A complete guide to rear derailleurs

Derailleur compatibility, tooth capacity and much more explained

Is it time to upgrade or replace your rear derailleur? Do you sit awake at night worrying about what a tooth capacity is? Or have you ever simply wanted to know absolutely everything there is to know about buying a rear derailleur or thought which rear derailleur do I need? If so, you’ve come to the right place.


While we certainly don’t recommend you break out this hot derailleur chat at your next party, this is undoubtedly useful information if you’re looking to buy or upgrade a rear derailleur.

We must stress that this article only covers rear derailleurs because including front derailleurs would make this guide far too unwieldy. Plus, if you’re to believe SRAM, the front derailleur is dead anyway.

Which brand of derailleur should I buy?

Which rear derailleur do I need? Shimano, SRAM and Campagnolo are the three main players in the drivetrain market

As a general rule of thumb, it’s best not to mix and match drivetrain components from different brands. While things like cranks, chains and cassettes are largely inter-compatible between brands, shifters and derailleurs generally speaking aren’t.

In brief, Shimano, SRAM and Campagnolo all use different cable pull ratios (the amount that a derailleur moves for every millimetre of cable pulled through by the shifter), and mixing parts will result in very poor shifting.

There are of course exceptions and there are lots of bodged, Sramshimpagnolo mashups on the ‘net, but for the sake of simplicity we suggest you stick to the same brand as your shifters when buying a rear derailleur.

How many gears does my bike have?

Once you’ve settled on the brand, you must now determine the number of gears that your bike has.