Derailleur gears a detent or ratchet mechanism which stops the gear lever, and hence the cable and the derailleur, after moving a specific distance with each press or pull. Indexed shifters require re-calibration when cables stretch and parts get damaged or swapped. On racing bicycles, 10-gear rear cassettes appeared in 2000, and 11-gear cassettes appeared in 2009. Most current mountain bicycles have either. Many modern, high-end mountain bikes have begun using entirely one chain ring drivetrains, with the industry constantly pushing the number of rear cogs up and up, as shown by SRAM's Eagle groupsets (1 by 12) and Rotor's recent 1
Derailleur gears lever chain tensioners mounted near or on the downtube. However, these systems, along with the rod-operated Campagnolo Cambio Corsa were eventually superseded by "parallelogram derailleurs". In 1937, the derailleur system was introduced to the Tour de France, allowing riders to change gears without having to remove wheels. Previously, riders would have to dismount in order to change their wheel from downhill to uphill mode. Derailleurs did not become common road racing equipment until 1938 when Simplex introduced a cable-shifted derailleur. In 1949 Campagnolo introduced the Gran Sport, a more refined version of the then already existing, yet less commercially successful,