Adjust a rear derailleur: limit screws, indexing and the hanger check

The hanger is the small part that holds the derailleur to the frame. Park Tool says a bent hanger causes poor shifting that limit screws and indexing generally can't fix (Park Tool). With an alignment gauge, a hanger is straight when three points 90 degrees apart on the rim are within 3 mm of each other (Park Tool hanger).

If you have more than one chainring, shift to the largest. Shift the rear to the smallest cog, then keep clicking the shifter until no clicks are left (Park Tool).

If the chain won't reach the smallest cog, turn the H screw counterclockwise a few turns and pedal. If it still won't, turn the barrel adjuster clockwise a few turns. Then turn the barrel adjuster about two more turns clockwise so the cable doesn't hide the limit setting (Park Tool).

If the screws aren't marked, turn each one while watching the derailleur. In this gear, the one that moves the derailleur is the H screw (Park Tool).

Tighten it until the chain starts to rub the next cog while you pedal, then back it out a quarter turn at a time until the noise stops. If two settings are equally quiet, use the tighter one. Turn the barrel adjuster back counterclockwise a couple of turns (Park Tool).

Pedal and shift one click.

• If the chain doesn't move, return to the smallest cog and turn the barrel adjuster counterclockwise one full turn, then try again.
• If it jumps two cogs, return to the first cog and turn it clockwise one turn.
• Shift through each cog and listen. If one is noisy, turn the adjuster clockwise a quarter turn and test again.

If the barrel adjuster is almost all the way out, Park Tool threads it back in, backs it out one or two turns, and takes up cable slack at the pinch bolt (Park Tool).

Shift to the second largest cog, then the largest. A failed, slow or noisy shift means the L screw is too tight. Back it out a quarter turn at a time until the shift is clean (Park Tool). The L screw stops the chain going past the largest cog. To check it, push a little extra on the shifter in the largest cog. The derailleur should not move further in (Park Tool).

In the largest cog, Park Tool measures the gap between the top pulley and the cog and gives 5 to 6 mm for most road and mountain derailleurs. Tightening the B screw opens the gap. If you change it a lot, recheck the indexing. Don't fuss over it if the bike already shifts well (Park Tool).

Ride and shift through every gear. If the chain still slips under hard pedaling, it may be worn (Park Tool chain wear). See check and replace a chain.