How do I index my bike gears?

The fixed order — hanger, limit screws, cable tension, barrel adjuster — and why doing it in any other order means starting again.

Difficulty: intermediate
Time: 40 min
Read: 5 min

Short answer

Check the hanger is straight, shift to the smallest sprocket and release the cable, set the high limit screw, pull the cable taut with the barrel adjuster wound almost in, then shift up one gear at a time and tune with the barrel adjuster. Set the low limit screw last, with the chain on the biggest sprocket.

Indexing is the adjustment that makes one click of the shifter move the chain exactly one sprocket. It fails in a specific order, and it is fixed in a specific order — every step depends on the one before, which is why random adjustment of the barrel never quite gets there.

What you'll need

Allen keys
A small cross-head screwdriver
Cable cutters (optional)
A repair stand, or something to hold the bike upright (optional)

Indexing a rear derailleur, in order

Step by step

Check the derailleur hanger is straight.Stand behind the bike and sight down: the two jockey wheels and the sprockets should all lie in the same plane. A hanger bent inwards by the bike falling over makes correct indexing impossible, and nothing you do with the cable will compensate. Hangers are designed to bend rather than let the frame break, and a shop can straighten one with a gauge.
Shift onto the smallest rear sprocket and smallest chainring.Click the shifter all the way down while pedalling. Everything after this is set from that position.
Release the cable.Undo the pinch bolt at the derailleur so the cable is completely slack. The derailleur is now sitting where its own spring puts it, which is the reference position for the limit screws.
Set the high limit screw.The screw marked H. Adjust until the upper jockey wheel sits directly under the smallest sprocket — not outboard of it, which throws the chain off into the frame, and not inboard, which stops the chain reaching top gear.
Wind the barrel adjuster almost fully in.Then out one full turn. This leaves you adjustment available in both directions once the cable is clamped, which is exactly what you will need.
Pull the cable taut and clamp it.Pull it firmly by hand or with pliers so there is no slack, seat it in the groove at the pinch bolt, and tighten. Taut, not tensioned — you are removing slack, not pulling the derailleur.
Shift up one sprocket and watch.If the chain hesitates or will not climb, there is not enough cable tension: wind the barrel adjuster out, anticlockwise, a quarter turn at a time. If it jumps two sprockets or is noisy in the next gear up, there is too much: wind it in.
Work through every gear in both directions.Up one at a time, then down one at a time, under light pedalling load rather than freewheeling. Shifting up and shifting down are affected differently by tension, and a setting that is perfect one way and rough the other usually needs a quarter turn.
Set the low limit screw last.With the chain on the largest sprocket, adjust the screw marked L so the derailleur can reach that sprocket but cannot travel any further inboard. This is the screw that stops a derailleur going into the spokes, which destroys the wheel, the derailleur and often the frame hanger in one go.
Check the B-tension if shifting is still rough at the big end.The screw on the back of the derailleur body sets how far the upper jockey wheel sits from the sprockets. Too close on the largest sprocket makes shifting noisy and can jam.
Ride it and expect one more adjustment.A new cable settles in the first few rides. A quarter turn of the barrel after the first ride is normal, not a sign that something is wrong.

Tips

Quarter turns. A full turn of a barrel adjuster is a big change and it is why people go round in circles.
The barrel adjuster wound out moves the chain towards the larger sprockets. Remembering that one fact makes roadside adjustment easy.
If the indexing was fine yesterday and terrible today, suspect the hanger — something has knocked it.

Common mistakes

Adjusting the barrel before checking the hanger — A bent hanger changes the spacing between the derailleur and every sprocket unevenly. No single cable tension can be right across the cassette, so you chase the adjustment for an hour and never fix it.
Setting the low limit screw casually — It is the only thing stopping the derailleur being pulled into the spokes. That failure wrecks the wheel, the derailleur and usually the hanger, and it happens at the worst moment, under load on a climb.

If it doesn't work

Chain will not climb to a bigger sprocket

Cause: Not enough cable tension — Fix: Wind the barrel adjuster out a quarter turn at a time until it shifts cleanly.

Chain shifts up easily but hesitates coming down

Cause: Too much cable tension, or a sticky cable — Fix: Wind the barrel in a quarter turn; if it is still slow, fit a new inner and outer cable.

Rattling in the smallest sprocket only

Cause: High limit or tension slightly out at the top of the range — Fix: Slacken tension a fraction, then confirm the H limit puts the jockey wheel directly under the small sprocket.

Chain jumps off into the spokes

Cause: Low limit screw not set — Fix: Stop riding, check for damage, and set the L screw so the derailleur cannot travel past the largest sprocket.

Indexing keeps drifting out over a few weeks

Cause: New cable bedding in, or a frayed cable and worn outers — Fix: One quarter-turn adjustment after the first ride is normal; repeated drift means replacing inner and outer cables.

Questions people ask

Which way does the barrel adjuster go?

Unscrewing it, out of the derailleur, increases cable tension and moves the chain towards the larger sprockets. Screwing it in reduces tension and moves the chain towards the smaller ones.

The gears skip in every gear, not specific ones. Is that indexing?

No. Skipping under load across the whole cassette is wear — a stretched chain, a worn cassette or both — and indexing will not cure it.