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 (26)

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.

what size cage

By: on 18 March 2018
Hi Guys i am running a shimano XT set up on my 2012 Trek superfly 100 elite ( 2x10 38T front and 11/36 rear cassette) I want to change to a 1x10 and wondered what size rear shimano mech i would need to run the same 38T large front sprocket up front and same 11/36 rear cassette. hope someone can help with this upgrade question? many thanks

Mountain Bikes Direct Response
Hi there, Thanks for getting in touch! The easiest thing to do is to keep the same derailleur you have now and take a few links out of the chain. Otherwise, with an 11/36 cassette and single ring up front, you can run a short cage derailleur. Cheers,

which cage length

By: on 1 March 2018
I have a 32T front chainring with 11-42 10 speed cassette, wanna use a SRAM GX 2x10 10 Speed Type 2.1 Mountain Bicycle Rear Derailleur, wondering if a medium cage will work or do i need a long cage, and what the difference in cage lenght between the two?

Medium or long cage?

By: on 23 February 2018
Hey there, I have a 2006 Specialized Rockhopper that I am rebuilding, and I'm running a 1x9 setup. It has a 34T front chain ring and a 11-36 rear cassette, I'm going SRAM X-5 shifter and derailleur and would like to know if I should buy a medium or long cage.

xt cassette

By: on 22 February 2018
hey guys just bought a long derailleur cage and was just wondering if I could run it with a 32 tooth up front and a 11-46t cassette in the rear and its 1x11

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