Bicycle gear mechanism. The peddle and chain wheel (top right corner) are turned by the rider. This turning force is transferred to the rear wheel by a toothed chain (running centre right to bottom left). The chain runs through the cogged gears (freewheel, centre left) and turns the back wheel, propelling the bicycle forward. The gears have different ratios that can change one revolution of the chain wheel into less than, more than or exactly one revolution of the rear wheel. The gears are changed by means of a derailleur (lower left) which moves the chain between gears.