Derailleurs are the mechanisms that shift the chain on your drivetrain. The rear derailleur shifts the chain into a smaller or larger sprocket on the cassette which attaches to your rear wheel. The front derailleur moves your chain between the big and small chainring on your crankset. Both are crucial to the functionality of your bike and allow you to choose an easier gear when cycling uphill and a tougher when riding downhill or at high speeds.
Here at ProBikeKit, we stock derailleurs from the three biggest component manufacturers; Shimano, Campagnolo and Sram. We also sell spare parts such as jockey wheels and gear cables as well as wires for electronic groupsets such as Shimano Di2.