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's front derailleur derails the chain from one chainring to another which is attached to the bicycle's crankset. Wizbiker.com stocks top, bottom, dual pull mountain bike front derailleurs and braze-on and band-clamp road bike front derailleurs and the associated spare parts online in India. Make sure you buy compatible bicycle derailleur for your ride. And, it pays to get it installed and adjusted by the expert bike mechanic as bicycle derailleurs can be tricky sometimes.