What a derailleur does and when to replace it

A derailleur is the mechanism that moves your chain from one gear to another. The rear derailleur hangs below your cassette (the stack of gears on your back wheel) and pulls the chain sideways as you shift. The front derailleur sits above your crankset (the gears near the pedals) and does the same job for those larger gears. When a derailleur stops shifting smoothly, makes grinding noises, or visibly bends after a crash, replacement is usually the only fix.

You can replace either derailleur yourself with basic tools if you are comfortable with bike maintenance. The rear derailleur is more common to replace because it hangs lower and takes more impacts. The front derailleur is simpler mechanically but requires more precise adjustment. Both jobs take 30 minutes to an hour once you have the right parts.

Key Takeaways

  • Rear derailleurs are held on by a single bolt to the derailleur hanger (a small metal tab on your frame) and connected to your shifter by a cable you can reuse.
  • You need to know your derailleur type (Shimano, SRAM, or Campagnolo) and speed (how many gears it controls) before you buy a replacement.
  • Removing the old derailleur means unbolting it from the hanger, loosening the cable, and removing the chain from the pulley wheels.
  • Installing the new one reverses those steps, but you will need to adjust cable tension and limit screws so the chain shifts cleanly without falling off.
  • If your derailleur hanger is bent, you must straighten or replace it first, or the new derailleur will not work.

Identifying your derailleur type and gathering tools

Before you buy a replacement, look at your current derailleur or your bike's documentation to find the brand and speed. The brand is almost always Shimano, SRAM, or Campagnolo. The speed refers to how many gears your cassette has — a 10-speed derailleur works with a 10-gear cassette, an 11-speed with 11 gears, and so on. Mixing brands or speeds will cause poor shifting or the chain to fall off.

You will need a Phillips or flathead screwdriver (depending on your shifter cable anchor), a 5 mm Allen wrench (for the derailleur hanger bolt), and possibly a 2.5 mm Allen wrench if your derailleur uses a smaller cable anchor bolt. A chain tool is helpful but not essential for this job. Have a clean rag nearby because derailleurs collect grease and dirt.

If you are replacing the rear derailleur, check whether your derailleur hanger is straight. Sight along the hanger from behind the bike — it should form a straight line with the cassette. If it bends inward or outward, you must straighten it with a derailleur hanger alignment tool or replace it before installing the new derailleur. A bent hanger will make any new derailleur shift poorly.

Removing the old rear derailleur

Shift your chain onto the smallest cog (the innermost gear on your cassette) and the smallest chainring at the front. This puts slack in the cable and makes removal easier. Locate the cable anchor bolt on the derailleur body — it is usually a small bolt where the cable enters the derailleur. Loosen this bolt with your screwdriver until the cable slides free. You do not need to remove the cable from the shifter; just let it hang.

Next, locate the derailleur hanger bolt. This is a single bolt that attaches the derailleur to a small metal tab (the hanger) welded to your frame, just below the cassette. Use your 5 mm Allen wrench to unscrew it completely. The derailleur will drop away. Set it aside — you may need it for reference when installing the new one.

Look at the two small pulley wheels (the upper and lower jockey wheels) where the chain runs. If the chain is still threaded through them, you will need to remove one link of the chain to free the derailleur completely. Use a chain tool to push out a pin, separate the chain, and slide the derailleur away. If you are keeping the same chain, mark which direction the chain runs through the pulleys so you can thread it the same way on the new derailleur.

Installing the new rear derailleur

Slide the new derailleur into position so the upper jockey wheel sits just below the smallest cog on your cassette. The derailleur hanger bolt hole should align with the hanger tab on your frame. Insert the bolt and tighten it firmly with your Allen wrench — it should be snug but not so tight that you strip the threads. Do not overtighten.

If you removed the chain, thread it back through the upper and lower jockey wheels in the same direction it came out. Rejoin the chain using your chain tool, or if the chain is new, connect the ends with a quick link (a removable connector that comes with most chains). Make sure the chain sits properly in both pulleys and does not rub.

Pull the cable taut and insert it into the cable anchor bolt hole. Tighten the anchor bolt by hand first, then snug it with your screwdriver. Do not crank it down yet — you need to adjust the cable tension in the next step. Shift through all your gears a few times to see how the derailleur responds. If the chain struggles to move up to larger cogs, the cable is too loose. If it struggles to move down to smaller cogs, the cable is too tight.

Adjusting cable tension and limit screws

Loosen the cable anchor bolt slightly and pull the cable a bit tighter if the chain is slow to shift up, or let out slack if it is slow to shift down. Retighten the anchor bolt and test again. This usually takes two or three tries. The goal is for the chain to snap cleanly from one cog to the next without hesitation or overshifting.

Next, adjust the two limit screws on the derailleur body. These are usually labeled H (high) and L (low). The H screw stops the derailleur from pushing the chain onto the smallest cog and off the cassette. The L screw stops it from pushing the chain onto the largest cog and off the other side. Shift to the smallest cog and look at the upper jockey wheel — it should sit directly below the smallest cog with a small gap (about 1 mm). If the wheel is too close or too far, turn the H screw clockwise to move the derailleur inward or counterclockwise to move it outward. Shift to the largest cog and repeat with the L screw.

Once both limit screws are set, shift through all your gears several times. The chain should move smoothly without noise, hesitation, or overshifting. If one gear still feels rough, you may need to fine-tune the cable tension again. This is normal — derailleur adjustment is iterative.

Replacing a front derailleur

Front derailleurs are less common to replace but follow a similar process. Shift the chain onto the smallest chainring to create slack. Loosen the cable anchor bolt and let the cable hang. Locate the mounting bolt or bolts — front derailleurs usually clamp around the seat tube (the vertical tube below the saddle) with a single bolt, or bolt directly to the frame. Unscrew these bolts and slide the derailleur off.

Install the new derailleur by reversing the process. The derailleur cage should sit about 2 to 3 mm above the largest chainring when you sight from the side. Tighten the mounting bolt, thread the cable, and adjust the H and L limit screws so the cage does not push the chain off either side. Front derailleurs are more finicky about positioning, so take your time getting the height and angle right before you tighten everything down.

When to call a bike shop instead

If your derailleur hanger is bent and you do not have an alignment tool, a bike shop can straighten it for $15 to $30. If you are unsure about your derailleur type or speed, a shop can identify it from your bike. If you have tried adjusting cable tension and limit screws multiple times and the derailleur still does not shift smoothly, the problem may be a bent derailleur body, a damaged cable, or a worn cassette — all things a shop can diagnose quickly.

Replacing a derailleur is straightforward if you are comfortable with basic wrenching, but it does require patience with adjustment. If you are not confident, a shop can do the job for $60 to $120 in labor, plus the cost of the part.

Frequently Asked Questions

Can I use a different brand derailleur than the one I have?

No. Shimano, SRAM, and Campagnolo derailleurs use different cable pull ratios, meaning the cable moves a different distance for each click of the shifter. Mixing brands will cause poor shifting or the chain to fall off. You must match the brand of your shifter.

What if I do not know what speed my bike is?

Count the cogs on your cassette (the gears on your back wheel). If there are 10, you have a 10-speed bike. If there are 11, you have an 11-speed. You can also look at your shifter — it usually has the speed printed on it, or count the number of clicks as you shift through all gears.

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

Not usually. If the cable is frayed, kinked, or visibly damaged, replace it. Otherwise, reuse it. A cable that works with your old derailleur will work with a new one of the same brand and speed.

How tight should the derailleur hanger bolt be?

Tight enough that the derailleur does not move when you shift, but not so tight that you strip the threads. If you are using a 5 mm Allen wrench, snug it firmly — you should feel resistance but not have to strain. If you are unsure, err on the side of slightly looser and tighten more if the derailleur rocks.

Why does my new derailleur shift poorly even after I adjusted it?

The most common cause is a bent derailleur hanger. Sight along the hanger from behind the bike — if it is not straight, straighten it or replace it. Other causes include a worn cassette, a damaged cable, or the derailleur not being fully seated on the hanger bolt. Check all three before assuming the new derailleur is faulty.