What a derailleur does and why it needs adjustment

Your derailleur is the mechanism that moves your chain between gears. It has two parts: the front derailleur (near the pedals) and the rear derailleur (above the back wheel). When you shift gears, the derailleur pushes or pulls the chain sideways onto a different sprocket. Over time—from riding, temperature changes, or a bump—the derailleur moves slightly out of position, and the chain starts to skip, rub, or struggle to shift.

You can fix most derailleur problems yourself with basic tools: a Phillips screwdriver, a 5 mm Allen wrench, and sometimes a barrel adjuster (a small cylindrical knob on the derailleur cable). The adjustment takes 15 to 30 minutes and costs nothing. If the derailleur is bent or the cable is frayed, you will need a replacement, but adjustment alone solves the majority of shifting problems.

Key Takeaways

  • Start by checking that the derailleur is straight and the cable is tight, because a bent derailleur or loose cable cannot be fixed by adjustment alone.
  • The limit screws (marked H and L) stop the chain from falling off the smallest and largest sprockets; adjust these first before touching the barrel adjuster.
  • The barrel adjuster fine-tunes cable tension to fix slow or hesitant shifts without moving the derailleur itself.
  • Test each gear while pedaling gently, and make small turns—a quarter turn at a time—because large adjustments overshoot the problem.
  • If the chain still skips or rubs after adjustment, the cable may be damaged, the sprockets may be worn, or the derailleur may be bent and need replacement.

Check the derailleur for damage and cable tension

Before you adjust anything, look at the derailleur from behind the bike. The derailleur arm should be parallel to the chain, not bent inward or outward. Spin the wheel by hand and watch the derailleur—it should move smoothly without rubbing the chain or frame. If the derailleur is visibly bent or the arm does not move freely, it needs replacement, not adjustment.

Next, check the cable. Follow the thin metal cable from the shifter (on the handlebar) down to the derailleur. Squeeze the cable between your fingers; it should feel tight, not loose or saggy. If the cable is loose, turn the barrel adjuster (the knob where the cable enters the derailleur) counterclockwise one full turn to tighten it. If the cable is frayed, kinked, or visibly damaged, it must be replaced before adjustment will work.

Adjust the high-gear limit screw (H screw)

The high-gear limit screw (marked H) prevents the chain from falling off the smallest sprocket at the back. Shift to the highest gear (smallest sprocket) by hand, using the shifter. Look at the derailleur from behind; the inner plate of the derailleur should be almost touching the chain, but not rubbing it.

If the chain rubs the inner plate, turn the H screw clockwise one quarter turn. If the chain falls off the sprocket or the derailleur is too far away, turn the H screw counterclockwise one quarter turn. Shift back and forth between the two highest gears a few times to test. Repeat in quarter-turn increments until the chain sits cleanly on the smallest sprocket without rubbing or falling.

Adjust the low-gear limit screw (L screw)

The low-gear limit screw (marked L) prevents the chain from falling off the largest sprocket at the back. Shift to the lowest gear (largest sprocket) using the shifter. Look from behind; the outer plate of the derailleur should almost touch the chain, but not rub it.

If the chain rubs the outer plate, turn the L screw clockwise one quarter turn. If the chain falls off the sprocket or the derailleur is too far away, turn the L screw counterclockwise one quarter turn. Shift back and forth between the two lowest gears several times. Continue adjusting in quarter-turn increments until the chain sits cleanly on the largest sprocket without noise or slipping.

Use the barrel adjuster for fine-tuning shifts

Once both limit screws are set, the derailleur should move across the full range of gears without the chain falling off. Now you will fine-tune the cable tension so shifts happen smoothly. Pedal gently and shift up one gear at a time, listening for hesitation or delay. If the chain is slow to move to a smaller sprocket, the cable is too loose; if it is slow to move to a larger sprocket, the cable is too tight.

To tighten the cable, turn the barrel adjuster counterclockwise one quarter turn. To loosen the cable, turn it clockwise one quarter turn. Shift through all the gears after each adjustment. The goal is for the chain to move when ready to each sprocket without rubbing or hesitation. This usually takes three to five small adjustments. If you turn the barrel adjuster more than two full rotations in one direction, stop and re-tension the cable at the derailleur body instead, because the barrel adjuster has a limited range.

Test the full range of gears

Ride the bike slowly in a safe area and shift through every gear combination, pedaling gently each time. Listen for chain rub or skipping. Shift up and down repeatedly to make sure each transition is smooth. Pay special attention to the highest and lowest gears, where problems show up first.

If the chain still skips or rubs after you have adjusted both limit screws and the barrel adjuster, the problem is usually one of three things: the cable is damaged and needs replacement, the sprockets are worn and need replacement, or the derailleur itself is bent or damaged. At that point, take the bike to a shop or replace the cable and derailleur as needed.

Common mistakes and how to avoid them

The most common mistake is turning the limit screws too far. A quarter turn is usually enough; a full turn often overshoots and creates a new problem. Write down which screw you are adjusting and which direction, so you can undo a bad turn if needed.

Another mistake is adjusting the barrel adjuster when the limit screws are wrong. If the chain is falling off the sprockets, no amount of barrel adjuster turning will fix it—you must set the limit screws first. The barrel adjuster only fine-tunes the cable tension; it cannot move the derailleur far enough to stop the chain from falling.

A third mistake is ignoring a loose cable. If the cable is visibly slack or the shifter feels mushy, tighten it at the cable anchor bolt (where the cable attaches to the derailleur body) before adjusting anything else. A loose cable makes every other adjustment pointless.

Frequently Asked Questions

What is the difference between the barrel adjuster and the limit screws?

The limit screws (H and L) control how far the derailleur can move in each direction and stop the chain from falling off. The barrel adjuster controls cable tension and fine-tunes the timing of shifts. Adjust the limit screws first, then use the barrel adjuster to smooth out the shifts.

My chain falls off the smallest sprocket when I shift to the highest gear. What do I do?

Turn the H screw counterclockwise one quarter turn to move the derailleur outward. Test the shift again. If the chain still falls off, turn it another quarter turn. Do not turn more than one full rotation without testing, or you will overshoot and have to undo your work.

The chain rubs when I am in the middle gears. Is that a derailleur problem?

Usually not. Rubbing in the middle gears often means the limit screws are set too far inward, or the cable is too loose. Make sure both limit screws are adjusted correctly first. If rubbing continues after that, the derailleur may be slightly bent and need replacement.

How often should I adjust my derailleur?

Most bikes need a derailleur check every few months if you ride regularly, or after a crash or hard bump. If shifts are smooth and the chain does not fall off or rub, no adjustment is needed. Cables stretch over time, so you may need a barrel adjuster turn once or twice a year.

Can I adjust the front derailleur the same way?

The front derailleur uses the same H and L limit screws and a barrel adjuster, but the cable routing and adjustment feel different because the front derailleur moves side to side rather than up and down. The same quarter-turn rule applies: make small adjustments and test after each one.