they will move in the same direction.

A drive gear and driven gear separated by an idler gear.  Directional arrows show drive and driven gears move in the same direction and the idler gear in the opposite direction.