The bike front derailleur (or front mech) is used to move the chain side to side from one chainring to the other. The are many types of front derailleurs available. From braze on to clamp on or for double or triple chainrings.. There are plenty of different factors to take into account when changing your front derailleur. And don't forget you can use a braze on front derailleur with a clamp adapter if you don't have the boss on your frame!