How do you align bike derailleurs?

Answer 1

First, you have to make sure the frame's dropout or derailleur hanger is not bent. The rear derailleur must be on the same vertical plane as the dropout, perpendicular to the axle. Your derailleur has two limit screws: A high limit, labeled "H", and a low limit, labeled "L". The "H" screw must be screwed in far enough to prevent the chain from descending beyond the highest gear, but should not be screwed in so far that the chain does not easily drop into the highest gear. The "L" screw should be screwed in far enough to prevent the derailleur from shifting into the spokes, but should be backed off enough to allow the derailleur to easily push the chain up into the lowest gear. Beyond this, the derailleur's accuracy depends on cable tension. If the shifter cable is too tight, it will overshoot gears. If the cable is too loose, it will not pull up into lower gears and cause the chain to chatter.