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 (41)

Good day

By: on 11 September 2018
im running a 2x10 hardtail XC with a 100mm travel fork.. i have a 39t-26t front and 11-36 cogs and currently on long cage RD but planning to put a medium cage RD.. i need to know if is possible? any suggestions?

Mountain Bikes Direct Response
Hey mate, If you are still going to run a 2x setup then you would need a long cage with those ratios. A medium cage would just be out of range for most rear derailleurs. Hope this helps,

Cage length

By: on 11 September 2018
Hi there, I need to replace a busted SRAM X7 rear mech on an all-mountain bike with the following: 1/9 drivechain (converted from 18 sp), 34t chainring and 11/34 cassette. What cage length would you recommend? Thanks

Mountain Bikes Direct Response
Hi Andrew, Thanks for checking in! While technically you fall under the 'short' cage limit, I would go with a medium cage. Cheers, MTB Direct.

SRAM xo1 7 speed DH x horizon

By: on 8 September 2018
Hey Guys, I am building a new downhill bike. I have a 36 tooth chain ring and a SRAM xg 795 xo1 DH 7 speed 10-24 cassette. Looking to buy the SRAM xo1 7 speed DH X Horizon derailleur. It comes, in a medium or short length. Which should I buy? Thanks! Craig

Mountain Bikes Direct Response
For a 10-24 you definitely only need a small cage. Small is better for DH as well so you have less chance of hitting it.

SRAM GX rear mech - largest cassette without drama

By: on 2 September 2018
Hi. I have a SRAM X5 rear mech (not sure which length) and chainrings/cassette as follows: Front - 22/34 and Rear - 11-36t. It's a 10 speed. I am building a new wheelset and want to put another cassette onto the rear wheel rather than buy a 2nd one of what is on the existing wheel. Using the formula ((34-22) + 36-11)) I get 37. Could I go with a Sunrace 11-40t (CSMX3 ? Or will this cause issues if it's a medium cage that I have? I've been to the SRAM webpage and downloaded the compatibility map but trying to nut that out gives me a headache... Cheers for any help you can offer.

Mountain Bikes Direct Response
Hi mate, with a two front chainrings, and a wide range cassette, you'll need a long cage derailleur to be able to cope with that. Cheers

42 tooth front ring

By: on 30 August 2018
Hi, length of chain needed . for Shimano slx Super long cage, 10 spd front 46 tooth. rear 11X42.

Mountain Bikes Direct Response
Hi Thomas, Thanks for checking in. Unfortunately, there's no easy way to determine chain length without actually sizing a chain on the bike. This is due to different factors such as chainstay length which is unique to different models. There are a few popular methods and I would recommend checking out Park Tool's website. They an in depth write up with pictures on each method. Cheers, MTB Direct.

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