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 is relatively uncommon today. In mountain biking and off-road cycling, the most critical gear changes occur on uphill sections, where riders must cope with obstacles and difficult turns while pedalling under heavy load. This derailleur type provides an advantage over high normal derailleurs because gear changes to lower gears occur in the direction of the loaded spring, making these shifts easier during high load pedalling. The distance between the upper and lower pulleys of a rear derailleur is known as the cage length. Cage length, when combined with the pulley size, determines the capacity of a derailleur to take up