If you have more than just a chain ring, you will need a derailleur. This is the counterpart on the chainwheel to the rear derailleur. A derailleur system with multiple chain rings works only with a front derailleur, since the chain is pushed or pulled on the driven chain ring. As with all bicycle parts, when selecting the derailleur, make sure that it is perfectly matched to the other components of the shifting. Thus, the used chain width should be consistent, matching ratio of the chain rings and matching lever for the number of speeds. When selecting the derailleur pay attention to its mounting position and how the derailleur cable routing of the bicycle is mounted.