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 (21)

Xt cassette

By: on 14 February 2018
Hi guys I’m just wondering what sized derailleur cage I should run for a 32t front cog and a 11x46t cassette. Cheers

Mountain Bikes Direct Response
HI There, Thanks for checking in. If you're running an 11-speed 11-46t Shimano drivetrain, you can run a medium cage with 1x. Let us know if you're running a different setup. Cheers,

Bit confused. What size?

By: on 2 February 2018
I have a 11 speed 10-42T cassete, and a 30T chain ring. What size would be the best? Does the cage size effect chain tension? I ride enduro. Thanks!

Mountain Bikes Direct Response
Hi Ethan, Thanks for getting in touch. If you're using a Sram derailleur, you don't have to worry as those come in one cage Length. As for Shimano, you'll want the medium cage. Different cage lengths are optimised to take up excess slack in the chain as you shift through your gears. The wider the range, the longer the cage has to be. Since Sram MTB 11-speed was designed around the 10-42 cassette, they only need to have one cage length. Cheers,


By: on 28 January 2018

Mountain Bikes Direct Response
Hi mate, Sounds like a medium length cage would be perfect for your settup.

Shimano Dura Ace 7401 on '87 Bridgestone MB-1

By: on 27 January 2018
Hello! Thank you for the article - very useful! I'm looking to build up an '87 Bridgestone MB-1 with Dura Ace 7401 components. My chainrings are 50-38-28 and freewheel is 12-19. Using your formula I'm coming up with 29T capacity... Does that mean I could get away with using the Dura Ace 7401 short cage derailleur? Thanks for any help!

Mountain Bikes Direct Response
Short cage seems like it will do the job, but there's no harm in a medium cage either, so you're probably sweet on both!

1x10 conveersion

By: on 8 January 2018
Hi Would just like to know what cage size derailleur I would need for this 1x10 conversion 34T front 10 speed 11-42T cassette Many thanks

Mountain Bikes Direct Response
Hey mate, you'll need a long cage for that set up :)

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