How to replace your rear derailleur

This might seem like an intimidating task, but with the right steps and a few tools, it’s an easy skill to master

Clock09:00, Monday 13th November 2023

Your rear derailleur manages your chain tension and allows you to change gear on your cassette. They're great, but hanging so close to the ground, it's easy for them to get banged, bent and broken. Thankfully, replacing them is pretty simple. All you need is a few tools (see our tool list on the right) and half an hour or so.

While you're at it, you might as well replace the gear cable too. A new cable will give crisper shifting, and this is a good opportunity to sort two maintenance jobs in one session.

In this guide, we're using a Shimano rear derailleur as an example, but the steps are broadly similar for other brands too.

Have you bent your derailleur, or just the hanger?

If you're switching your derailleur because your derailleur looks like it has bent, consider replacing your derailleur hanger first. These are designed to bend easily on impact, to protect your derailleur and frame. So, if you've had a knock, the chances are it's your hanger that needs replacing, not the derailleur itself.

Read more: Complete guide to road bike groupsets

Tools Needed

Squeeze the master link to open the chain

Step 1

Remove the chain

Begin by removing the chain. If you have a master link, you can use a pair of pliers to remove it. If not, use a chain tool to break the chain.

Read more: How to remove and replace a chain

Undo the clamp, pull out the cable, then pull it from the derailleur

Step 2

Remove the cable

Remove the cable. Do this by pulling the crimp end off with your pliers, then undo the cable clamp bolt with an Allen key (hex key).

Unscrew the bolt and remove the old derailleur

Step 3

Remove the derailleur

Once the cable is out of the way, you can remove the derailleur. Take a 5mm Allen key and undo the main mounting bolt, applying firm but even pressure to remove the derailleur from the hanger.

Be careful not to cross-thread the bolt when attaching the derailleur

Step 4

Attach the new derailleur

Add a little grease to the thread of the new derailleur, then mount it with the 5mm Allen bolt. It’s very important to make sure that the angle of the derailleur is right. For shimano derailleurs, the small, swingable piece on top should be horizontal to the ground, with the little nub above the notch on the mech hanger. Unless otherwise stated, torque the bolt to 8-10Nm for SRAM and Shimano, or 15Nm for Campagnolo.

Make sure to thread the chain the right way around the jockey wheels

Step 5

Reinstall the chain

Thread the chain back on, making sure to route it correctly through the jockey wheels and the front derailleur, and that the text printed on the links is facing you. Once it’s in, reattach with a master link. Just note that most master links aren’t designed to be reused.

Thread the gear cable through and clamp it in place

Step 6

Install the gear cable

This is a good opportunity to put in a new gear cable, but if you are using the old one, insert the gear cable back through the micro adjuster and under the cable clamp bolt. Pull the cable tight and tighten down the clamp bolt. Cut it to length, put a cap on the end and crimp it on.

Some derailleurs use an Allen key, others use a screwdriver

Step 7

Set the H (high) limit screw

All that’s left to do is index the gears, starting by setting the limit screws. These screws determine how far inboard and outboard the derailleur can move and stop the chain going over the largest sprocket into your spokes, or off the smallest sprocket and into your frame.  With the chain on the smallest sprocket of the cassette, turn the H screw until the top jockey wheel lines up with the smallest sprocket on the cassette.

Change into the biggest cog to adjust the L limit

Step 8

Set the L (low) limit screw

Now shift the derailleur to the largest sprocket. Adjust the L screw until the top jockey wheel is in line with the largest sprocket on the cassette. You might need to push the derailleur by hand to find where the limit is set.

The B-tension screw adjusts the angle of the derailleur

Step 9

Set the B-tension screw

Staying in the biggest sprocket, adjust the long B-tension screw until the outside edge of the top jockey wheel sits around 5mm below the outside edge of the cassette teeth.

Adjust the cable tension with the thumb screw

Step 10

Set the cable tension

To set the cable tension, move to the smallest cog then shift one gear up. If the chain chatters but doesn’t move, turn the barrel adjuster counter-clockwise until it does.

Test your gears one by one before you hit the road

Step 11

Check it's all working

Run through all your gears to make sure the rear derailleur is working. If it's struggling to change into the bigger cogs, turn the barrel adjuster anti-clockwise to add cable tension, until the chain moves. If it's struggling to change into the smaller cogs, turn the barrel adjuster clockwise to reduce the cable tension, until the chain changes into the smaller cogs without delay.

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