What you need to do before you start

Replacing a rear derailleur means removing the old one completely and bolting a new one in its place. You will need to disconnect the cable that runs from your shifter to the derailleur, unbolt the derailleur from the frame, and then reverse the process with the new part. The whole job takes 30 to 45 minutes if you have done it before, or an hour to 90 minutes if this is your first time.

Before you begin, shift your chain to the smallest cog on the cassette and the smallest chainring up front. This puts slack in the cable and makes removal easier. Lay out your tools in order so you do not have to hunt for them mid-job: a Phillips or hex screwdriver (depending on your derailleur), a cable cutter if you are replacing the cable too, and a 5 mm hex key for the mounting bolt.

Take a photo of the cable routing before you disconnect anything. The cable path matters — it has to follow the same route on the new derailleur, or the shifting will not work correctly. If your derailleur is very old or the cable is frayed, buy a new cable at the same time; old cables often break during removal.

Key Takeaways

  • Shift to the smallest cog and smallest chainring before you start, so the cable has slack and the derailleur sits in its easiest position.
  • Take a photo of how the cable routes before you disconnect it, because the new derailleur must follow the same path.
  • The mounting bolt is usually a 5 mm hex key, but check your derailleur manual because some older models use a Phillips screw instead.
  • After you bolt the new derailleur on, you will need to adjust the cable tension and the limit screws so the chain shifts smoothly across all cogs.

Removing the old derailleur

Start by loosening the cable anchor bolt — the small bolt that clamps the cable to the derailleur body. You will see it on the side of the derailleur, usually marked with an "L" or "R" for left or right. Turn it counterclockwise until the cable is loose enough to slide out by hand. Do not remove the bolt completely; just loosen it enough that the cable pulls free.

Next, locate the mounting bolt on the derailleur. This is the bolt that goes through the derailleur and into the frame's derailleur hanger — a small metal tab on the right side of the rear wheel. Use your 5 mm hex key to turn it counterclockwise. The derailleur will drop away once the bolt is out. Keep the bolt in a safe place; you may be able to reuse it on the new derailleur, though some models require a new one.

If the cable is damaged or you are replacing it anyway, cut it now with a cable cutter. If you are keeping the old cable, thread it out carefully so you can see exactly how it routed. Damaged cables fray at the cut end, so wrap the end with electrical tape to keep the strands together while you work.

Installing the new derailleur

Before you bolt anything down, hold the new derailleur up to the frame and check that the mounting bolt hole lines up with the derailleur hanger. The hanger is a small metal tab, and the bolt must go straight through it. If the hole does not line up, the derailleur is the wrong model for your frame.

Slide the mounting bolt through the derailleur and into the hanger, then tighten it by hand first. Do not use the hex key yet — hand-tight is enough to hold it in place while you route the cable. Once the cable is in, you can tighten the bolt fully with the hex key. Tighten it until it is snug but not so hard that you strip the threads; a quarter turn past snug is usually right.

Now thread the cable through the cable guide on the derailleur body, following the same path your old cable took. The cable should run smoothly without kinks or sharp bends. If your new derailleur came with a cable guide that is different from the old one, check the manual to see if the routing changes. Some derailleurs have the guide on the outside of the body; others have it on the inside.

Connecting and tensioning the cable

Pull the cable tight by hand and slide it under the cable anchor bolt. Tighten the anchor bolt by hand first, then use your screwdriver to turn it clockwise until the cable is clamped firmly. The cable should not slip when you tug it, but it should not be so tight that you cannot move the derailleur by hand.

Shift the lever on your handlebars to see if the derailleur moves. If it does not move at all, the cable is too tight. If it moves but feels sluggish, the tension is close but may need a small adjustment. You will fine-tune this in the next step.

Once the cable is connected, sit on the bike and shift through all the gears. The chain should move smoothly from the smallest cog to the largest without hesitation or grinding. If the chain struggles to move up to larger cogs, the cable is too loose — loosen the anchor bolt slightly and pull more cable through before tightening it again. If the chain struggles to move down to smaller cogs, the cable is too tight — loosen the anchor bolt and let some cable out.

Adjusting the limit screws

The limit screws prevent the derailleur from moving too far in either direction. Without them, the chain can fall off the cassette or get jammed between the cassette and the frame. Most derailleurs have two limit screws, marked "H" for high gear (smallest cog) and "L" for low gear (largest cog).

Shift to the smallest cog and look at the derailleur from behind the bike. The derailleur's inner plate should be parallel to the smallest cog, not tilted toward it or away from it. If it is tilted, turn the "H" screw clockwise to move the derailleur away from the cog, or counterclockwise to move it closer. Turn the screw a quarter turn at a time and check the alignment after each turn.

Now shift to the largest cog and check the alignment again. The derailleur's outer plate should be parallel to the largest cog. Adjust the "L" screw the same way — clockwise to move away, counterclockwise to move closer. Once both limit screws are set, the chain should not fall off in either direction, even if you shift hard or the bike bounces over rough ground.

Testing and fine-tuning

Take the bike for a short ride on flat ground and shift through all the gears several times. The chain should move smoothly and quickly without grinding, hesitation, or noise. If the chain hesitates when moving up to larger cogs, the cable is slightly too loose — find the barrel adjuster on your shifter (a small cylinder where the cable enters the lever) and turn it counterclockwise a quarter turn. This adds tension without loosening the anchor bolt.

If the chain hesitates when moving down to smaller cogs, turn the barrel adjuster clockwise a quarter turn. Make small adjustments and test after each one; a quarter turn is usually enough to fix a hesitation. If the chain still grinds or skips after adjusting the barrel adjuster, the limit screws may need another small tweak, or the cable may be damaged and need replacement.

Once shifting is smooth across all gears, the job is done. Wipe down the derailleur and chain with a dry cloth to remove any dust or grease from handling, and you are ready to ride.

When to replace the cable at the same time

If your old cable was frayed, kinked, or hard to move, buy a new one and install it while the derailleur is off the frame. A new cable costs 8 to 15 dollars and takes only a few extra minutes. Slide the old cable out of the housing, measure its length, and cut a new cable to the same length. Thread the new cable through the housing and derailleur, then connect it the same way you would an old one.

If the cable housing (the plastic tube the cable runs through) is cracked or leaking grease, replace that too. Housing costs 3 to 8 dollars per meter, and most rear derailleur cables need 1 to 2 meters. Measure the old housing before you remove it, cut the new housing to length with a cable cutter, and slide the new cable through it before connecting to the derailleur.

Frequently Asked Questions

What if the derailleur hanger is bent?

A bent hanger will make the derailleur sit at an angle, and no amount of adjustment will fix it. You can buy a replacement hanger for 15 to 40 dollars — they are frame-specific, so check your frame model before ordering. Unbolt the old hanger and bolt the new one on, then install the derailleur as normal.

Do I need to remove the chain to replace the derailleur?

No. The chain can stay on the bike. Just shift to the smallest cog so there is slack, and the derailleur will come off without touching the chain. When you install the new one, the chain will sit naturally in the same position.

How do I know if my derailleur is the right speed?

Your derailleur must match the number of cogs on your cassette. A 10-speed derailleur works with a 10-cog cassette, an 11-speed with an 11-cog cassette, and so on. Count the cogs on your cassette or check your bike's manual. Buying the wrong speed will cause poor shifting and may not fit your frame.

What if the cable keeps slipping out of the anchor bolt?

The cable is probably too short or the anchor bolt is not tight enough. Loosen the bolt, pull the cable as tight as you can by hand, and tighten the bolt firmly. If the cable still slips, it may be damaged or frayed — replace it with a new one.

Can I reuse the old mounting bolt?

Usually yes, but check the manual for your new derailleur. Some models use a different bolt size or thread type. If the old bolt does not fit, the new derailleur will come with a replacement, or you can buy one at a bike shop for 2 to 5 dollars.