The rear derailleur, or rear mech as you will more often hear it called, is the device on the bottom rear right hand side of the of the bike frame which moves the chain up and down the cogs, or cassette, on the back wheel, ie changes the rear gears. It is operated by a cable, or electronically on some modern systems, which pulls the mech a certain distance into or away from the frame depending on the amount of force used to pull or release the cable. Almost all rear mechs these days are ‘indexed’ in that the mech moves the chain a fixed distance up or down the cogs with one push of the gear lever.
There are three companies which produce road rear mechs; Shimano, Campagnolo and SRAM, which may look and operate very similar, are unfortunately incompatible with each other’s systems. This is because each system has a certain amount of movement from the gear lever and differs slightly with each manufacturer. As an aside, they do work with each other to a certain degree but it is not as slick and not recommended for long term use.
Each manufacturer also recommends only using the correct rear mech for each groupset as some mechs may not work with differing numbers of gears, i.e. Dura Ace 11 speed must use an 11 speed rear mech as it has narrower jockey wheels. You can mix rear mechs from the same manufacturer as long as they are designed for the same number of gears, i.e. you can use a 10 speed Shimano Tiagra rear mech with an Ultegra 10 speed groupset as they are both 10 speed compatible. Some older rear mechs will work across different numbers of rear gears but it can be tricky to set up and so always easier to run a matching mech with the specified amount of gears.
Shimano are the world’s biggest cycle groupset manufacturer making rear mechs for all types of bikes and riders. From professional riders to shopping bikes, Shimano make a rear mech for it and all are commercially available to buy so if you fancy using the same rear mech as 2013 Tour de France winner Chris Froome, Shimano Dura Ace 11 speed, you can easily buy that part although it is very expensive.
The hierarchy of Shimano road rear mechs from professional to entry level is as follows:
Campagnolo are the world’s oldest manufacturer of cycle gears and the company which actually invented the rear derailleur. They produce professional level components for some of the world’s top cycling teams down to weekend warrior suitable gears with prices to suit
The hierarchy of Campagnolo road rear mechs from professional to entry level is as follows:
SRAM are the newest groupset manufacturer and have come up with a series of ingenious inventions which have quickly gained a wide fan base from professional to recreational with their alternative view on operating systems.
The hierarchy of SRAM road rear mechs from professional to entry level is as follows: