Bicycle Gears Skipping or Not Shifting Cleanly

If your derailleur is skipping or gears aren't engaging cleanly, the cause is almost always a ADJUSTMENT issue — not expensive wear. Check the cheap, most common cause first and do NOT rush to replace the chain or cassette (that's rarely necessary and costly). Especially after a crash, the indexing (cable tension) is usually off or the derailleur hanger is bent. Follow these steps: 1) Adjust the indexing: Place the chain on the middle sprocket, then make small adjustments at the barrel adjuster (the small adjuster screw on the derailleur or shifter) in one-eighth to one-quarter turn increments until the gears shift cleanly and quietly — if it skips when shifting up, add a little cable tension; if it skips when shifting down, release some tension. 2) Check the derailleur hanger: the small aluminium bracket on the rear right where the derailleur mounts — if it's bent (e.g. after a crash), the shifting will never run cleanly; carefully straighten it by hand or have it aligned at a shop with a proper alignment tool, and replace it if it's severely deformed. 3) Only adjust the limit screws (H and L) in small increments if the chain is overshooting at the top or bottom end of the cassette. Only once the indexing, derailleur hanger, and cable tension are all correct and shifting is still poor should you consider a worn chain or sprockets — and check with a chain wear indicator tool first.