What a derailleur does and why you might replace one
A derailleur is the mechanism that moves your chain between sprockets on your cassette (the stack of gears at the back wheel) or chainrings (the gears at the pedals). When it stops shifting smoothly, skips gears, or gets bent from a crash, replacing it is often simpler than repairing it. You can install a new derailleur yourself with basic tools if you follow the steps in order—the job takes 30 to 45 minutes once you have the right part.
Before you start, confirm you are replacing the rear derailleur (the one at the back wheel) or the front derailleur (the one at the pedals). This guide covers the rear derailleur, which is the more common replacement. The front derailleur follows similar logic but attaches differently. Also check your bike's manual or the derailleur packaging to verify the new one matches your bike's drivetrain—Shimano, SRAM, and Campagnolo parts are not interchangeable, and the speed (8-speed, 9-speed, 10-speed, and so on) must match your cassette.
Key Takeaways
- A rear derailleur bolts to a hanger on the frame behind the cassette and connects to your shifter cable, so you need the correct replacement part for your drivetrain brand and speed.
- Remove the old derailleur by loosening the cable anchor bolt, unbolting the derailleur from the frame hanger, and setting the chain on the smallest sprocket to release tension.
- Install the new derailleur by bolting it to the hanger, threading the cable through the anchor bolt, and adjusting the limit screws so the chain does not fall off either end of the cassette.
- Cable tension and limit screw adjustment take trial and error—shift through all gears and fine-tune the screws until the chain moves smoothly without noise or hesitation.
- If the derailleur hanger is bent, you will need a hanger alignment tool or a trip to a bike shop, because a bent hanger will make any new derailleur misalign.
Tools and parts you will need
Gather these items before you start. You need an Allen wrench set (usually 4 mm, 5 mm, and 6 mm sizes fit most derailleurs), a Phillips or flat-head screwdriver for the limit screws, and a cable cutter if you are replacing the cable as well. A bike stand or way to prop the bike upside down makes the work much easier—you need both hands free and the rear wheel spinning.
You also need the correct replacement derailleur. Write down your bike's brand (Shimano, SRAM, or Campagnolo), the speed (count the sprockets on your cassette), and whether your current derailleur is short-cage or long-cage (a longer cage holds more chain slack and works with larger cassettes). Take a photo of your current derailleur and bring it to a bike shop if you are unsure. A new derailleur costs between $40 and $150 depending on the brand and model.
If your derailleur hanger—the small metal tab bolted to the frame—is bent, you will need a derailleur hanger alignment tool (around $30 to $60) or a professional repair. A bent hanger will cause any new derailleur to misalign, so check this before you buy a replacement.
Remove the old derailleur
Shift the chain onto the smallest sprocket at the back and the smallest chainring at the front. This releases tension on the cable and makes the derailleur easier to remove. If the chain is stuck or the shifter is broken, use your hand to pull the derailleur forward and guide the chain onto the smallest sprocket manually.
Locate the cable anchor bolt—a single bolt on the derailleur body where the cable enters. Loosen it with an Allen wrench (usually 4 mm or 5 mm) and slide the cable out. You do not need to remove the cable from the shifter yet; just free it from the derailleur.
Find the mounting bolt that attaches the derailleur to the frame hanger. It is a single bolt on the upper part of the derailleur body. Loosen it with an Allen wrench and remove the derailleur completely. Set it aside. Inspect the hanger for bends—it should be a straight vertical tab. If it is bent, straighten it with a hanger tool or take the bike to a shop before installing the new derailleur.
Install the new derailleur and thread the cable
Position the new derailleur so the mounting bolt hole aligns with the frame hanger. Insert the bolt and tighten it by hand first, then use an Allen wrench to snug it firmly—do not over-tighten, as you may strip the bolt. The derailleur should sit parallel to the cassette with the pulley wheels (the two small wheels that guide the chain) aligned vertically behind the sprockets.
Thread the cable through the cable anchor bolt. Pull the cable tight by hand—do not use the shifter yet—and tighten the anchor bolt to hold it in place. The cable should be taut but not so tight that it pulls the derailleur out of position. Trim any excess cable with a cable cutter and cap the end with a ferrule (a small metal sleeve) to prevent fraying.
Spin the rear wheel and shift through all the gears using the shifter. The chain should move smoothly across the cassette without noise, hesitation, or falling off. If it does not, move to the next section to adjust the limit screws.
Adjust the limit screws for smooth shifting
The two limit screws control how far the derailleur can move in each direction. They are usually labeled H (high gear, smallest sprocket) and L (low gear, largest sprocket). Locate them on the derailleur body—they are small screws with a Phillips or flat head. Turning them clockwise restricts movement; turning them counterclockwise allows more movement.
Shift to the smallest sprocket and watch the chain. If it falls off the inside (toward the frame), turn the H screw clockwise a quarter turn and try again. If the chain hesitates or does not reach the smallest sprocket, turn the H screw counterclockwise a quarter turn. Make small adjustments and test after each one.
Shift to the largest sprocket and repeat the process with the L screw. If the chain falls off the outside (away from the frame), turn the L screw clockwise. If it hesitates or does not reach the largest sprocket, turn the L screw counterclockwise. Once the chain moves smoothly at both ends, shift through the middle gears and listen for noise or hesitation. Fine-tune the cable tension by loosening the anchor bolt slightly, pulling or releasing a small amount of cable, and re-tightening.
Test the derailleur and troubleshoot common problems
Spin the wheel and shift through all gears slowly. The chain should move one sprocket at a time without noise, grinding, or delay. If the chain struggles to move up to smaller sprockets, the cable may be too loose—loosen the anchor bolt, pull more cable through, and re-tighten. If the chain struggles to move down to larger sprockets, the cable may be too tight—loosen the anchor bolt, release a small amount of cable, and re-tighten.
If the chain still falls off or makes noise after adjusting the limit screws and cable, check that the derailleur hanger is straight and the derailleur is parallel to the cassette. A bent hanger or misaligned derailleur will cause problems no matter how much you adjust. If you suspect a bent hanger, use a hanger alignment tool or take the bike to a shop.
After a few rides, check the cable tension again. New cables stretch slightly, so you may need to tighten the anchor bolt a quarter turn to keep the shifting smooth. This is normal and takes only a minute.
When to call a professional
If the derailleur hanger is bent, a professional repair is faster and safer than attempting to straighten it yourself. A bike shop can align the hanger with a specialized tool in 15 minutes. If the frame itself is cracked or the hanger bolt hole is stripped, the frame may need welding or replacement—this is beyond a home repair.
If you have adjusted the limit screws and cable tension multiple times and the chain still falls off or makes noise, the derailleur may be damaged or the cassette may be worn. A shop can diagnose this and recommend whether to replace the derailleur, cassette, or both.
Frequently Asked Questions
Can I install a derailleur without removing the chain?
Yes. You do not need to remove the chain to replace the derailleur. Shift the chain to the smallest sprocket to release tension, then unbolt the old derailleur and bolt on the new one. The chain will stay on the bike the entire time.
What if the cable is frayed or damaged?
Replace the cable at the same time. Remove the old cable from the shifter and derailleur, thread a new cable through the same path, and attach it to the new derailleur. New cables cost $5 to $15 and make shifting smoother.
How tight should the mounting bolt be?
Snug it firmly with an Allen wrench so the derailleur does not move when you shift, but do not use all your strength. Over-tightening can strip the bolt or crack the hanger. If the derailleur moves during shifting, tighten the bolt a quarter turn and test again.
Why does my chain skip after installing a new derailleur?
Skipping usually means the cable is too loose or the limit screws need adjustment. Tighten the cable slightly by loosening the anchor bolt, pulling more cable through, and re-tightening. If that does not work, adjust the limit screws as described above.
Do I need to replace the cassette when I replace the derailleur?
No. A derailleur and cassette are separate parts. Replace the derailleur only if it is bent, broken, or worn. If the chain skips on multiple sprockets, the cassette may be worn and need replacement, but this is a separate job.