AI 速览
- Indexing is adjusting cable tension so the derailleur moves the chain to exactly the right position for each gear. 90% of shifting problems are cable tension, not component wear
- The process: shift to smallest cog, turn the barrel adjuster to fine-tune, test each gear up and down, adjust until every shift is crisp. No special tools — just your hands and a Phillips screwdriver
- The H and L limit screws set the derailleur's travel range — only touch them if the chain is falling off the cassette or into the spokes. They are NOT for fine-tuning shifting
- If indexing doesn't fix it: check cable stretch (replace old cables), bent derailleur hanger (common after transport/crashes), or worn chain/cassette (check with a chain checker)
/ 01
When to index
Index your gears when: shifts are slow or hesitant (the chain takes a second to move), shifts skip (the chain jumps past a gear), the chain rubs the derailleur in certain gears, or after replacing a cable, cassette, or chain. Most shifting problems are cable tension — the cable stretches slightly over time, and the derailleur's position drifts. A 10-minute re-index fixes it.
When NOT to index: if the chain is falling off the cassette entirely (that's a limit screw issue), if the derailleur is visibly bent (straighten or replace the hanger first), or if the chain/cassette is worn (check with a chain checker — a worn drivetrain won't index cleanly no matter what).
/ 02
The 10-minute process
1. Shift to the smallest cog (hardest gear) on the rear. This removes cable tension. The chain should sit centered on the smallest cog. If it's rubbing the derailleur cage, turn the barrel adjuster (the knurled knob where the cable enters the derailleur) counterclockwise 1/4 turn to move the cage inward.
2. Pedal by hand (or have a friend hold the rear wheel up) and click ONE shift up. The chain should move to the next cog crisply. If it hesitates, the cable is too loose — turn the barrel adjuster counterclockwise 1/4 turn and try again. If it overshoots (jumps two gears), the cable is too tight — turn clockwise 1/4 turn.
3. Continue clicking up through every gear, adjusting the barrel adjuster in 1/4-turn increments until every upshift is crisp. The barrel adjuster is your only tool — small turns, test, repeat.
4. Shift back down through every gear. Downshifts should be equally crisp. If they hesitate, add a tiny bit of cable tension (counterclockwise). The sweet spot is where both up and down shifts are clean.
5. Ride it. Real-world shifting under load (standing, sprinting) is harder than hand-pedaling. If a shift skips under load but not in the stand, add 1/8 turn of tension.
/ 03
The H and L limit screws — what they do (and when to touch them)
The two limit screws on the rear derailleur (labeled H and L, or high and low) set the derailleur's maximum travel range. The H screw prevents the chain from falling off the small cog (into the frame). The L screw prevents it from going past the largest cog (into the spokes).
Do NOT use limit screws for fine-tuning shifting — they are range-setters, not tension adjusters. Only adjust them if: (a) the chain falls off the smallest cog (turn H clockwise 1/4 turn), (b) the chain goes into the spokes on the largest cog (turn L clockwise 1/4 turn), or (c) you've installed a new derailleur or cassette with a different range. If your shifts are slow but the chain stays on, it's a cable tension issue — use the barrel adjuster.
/ 04
When indexing doesn't fix it
If indexing doesn't resolve the problem, check: (1) Cable condition — frayed, rusted, or stretched cables don't index cleanly. Replace cables and housing every 1–2 years. (2) Derailleur hanger alignment — a bent hanger (from a crash, transport, or even leaning the bike wrong) makes shifting impossible to index. A hanger alignment gauge ($25) or a trip to a shop fixes it. (3) Drivetrain wear — a chain at 0.75%+ stretch or a worn cassette won't shift cleanly because the teeth are hooked. Check chain wear first; replace chain+cassette if needed.
/ 来源