A derailleur is a bicycle component that allows the rider to change gears. It is made up of a cage that holds the chain in place, and a set of pulleys that guide the chain onto different sprockets on the rear wheel. Derailleurs are essential for any bike that has more than one gear, as they allow the rider to adjust the gear ratio to suit the terrain and their own fitness level.
There are two main types of derailleurs: rear derailleurs and front derailleurs. Rear derailleurs are mounted on the rear dropout of the bike, and they move the chain between the different sprockets on the rear wheel. Front derailleurs are mounted on the downtube of the bike, and they move the chain between the different chainrings on the front crankset.
