what are derailleur cables
A derailleur cable doesn't need to pull as hard as a brake cable but - if the system is indexed - it needs to transmit energy precisely from the shifter to the derailleur. Select the position that pulls the least derailleur cable. Usually the highest gear at the rear and the lowest gear at the front. Thread the cable into the shifter. This is easiest if you take careful note of the path of the old derailleur cable before removing it.