Bicycle gears skipping or not shifting cleanly

The cause is usually a loose or stretched gear cable, incorrectly set limit screws, or a bent derailleur hanger. Check the gear cable tension: turn the barrel adjuster on the shifter or derailleur clockwise to increase tension. Place the chain on the middle sprocket and adjust at the derailleur barrel adjuster in 1/4-turn increments until the chain runs quietly. The limit screws (H and L) define the end positions – make only small corrections. If the derailleur hanger (the small aluminium bracket at the rear right) is bent, it can be carefully straightened – have it replaced if severely deformed.