Derailleur gears several standards in use, and in each the product of the derailleur's actuation ratio and the length of cable pulled must equal the pitch of the rear sprockets. The following standards exist. Some rear derailleurs, especially for mountain bikes, incorporate a clutch to keep the lower length of chain in sufficient tension to prevent the chain from striking the bottom of the chain stay, called "chain slap", and which can damage the chain stay. Clutches are also helpful in preventing the chain from derailing from the chain ring on systems without a front derailleur. The front derailleur only has to
Derailleur gears = 45T; medium = 33T SRAM long = 43T; medium = 37T; short = 30T Benefits of a shorter cage length: There are at least two methods employed by rear derailleurs to maintain the appropriate gap between the upper jockey wheel and the rear sprockets as the derailleur moves between the large sprockets and the small sprockets. The actuation ratio is the ratio between the amount of shifter cable length and the amount of transverse derailleur travel that it generates. Shift ratio is the reciprocal of actuation ratio and is more easily expressed for derailleurs than actuation. There are currently