How to adjust your rear derailleur (fix bad shifting)
If your shifting has gone vague — slow to move, hesitating, or clattering between gears — the rear derailleur usually just needs a small tune. Most of the time it's cable tension, which you can fix in a minute with the barrel adjuster. Here's the full picture.
First, rule out the simple stuff
Before adjusting anything, check the chain isn't worn out and the derailleur hanger isn't bent (if shifting is bad across every gear and won't tune, suspect the hanger — see chain skipping). A clean, healthy drivetrain tunes far more easily.
The barrel adjuster: 90% of fixes
The barrel adjuster is the small dial where the cable enters the derailleur (or the shifter). It fine-tunes cable tension, and cable tension is what indexing depends on.
- Chain slow to climb to a bigger cog (harder gear → easier): not enough tension. Turn the barrel adjuster counter-clockwise a quarter-turn at a time.
- Chain slow to drop to a smaller cog, or rattling/over-shifting: too much tension. Turn it clockwise a quarter-turn at a time.
Shift up and down through the gears after each small turn until it's crisp.
The limit screws (H and L)
The two small screws marked H and L set how far the derailleur can travel — they stop the chain from throwing off either end. You only touch these if the chain overshifts into the spokes or off the smallest cog.
- H (high) limits travel at the smallest cog. If the chain won't quite reach the smallest cog or threatens to fall off the outside, adjust H.
- L (low) limits travel at the largest cog. If the chain won't reach the biggest cog, or risks going into the spokes, adjust L.
Turn in small increments, checking the chain sits centered on the end cog without trying to go past it.
The B-tension screw
This sets the gap between the top derailleur pulley and the cassette. Too close and shifting is noisy and notchy; too far and it's sluggish. Adjust so the upper pulley clears the largest cog by a few millimeters when you're in that gear. Most bikes rarely need this touched.
If a tune doesn't fix it
Crisp shifting depends on a healthy chain. If you've tuned the cable and it still skips under load, the chain and cassette are the usual suspects — see why your chain skips. Staying ahead of chain wear prevents most shifting problems before they start.