MTB rear derailleurs: precise gear changes at the rear wheel
The rear derailleur is responsible for changing the gear from one cassette sprocket to the next at the rear wheel. Mountain bike rear derailleurs must therefore be accurate, so you're always in the gear you want to be. They also should be quick and, above all things, resilient. Nowadays, many MTB rear derailleurs are so-called 'clutch' derailleurs. These have a clutch to keep tension on the derailleur at all times, keeping the chain taught when riding over bumpy terrain, thus avoiding chain drop. When you buy a rear derailleur, make sure it's compatible with the number of gears and size of your rear cassette: for big cassettes, you'll want a long-cage rear derailleur that can comfortably get the chain onto the biggest sprocket. Bear in mind that a long-cage rear mech means more hanging down from the bike and more to smash into trail obstacles. Try to get away with the shortest cage you can, while still being able to shift to all gears.