How to choose your MTB rear derailleur?

More difficult than it appears, choosing an MTB rear derailleur is fairly well-defined. Here is how to make the right choice.

Standards

SHIMANO or SRAM type
The number of gears: 9, 10, 11 or 12
Cage length: short, medium or long

Types of rear derailleurs

Reminder: SHIMANO and SRAM do not operate under the same standard. It is therefore not possible to mix components from these brands. (E.g. SRAM shifters with a SHIMANO derailleur).

Indeed, the cable pull ratio for SHIMANO is 2 to 1 (2 mm of cable pulled moves the derailleur by 1 mm), whereas for SRAM it is 1 to 1.

SHIMANO: 1 press a = 2 mm of cable b = 1 gear c

SRAM: 1 press a = 1 mm of cable b = 1 gear c

All components of an MTB drivetrain are designed to work under a specific speed standard, so you cannot pair a 9-speed derailleur with a 10-speed drivetrain, for example.

What cage length should you choose?

There are three cage lengths (The part of a derailleur that holds the two pulleys; this can be of different sizes depending on the drivetrain.): short, medium and long.
The shorter the cage, the more responsive and resistant it is. As a general rule, a long cage is recommended for triple drivetrains or doubles with an MTB cassette with a large sprocket range.

Once you have checked compatibility, you need to find the appropriate cage length. This is determined by the number of gears and the tooth counts of your drivetrain (cassette and MTB chainrings).

For SHIMANO, you will need to refer to the capacity (The calculation that determines the required cage length), calculated as follows:
(Maximum cassette sprocket – Minimum cassette sprocket) + (Maximum chainring – Minimum chainring). For example, with an 11/36 cassette and a 26/36 chainset, the calculation is (36-11)+(36-26) = 35. You will therefore need a derailleur with a capacity of 35.

For SRAM, you should consult the brand's compatibility chart.

So, simply make sure you choose a derailleur that is compatible with your gear shifters and fitted with a cage* suited to your drivetrain.

a Cage length

For a build from "scratch", you will have the choice between the two brands. Riders generally report a smoother and more forgiving feel with SHIMANO, or a more positive and snappy feel with SRAM.