# How does a bike work 1

## Simple machines

With the help of a gearshift, you can adapt the effort required when riding a bicycle to your physical capabilities and the different requirements of the terrain. There are different types of circuit. The so-called chain gears and the so-called hub gears are particularly widespread.

#### Derailleur

The structure of the widely used derailleur is particularly easy to understand.

On the crank side, up to three gears (chainrings) sit next to each other on one axis. Depending on the purpose of the bike (e.g. racing bike or mountain bike), the radii of the gears are slightly different. It is common not to specify the radius or diameter of a gear, but the number z of teeth (this number is proportional to the gear radius or diameter).
Mountain bikes usually have three gears on the crank side. A common combination is e.g. 48 | 36 | 26 teeth.

On the axle of the rear wheel there is a ring gear with up to ten gear wheels (pinions) arranged next to each other. A common combination for a mountain bike with nine gears on the rear axle is, for example, 11 | 12 | 14 | 16 | 18 | 21 | 24 | 28 | 32 teeth.

If a bicycle has 3 gears on the crank and 9 gears on the rear wheel, it theoretically has 3 ยท 9 = 27 different gears.

#### Gear ratio for the derailleur

The effort required to pedal is particularly low if you choose the sprocket with the lowest number of teeth on the crank and the sprocket with the highest number of teeth on the rear wheel (see the corresponding sample exercises).
Conversely, the effort is greatest when you select the largest sprocket on the crank and the smallest sprocket on the rear wheel.

If you set the gearshift so that the effort required is small, you have to crank more often to cover a certain distance than with a gearshift with great effort.