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 cassette on the back wheel, ie changes the rear gears. It is operated by a cable 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. 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.
A rear mech has a main body which contains the spring and frame bolt and a pivoting cage containing two jockey wheels underneath. This cage is available in three lengths; short, medium or long, with the general rule being the higher the number of teeth on the largest sprocket at the back the longer the cage you will need, ie 34 tooth equals long cage whereas 21 should use a short cage etc.
The rear mech is mounted to the frame via a ‘hanger’ which is usually bolted on, making it easily replaceable and designed to break off in the event of a crash to prevent damage to the frame. Shimano now make a Direct Mount version which is bolted directly to the frame without a hanger and is more common on bikes with a bolt through rear axle.
There are two main companies which produce off-road rear mechs; Shimano and SRAM, which although 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.
You can mix rear mechs from the same manufacturer as long as they are designed for the same number of gears, ie you can use a 10 speed Shimano XTR rear mech with an XT 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.
The hierarchy of Shimano off-road rear mechs from professional to entry level is as follows
Downhill / Freeride
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 off-road rear mechs from professional to entry level is as follows: