Rear derailleur adjustment: limit screws, cable and indexing

Before touching the screws: diagnosis

Before you start adjusting, check these three points. In 50% of cases, the problem comes from here, not from the derailleur itself.

• Is the derailleur hanger straight? A bent hanger throws off the entire adjustment. Look at the derailleur from behind: the jockey wheels must be perfectly aligned vertically with the cogs. If they are not, straighten or replace the hanger.
• Is the cable in good condition? A frayed, rusty cable or a cracked housing will never transmit tension correctly. When in doubt, replace both cable and housing.
• Is the chain worn? A chain stretched beyond 0.75% will skip on the cogs, even with a perfectly adjusted derailleur. Check with a wear indicator tool (see our article on chain wear).

Step 1 — Set the low limit screw (L)

The L screw (Low) limits the derailleur travel towards the largest cog (easiest gear).

• Shift to the largest rear cog and the smallest front chainring
• Look at the jockey wheels from behind: they must be perfectly aligned under the largest cog
• If the derailleur cannot reach the largest cog, loosen the L screw (counter-clockwise)
• If it overshoots and risks dropping into the spokes, tighten the L screw (clockwise)

Safety: an L limit that is too open can send the chain into the spokes. This kind of mistake can destroy a wheel and cause a serious crash.

Step 2 — Set the high limit screw (H)

The H screw (High) limits the derailleur travel towards the smallest cog (hardest gear).

• Shift to the smallest rear cog and the largest front chainring
• The jockey wheels must be aligned under the smallest cog
• Same logic: loosen if the derailleur does not reach it, tighten if it overshoots

Step 3 — Set the B-gap (B screw)

The B-gap is the distance between the upper guide pulley and the cog teeth. It is an often-forgotten adjustment, but it is crucial for shift quality.

• Shift to the largest rear cog
• Measure the distance between the top of the guide pulley and the bottom of the cog teeth
• The target: 5 to 6 mm (varies by derailleur — check the manual)
• Tighten the B screw to move the pulley away from the cog, loosen to bring it closer

A B-gap that is too small causes noise and rubbing. A B-gap that is too large makes shifts slow and imprecise, especially when climbing to larger cogs.

Step 4 — Cable tension and indexing

This is the final step, the one that makes shifting smooth and silent.

• Shift to a middle cog (centre of the cassette)
• Pedal while turning the barrel adjuster on the derailleur or shifter
• The chain hesitates going up (towards larger cogs) — increase tension (turn the barrel adjuster counter-clockwise as viewed from behind)
• The chain hesitates going down (towards smaller cogs) — decrease tension (turn clockwise)
• Adjust in quarter turns until each click of the shifter produces a clean single-cog shift

Tip: do the final adjustment on a workstand while pedalling. Then confirm by riding: vibration and chain load can reveal a slightly imperfect setting.

Quick summary

When to ask L'Atelier

If you have a specific derailleur (Shimano XT RD-M8100, SRAM GX Eagle, etc.), the exact B-gap values and procedures may differ. Ask:

"What B-gap for the Shimano XT RD-M8100-SGS derailleur?"

The assistant will find the exact value in the dealer's manual in seconds.

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