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 clamp around the seat tube, below the front derailleur, and at least one attaches to the front derailleur mount. Derailleurs require the chain to be in movement in order to shift from one ring or sprocket to another. This usually requires the rider to be pedalling, but some systems have been developed with the freewheel in the crankset so that the chain moves even when the rider is not pedalling. The Shimano FFS (Front Freewheel System) circa 1980 was the most widespread such system. Chain-drive systems such as the derailleur systems work best if the chain is aligned with the