What you need to do before you start

A derailleur is the mechanism that moves your chain between gears. If yours is bent, cracked, or not shifting smoothly, replacement is usually the fastest fix. Before you remove the old one, you need to know which derailleur you have — front or rear — and whether it matches your bike's drivetrain.

The rear derailleur is the cage that hangs below your cassette (the stack of gears on the back wheel). The front derailleur is the clamp that wraps around the chainrings near the pedals. Most shifting problems happen at the rear, so start there if you are not sure which one is broken.

Write down your bike's brand, model, and year if you know it. Take a photo of the derailleur from the side. You will need this information to buy the correct replacement — derailleurs are not universal, and installing the wrong one will not work.

Key Takeaways

  • Rear derailleurs are more common to replace than front ones, and the job takes 20 to 40 minutes with basic tools.
  • You must match the new derailleur to your bike's drivetrain speed (8-speed, 9-speed, 10-speed, and so on) or it will not shift correctly.
  • The derailleur bolts to your frame with one bolt; the chain and cable attach with quick-release or bolt connections you can undo by hand or with an Allen wrench.
  • After installation, you will need to adjust the cable tension and limit screws so the chain moves smoothly between gears without falling off.

Identifying your derailleur type and speed

Look at the back of your bike. Count the number of gears on the cassette (the stack on the rear wheel). If you see 8 cogs, you have an 8-speed drivetrain. If you see 10, you have a 10-speed. This number must match your new derailleur exactly — a 9-speed derailleur will not work on a 10-speed cassette.

Check the derailleur itself for a brand name and model number. Shimano, SRAM, and Microshift are the three main makers. The model number is usually stamped or printed on the cage. If you cannot read it, take a clear photo and search the image online, or bring the photo to a bike shop and ask them to identify it.

Write down the brand, model, and speed. You will use this to order the exact replacement. Buying the wrong speed is the most common mistake — the cable will not tension correctly, and shifting will be sluggish or fail entirely.

Tools and parts you will need

You need an Allen wrench set (usually 2 mm, 3 mm, and 5 mm sizes), a Phillips screwdriver, and a chain tool if you plan to remove the chain. A bike stand or a way to hang the bike so the rear wheel spins freely makes the job much easier — you can prop the frame on a workbench or ask someone to hold it.

Have the new derailleur on hand before you start. You will also need a small container to hold the bolts and screws you remove, so you do not lose them. If the old derailleur is damaged, you may be able to reuse its cable and mounting bolt, but it is safer to replace the cable too — old cables fray and stick.

Removing the old rear derailleur

Shift the chain to the smallest cog on the cassette and the smallest chainring at the front. This takes tension off the chain and makes removal easier. If the chain is stuck or the derailleur will not move, do not force it — stop and check that you have shifted all the way.

Locate the cable that runs from the shifter (on the handlebar) down to the derailleur. It connects with either a bolt or a quick-release lever. If it is a bolt, use the correct Allen wrench to loosen it — usually 2 mm or 3 mm. If it is a quick-release, flip or slide it open. Unthread the cable from the derailleur.

Find the mounting bolt where the derailleur attaches to the frame. It is a single bolt on the upper part of the cage, pointing toward the rear wheel. Use the correct Allen wrench (usually 5 mm) to remove it. The derailleur will hang by the chain. Lift the cage and unhook the chain from the smallest cog, or use a chain tool to break the chain if it is not removable. Slide the derailleur away from the bike.

Installing the new rear derailleur

Thread the chain through the new derailleur's cage, starting at the smallest cog. The chain should sit in the groove on the upper pulley (the wheel closest to the cassette). If the chain is broken or you removed a link, rejoin it now using a chain tool or a quick-link connector.

Align the mounting bolt hole on the new derailleur with the hole on the frame. Insert the bolt and tighten it by hand first, then use the Allen wrench to snug it — do not over-tighten, or you may strip the frame. The derailleur should hang freely and not rub the cassette.

Thread the cable through the cable anchor on the derailleur. Pull the cable tight by hand and find it with the bolt or quick-release. The cable should be snug but not so tight that the derailleur is pulled out of position. You will fine-tune this in the next step.

Adjusting cable tension and limit screws

Shift the front shifter to the smallest chainring. Slowly turn the pedals and watch the chain. It should move smoothly from the largest cog toward the smallest without hesitation. If it moves too slowly or gets stuck, the cable is too loose — turn the barrel adjuster (the threaded collar on the cable housing) counterclockwise one quarter turn and test again.

If the chain overshoots the smallest cog and tries to fall off the cassette, the cable is too tight. Turn the barrel adjuster clockwise one quarter turn. Make small adjustments and test after each one — large changes make the problem worse.

Locate the two limit screws on the derailleur body, usually marked H (high gear) and L (low gear). The H screw stops the chain from falling off the smallest cog. The L screw stops it from falling off the largest cog. Shift to the smallest cog, then loosen the H screw slightly if the chain tries to fall inward, or tighten it if the chain will not move all the way to the smallest cog. Repeat for the L screw at the largest cog. This takes trial and error — make one quarter-turn adjustments and test each time.

Testing the new derailleur

Spin the pedals and shift through all the gears, both front and rear. The chain should move smoothly without noise, hesitation, or jumping. Shift slowly — rushing makes it hard to hear grinding or feel resistance. If the chain struggles to move up to a smaller cog, loosen the cable slightly. If it overshoots, tighten it.

Ride the bike on flat ground and shift under light pedal pressure. The chain should respond when ready. Shift hard while climbing or sprinting — if the chain drops or struggles, the limit screws need adjustment. Stop and make small changes until shifting is smooth in all gears.

Check the derailleur cage for rubbing against the chain. It should clear the chain by at least a millimeter in all gears. If it touches, the mounting bolt may be loose or the derailleur may be bent — tighten the bolt or replace the derailleur if it is damaged.

Frequently Asked Questions

Can I replace just the cage if the derailleur is bent?

Some derailleurs have a replaceable cage, but most do not. Check your derailleur's manual or contact the maker. If the cage is not replaceable, you must replace the entire derailleur. A bent cage will not shift smoothly no matter how much you adjust it.

What if the new derailleur does not fit the mounting hole?

The hole may be stripped or the frame may be damaged. Try a slightly larger bolt with a washer underneath to spread the load. If that does not work, take the bike to a shop — they can drill a new hole or repair the frame.

Do I have to replace the cable when I replace the derailleur?

You do not have to, but old cables fray and stick, which makes shifting sluggish. Replacing the cable at the same time costs little and makes the new derailleur work better. If the old cable is clean and moves freely, you can reuse it.

How long does a derailleur usually last?

A derailleur lasts three to five years with regular riding and maintenance. Dirt, rust, and impacts shorten its life. Cleaning the chain and derailleur every month and shifting smoothly extends it.

What if the chain falls off after I install the new derailleur?

The limit screws are out of adjustment. Shift to the gear where the chain fell off and locate the corresponding limit screw (H for small cogs, L for large ones). Tighten it one quarter turn, test, and repeat until the chain stays on.