Rear Derailleur Parts and Front Derailleurs

In the simplest of terms, the bicycle derailleurs are the transmission of the drive system permitting the chain to move from cog to cog changing the gear ratio between the cassette and the chainrings.  Front Derailleurs require the chain to be in motion by pedaling which provide the movement of the chain required to change gear ratios on either the chainrings or the cassette. 

Rear Derailleur parts are most efficient when the chain is in a straight line with the rear cassette cog and the chainring, other wise the chain is at an angle causing friction in the drive system.

Rear derailleur’s sever two functions, first and foremost they drive the movement of the chain up or down the cassette changing the gear ratio, secondly it keeps tension on the chain talking up slack in the chain when you move from a large cassette cog to a smaller cassette cog.

Cage length is the distance between the upper and lower idler pulleys of the derailleur.  Cage length determines the capacity the derailleur has in taking up slack in the chain.  Rood bikes with tight cassette gearing require a shorter cage than mountain bicycle derailleurs that has a greater span in gear ratios of the rear cassette and also the chainrings on the crankset.

Front derailleur serves only one purpose and that is to move the chain from side to side to the across the chainrings.  Some cranksets have two chainrings while others are three.  Most common are two chainrings on road bikes and three chainrings on mountain bikes, though the exception can be made in both categories.