B-Screw, Barrel Adjuster, Limit Screws: The 3 Turns That Fix Shifting

Nine out of ten shifting problems that roll into the shop get fixed with three adjustments: the B-screw, the barrel adjuster, and the limit screws. Before you tear down your derailleur or start swapping cables, master these three turns. They solve 90% of shifting complaints.

The B-Screw: Setting Gap Distance

The B-screw (B-tension screw) controls how close your upper pulley sits to the cassette. Get this wrong, and shifting suffers no matter how perfect everything else is.

Too close: Chain may contact cassette cogs, creating grinding noise. Shifting into larger cogs becomes sluggish or incomplete.

Too far: Shifting becomes vague and slow. The derailleur can’t wrap enough chain around each cog for crisp engagement.

The adjustment: Shift to your largest cog. The gap between the upper pulley and the cog teeth should be approximately 5-6mm for most derailleurs (check your specific manufacturer’s spec). Turn the B-screw clockwise to increase gap, counter-clockwise to decrease.

Pro tip: if you’ve just installed a larger cassette than your derailleur was designed for, the B-screw is your primary compensation tool. More teeth on the big cog means you need more B-tension.

The Barrel Adjuster: Cable Tension Fine-Tuning

This is the adjustment you’ll use most often. The barrel adjuster—that ridged cylinder where the cable housing meets the derailleur or shifter—adds or removes cable tension in tiny increments.

Symptoms of incorrect cable tension:

• Slow upshifts (to easier gears): cable too loose
• Slow downshifts (to harder gears): cable too tight
• Chain hesitates mid-cassette before dropping into gear
• Skipping in certain gears but not others

The adjustment: Turn the barrel adjuster counter-clockwise (as viewed from behind the bike) to add tension, clockwise to reduce it. Work in quarter-turn increments, then test-shift.

The goal: instant response in both shift directions across the entire cassette range. If upshifts are crisp but downshifts lag, reduce tension slightly. If downshifts snap but upshifts hesitate, add tension.

Where to find it: Most road bikes have barrel adjusters at the shifter and/or where the cable enters the rear derailleur. Many mountain bikes have them inline on the cable housing. Use whichever is most accessible.

The Limit Screws: Keeping Chain on the Cassette

Limit screws prevent the derailleur from throwing your chain off the cassette—into your wheel or into the frame. There are two: the H-limit (high, small cog) and L-limit (low, big cog).

H-limit adjustment: Shift to the smallest cog. Looking from behind, the upper pulley should align directly beneath the smallest cog—not inboard, not outboard. Clockwise moves the limit inward (away from the frame), counter-clockwise moves it outward.

L-limit adjustment: Shift to the largest cog. Again, pulley should align directly beneath the cog. Clockwise moves the limit outward (away from the spokes), counter-clockwise moves it inward.

Critical safety note: Always set the L-limit with your wheel in the stand, then test carefully while riding. A derailleur that overshoots into your spokes destroys wheels and can cause crashes. If in doubt, err toward a tighter limit and accept slightly slower upshifts to the big cog.

The Sequence Matters

When troubleshooting shifting, address adjustments in this order:

1. Limit screws first: Ensure the derailleur can reach both extreme cogs without overshooting
2. B-screw second: Set proper pulley-to-cassette gap
3. Barrel adjuster third: Fine-tune cable tension for crisp shifts

Adjusting cable tension when limits are wrong wastes time. Getting B-tension set before limit screws is inefficient. Follow the sequence.

When These Three Won’t Fix It

If methodical adjustment of all three doesn’t resolve shifting problems, deeper issues exist:

• Cable contamination: Frayed, kinked, or corroded cables resist movement
• Housing damage: Kinked, compressed, or contaminated housing adds friction
• Hanger alignment: A bent derailleur hanger puts the entire system out of plane
• Worn components: Stretched chains, shark-fin cassettes, worn derailleur pivots

But start with the three adjustments. In my experience, fresh cables and housing solve half of “unsolvable” shifting problems, and proper adjustment solves most of the rest.

The Two-Minute Trailside Fix

Shifting goes bad mid-ride? Here’s your field protocol:

1. Stop and shift to the smallest cog (hardest gear)
2. Verify the derailleur isn’t bent or caught on anything
3. Give the barrel adjuster two clicks counter-clockwise
4. Test-shift through the range
5. Adjust in quarter-turn increments until smooth

Most mid-ride shifting issues are cable stretch or settling. The barrel adjuster compensates in seconds. No tools required, no drama needed.