A bicycle derailleur derails the chain from one chainring to another with the help of the rider's pedal action and the gear-shifting command to change gears. A bicycle rear derailleur derails the chain from one sprocket to another on the bicycle's cassette attached to the rear wheel. wizbiker.com stocks short, long and medium cage rear derailleurs for the road, mountain and hybrid bike online in India. Make sure you buy compatible bicycle derailleur for your bike. Please get it installed and adjusted by the expert bike mechanic as bicycle derailleurs can be tricky sometimes.