AI 速览
- Always set the limit screws before cable tension — indexing on a misadjusted high/low limit will never stay dialed.
- B-tension (the gap between upper jockey wheel and largest cog) is the most overlooked adjustment and the cause of most noisy, hesitant shifts.
- Shift cable housing that's cut 2 mm too long or compressed at the ferrule feels identical to a worn chain — replace housing before chasing indexing problems.
- A bent derailleur hanger is the silent killer of rear shifting; check it with a gauge before touching a single limit screw.
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The four-screw framework
Every modern rear derailleur is governed by four screws: high (H) limit, low (L) limit, B-tension (sometimes marked B or, on Shadow designs, the body angle screw), and the barrel adjuster that sets cable tension. The order in which you touch them determines whether you spend twenty minutes or two hours on the tune. The correct order — hanger alignment, limits, B-tension, then indexing — is the same on a 9-speed Sora derailleur and a 12-speed XTR.
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Step 0: hanger alignment and chain length
A hanger that is 3 mm out of alignment — invisible to the eye — will ghost-shift under load and chew through drivetrain parts. Bolt a hanger gauge (Park DAG-2.2 or equivalent) to the hanger, and rotate it through four positions around the rear wheel. Anything beyond 2 mm deviation at the indicator needs correcting; beyond 4 mm, replace the hanger.
Chain length is the second silent cause. With the chain on the small ring and small cog, the derailleur cage should sit at roughly 45 degrees, with a slight arc of chain above the lower pulley. Too short and the derailleur snaps when you shift into the big-big combination; too long and shifts get vague.
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Limit screws first, then B-tension
Shift to the smallest cog. Back out the high-limit screw until the upper jockey wheel sits directly below the smallest cog's centerline, then snug it. Shift (by hand on the derailleur if needed) to the largest cog and repeat with the low-limit. The limit screws only prevent overshoot — they are not for trim.
Rear derailleur adjustments — symptoms and fixes
| Symptom | Cause | Fix |
|---|---|---|
| Chain drops past small cog | H-limit too loose | Tighten H 1/8 turn |
| Chain stalls before big cog | L-limit too tight | Loosen L 1/8 turn |
| Noisy shifts under load | B-tension too tight | Loosen B 1/4 turn |
| Chain rubs upper jockey in big cog | B-tension too loose | Tighten B 1/4 turn |
| Ghost shifts under torque | Bent hanger or worn chain | Gauge hanger, measure chain |
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Indexing: cable tension last
With limits and B-tension set, shift to the smallest cog and click the shifter one gear harder. The chain should move up one cog cleanly. If it stalls, add tension with the barrel adjuster in quarter-turn increments until it shifts. If it over-shifts (tries to jump two cogs), back tension out by quarter-turns.
Walk up the cassette, one cog at a time, and back down. A correctly indexed derailleur shifts every step in both directions under a steady pedal load. The trim point — where a quarter turn of the barrel cleans up one direction but slows the other — almost always points to cable friction or a worn chain, not a limit problem.
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Di2 and wireless derailleurs
Electronic shifting removes cable stretch from the equation but keeps the same logic. Limit screws are still mechanical. B-tension on Shimano Di2 is set with the app and a specific cog; SRAM eTap AXS uses a separate chain-length gauge. The one new variable is firmware — keep derailleur firmware current, because each release shifts the auto-trim behavior. Indexing adjustments on Di2 are made with the barrel-less b-adjust in the app, in 0.25 mm steps.
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