How to replace your rear derailleur

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

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.