Derailleur gears chain slack. Cage length determines the total capacity of the derailleur, that is the size difference between the largest and smallest chainrings, and the size difference between the largest and smallest sprockets on the cogset added together. A larger sum requires a longer cage length. Typical cross country mountain bikes with three front chainrings will use a long cage rear derailleur. A road bike with only two front chainrings and close ratio sprockets can operate with either a short or long cage derailleur, but will work better with a short cage. Manufacturer stated derailleur capacities are as follows: Shimano long
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