MTB Rear Derailleurs are designed to maintain accurate shifting while not getting too clogged up with mud and trail debris. Innovations such as low profile / shadow derailleurs reduce the amount the gear sticks out and so are less susceptible to damage. Clutch rear derailleurs have a stronger sprung cage which reduces the amount that the chain bounces around. Derailleurs are available for 9 / 10 / 11 / 12 speed transmissions.