Bicycle chains transfer the torque from the crankset to the hub of the rear wheel. They are made of steel and consist of separate links, which fit exactly on the pinions. It is crucial that the length of the chain matches with the bike, so that it does not fit too tightly, but also is not too slack. Bicycle chains can wear out, especially those of bikes with a derailleur gearshift. The chains are available in several different grades of quality. For the mounting you need chain locks or chain pins.