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The wheel is a very old invention (3000 BC), but the ancestor of the bike only appears in 1850. The velocipede has neither gears, chain nor pedals. The rider pushes with his feet.

The modern bike appears with the "safety" bicycle in 1890, equipped with the following transmission system:

- Two pedals attached to a front sprocket (chain wheel) but not attached to any wheel. The pedals rotate through the principle of the lever.
- The rear sprocket, a toothed wheel fixed on the rear wheel.
- A chain connects the front sprocket to the rear sprocket and transmits the torque exerted on the pedals to the rear wheel.

The diameter difference between the front and rear sprockets plays an important role in the operation of the bicycle. The diameter ratio (also equal to the ratio of the number of teeth) is called the gear ratio. Pedaling pace and the gear ratio directly influence the speed, the energy expended and distance.

**Click** on the board at the upper left or on the sprocket to change the number of teeth.**Turn** the pedals or **control** the pedaling rate with the button slider.

- To understand the mechanical transmission system of a bicycle.
- To identify simple machines: lever (winch), gear (pulley).
- To defining the technical terms gear ratio, frame rate, development, transmission ratio and make the connection with the gears.
- To introduce the concept of load torque.

A bike has the following characteristics:

- Wheel diameter: 70 cm
- 3 rear sprockets : 20, 40, 52 teeth
- 2 front sprockets : 12, 24 teeth

This bike has 6 **speeds** because there are 6…