How to choose derailleur cage length

We were recently asked: "How do I tell if I need a medium or long rear derailleur?"

Excellent question! And while there are a few factors that impact your choice of cage length (Long, Medium or Short), you can work it out using a simple formula.

Why does it matter?

The derailleur cage is a swinging link, with two pulleys. As the chain moves between different size sprockets (front and rear), the amount of chain required changes. The derailleur cage picks up this slack.

A cage that's too short has the potential to rip your derailleur off your bike, when the cage reaches its capacity....


A cage that's too long adds unnecessary weight to your bike, runs closer to the ground (when it's not necessary) and there's also the potential for more dropped chains (a longer cage generally has less leverage, and therefore runs less spring tension).


Working it out:

[ Largest Front Ring - Smallest Front Ring ] + [ Largest Rear Ring - Smallest Rear Ring ]  =  Required Capacity

e.g. if you are running a 22-32-42T front crankset, and a 11-34T rear cassette.

[ 42 - 22 ] + [ 34 - 11 ] = 43T

Choosing a cage:

Then you can choose which cage length you need. General guidelines for maximum capacity for each brand are (though it does vary between models, so best to check the model-specific capacity):


Long = 45T
Medium = 33T
Short = 28T


Long = 45T
Medium = 37T
Short = 30T

So in the example above, you would opt for a Long Cage derailleur in either brand. (Be sure to check the rating for the exact model you are looking at).

The other factor to consider is if you are riding a long travel dual suspension bike - if the calculation is on the edge, you may wish to go for the longer option cage (as the rear suspension goes through travel, it may pull on the chain if the cage is too short). There is no hard and fast rule on this, however there is an easy way to test - once you have your new derailleur set up, shift to the largest ring(s) on your bike, release all the pressure out of your rear shock (or remove it, if you have a coil sprung bike), and then check your chain does not limit your travel.


If you're having any trouble choosing the right derailleur, then feel free to contact us and we'll sort it out!

To view our range of derailleurs, Click Here!


Comments (35)

Long or Medium

By: on 15 August 2018
hi, i have a 1/11 drivetrain, the cassette is an 11/42 and the chainring is 36T What Cage should i buy?

Mountain Bikes Direct Response
Hi Jonah, Thanks for checking in! If you're running a Shimano group, the medium cage will work great! Cheers, MTB Direct.

Sram GX

By: on 25 July 2018
Hi guys, i was wondering what’s gonna be the best cage for me with a 1x10 drivetrain, 30 tooth front and 11-36 rear with extra 42 tooth cog in the back? Medium or large cage? Thanks

Mountain Bikes Direct Response
Thanks for checking in! The Medium cage will work well for you. Let us know if you have any further questions. Cheers, MTB Direct.

Confused calculation, is it only for 2x or 3x ring

By: on 15 July 2018
Currently running a 1x10 XT with medium cage clutch mech setup, single 28T ring up front and 11-42T rear cassette on a short travel Camber 29Er. Using your calculation... 28T + (42-11T) = 59???? Or is it a different calculation for single rings? Looking to switch to SRAM, same front rear T's, do I need a long cage or is the medium I have currently, okay?

Mountain Bikes Direct Response
Hi mate, Kind of, except rather than adding in your chainring size, you use a '0', as there is no difference between your chainrings, cos there's only one. Cheers,

SRAM GX Eagle 1x10 on Full Suspension

By: on 5 July 2018
Hi.. I had a SRAM GX Eagle short cage (I believe) and had it ripped off the bike just like the picture above. This was on a 'used' bike I just bought(new to me).....It has an 11-36 PG1050 in the rear, but with the 11 tooth cog removed, and a 40t cog instead. I have a 28 tooth front chain ring (SRAM NX). When It ripped off the derailleur, I was climbing in first gear(28-40) on a light hill. and it sucked up a twig and that was it.... When I mentioned to my bike shop what happened, they ordered the long cage. I get a 27 from the formula (28-13) + (40-28) =27... so it looks like short cage should be ok.. With the new derailleur (SRAM GX Eagle LONG cage) and a new chain.. I now get a pop/skip under load in first gear...(the 40t)... all other gears are fine under any/all load. Do you think I need a new 40t ring due to the new chain? or is this a problem with the derailleur cage length? Can too large a cage length be a bad thing?

Mountain Bikes Direct Response
Hi mate, would you be able to send through a whole bunch of photos of the cassette and derailleur to We'll help get you sorted. Cheers,

Need help

By: on 1 July 2018
I have a specialized crave I bent my deralleur badly. The bike is 29er large frame using the above calculation it is 39 do I need large or medium?

Mountain Bikes Direct Response
Hello, I would recommend a large cage here as this is what was stock on those bikes. Cheers

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