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 move the chain side to side between the front chainrings, but it has to do this with the top, taut portion of the chain. It also needs to accommodate large differences in chainring size: from as many as 53 teeth to as few as 20 teeth. As with the rear derailleur, the front derailleur has a cage through which the chain passes. On a properly adjusted derailleur, the chain will only touch the cage while shifting. The cage is held in place by a movable arm which is usually implemented with a parallelogram mechanism to keep the cage properly aligned