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 (30)

medium or long

By: on 20 June 2018
I'm planning to buy 26/36 front and 11/46 10 speed rear cassette. What derailleur cage length should i use?

Mountain Bikes Direct Response
Hi Louie, If we go off the formula in the post above we can work it out like this: 36-26 = 10 46-11 = 35 Then: 35+10 = 45 You'll end up with 45T which means that you'd need a long cage with both Shimano and SRAM setups. Note that an 11-46 cassette has too much range for most 10 speed derailleurs. The SRAM GX long cage can only really handle a max of a 36t cog. The only one that would be suitable would be the Shimano Deore M6000 long cage as this can handle a max cog of 46t and has a max capacity of 43t. 11-46t cassettes are really only meant for 1x chainring setups so you'll get better performance out of an 11-42. I'd avoid using the 46t cog when you're in the big ring as it'll probably stretch things a bit too far. Feel free to shoot us an email if you have any further questions!

Xt shadow on 11-40 single

By: on 9 June 2018
Hi Guys i am running a shimano XT setup on my 2008 Merida hfs 3000 ( 3x9 22-32-44T front and 11/34 rear cassette). I want to change to a 1x9 and wondered what size rear shimano mech i would need to run with 36t at front and 11/40 rear cassette. many thanks

Mountain Bikes Direct Response
Hi mate, thanks for getting in touch. For an 11-40t cassette and running 1x, a medium cage derailleur is plenty. Cheers,

Medium or Long?

By: on 9 June 2018
Running a 11x46 Shimano XT group set on 2016 SC Nomad w long cage--good? Used medium XT setup on other Turner 11x42, but thought more needed with 46 even if possibly more strikes (like I don't peddle strike as it is)

Mountain Bikes Direct Response
Hi mate, thanks for checking in! A medium cage XT derailleur is actually plenty long enough to fit on to an 11-46t cassette, and reportedly shifts better than a long cage. Cheers,


By: on 27 May 2018
I have a sram nx x sync crankset single 34t that I want to fit to my 2014 giant talon 0. Rear cassette is 11-36. Was wondering if I had to change my shimano xt shadow derailleur. Thanks

Mountain Bikes Direct Response
Hi John, that sounds like it'll be totally fine. I'd recommend that you check your chain length to make sure that it neither too short or too long with the new setup. Cheers, MBD

Long or medium?

By: on 11 April 2018
Hello, I plan on running 11/50 1x11 with a 32T front sprocket on my 2014 specialized crave 29er. Should I go long or medium on the RD? Thank you! Matthew

Mountain Bikes Direct Response
Hi mate, You'll definitely need a long cage for a settup like that.

Leave a comment

Comments have to be approved before showing up