Changing the rear derailleur might be more complicated that it first looks so today we are going to look at how to replace rear derailleur and how to choose one that is compatible.
– Because it’s damaged or seized.
– Because it’s worn out.
– Because you may want to upgrade it.
How to replace rear derailleur with a compatible one
Bike type compatibility.
Although road and MTB derailleurs differ, in some cases you can use a mountain bike derailleur on a road bike and vice versa.
However, to eliminate the possibility of gear shifting issues, use one that has been designed for the particular type of bike.
Rear derailleurs are designed to work with a certain number of gears so make sure that the one you are fitting matches the number of rear cogs.
The main bike part manufacturers design their components to work best within their specific groupsets.
Because of that certain components such as derailleurs and shifters, might not work mixed with another brand causing poor indexing.
Certain brands such as Shimano, offer a range of compatible derailleurs such as 105, Ultrega and Dura ace.
Top-of-the-range derailleurs are made out of more durable and lighter materials than the cheaper options. They also have a finer finish and look more expensive. However, this can only really be appreciated in high performance riding.
Cage length compatibility.
Derailleurs have different cage lengths depending on how much chain is needed when moving between higher and lower gears. A smaller ratio, e.g. road bikes require a short cage, and a bigger ratio .e.g. Mountain bikes, need a longer cage. There are 3 cage lengths:
Short cage – SS
Medium cage – GS
Long cage – SGS
Finding the compatible cage length
To determine the correct length for your bike, first check the maximum low sprocket capacity on your new rear derailleur. To do this, find a code or serial number. It is either on the box, the back of the derailleur body or the back of the inner cage.
Use this code to check the maximum low sprocket capacity. We’ve added some useful links in the description below this video.
If the amount of teeth on the biggest cog on the cassette is greater, then you will need to use a longer cage derailleur.
Calculating maximum drivetrain capacity
Now check the maximum drivetrain capacity. This is the amount of chain that the derailleur arm can handle. You can use this formula to calculate it.
First work out the Maximum Chainring Difference. Count the number of teeth on the biggest and smallest chainrings.
In our example the larger chainring has 40 teeth and the smaller 28 teeth. Subtract the lower number from the higher number.
Now it’s time to calculate the Maximum Cassette Cog Difference. Count the number of teeth on the biggest and smallest sprockets on your new cassette.
In our example the biggest sprocket is 36 teeth and the smallest one 11 teeth. Subtract the lower number from the higher number. Add these two numbers together.
If this number is greater than the maximum drivetrain capacity then the replacement rear derailleur won’t be able to function properly at the extremes and you will need to use a longer cage mech.
Not sure what is the maximum drivetrain capacity for your derailleur? Send us a message in the comments below and we will advise you.
How to replace rear derailleur.
- Shift the derailleurs to the highest gear at the back and the lowest at the front, to assist chain removal.
- Split the chain and remove from the jockey wheels. If you’re not sure how to remove and fit a chain then you can find a step by step guidance in our article here.
You can also watch our video tutorial here:
If you don’t have a quick link fit one now to make re-joining the chain easier.
- If you see a nipple at the end of the cable then cut it off.
- Loosen the cable and remove it from the derailleur.
- Remove the derailleur.
- Take the replacement derailleur and apply a small amount of grease to the thread.
- Position it so that bolt is against the derailleur hole and so that the “B” tension screw or tab sits above the tab on the hanger.
- Gently start to screw the derailleur bolt but don’t force it, as you could cross-thread the hanger. If it doesn’t screw in easily then remove and repeat the process.
- Do the final tighten. If you don’t have a torque wrench then make sure that the bolt is tight. However, if you have one then tighten it to about 9NM.
- Refit the chain.
- Now set up the limit screws. We have a great article here to help you with this task.
you can also watch our video tutorial here:
- Refit the cable. In some cases the length of the outer casing may differ between the old and new derailleur and the inner wire may be frayed. In these cases replace the inner cable and outer casing.
- Finally, adjust the gears. You can find great tips on how to adjust gears in our article here.
You can also watch our tutorial here:
Only a compatible rear derailleur will work with your gears. To find a correct rear mech replacement check:
– the type of bike you are using. Not all MTB derailleurs will work with drop bar shifters and vice versa.
– the number of rear cogs. Rear mechs work with certain number of cogs. Make sure that the one you are fitting matches the number of rear gears.
– the brand and the model. Some brand rear derailleurs like Shimano, Sram and Campagnolo will not work when mixed with another brand gear shifter.
– cage length. Bikes with a smaller gear ratio e.g. road bikes require a short cage, and a bigger ratio .e.g. Mountain bikes, need a longer cage.