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):

Shimano 

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

SRAM

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 (48)

Medium or Long Cage?

By: on 28 November 2018
Hi, I have a CX bike with 40T front chainring and 11-46T casette on the rear. Planning on replacing my old 105 11 speed road specific RD to SLX m7000. Which cage length should I get?

Mountain Bikes Direct Response
Hi there, The medium cage (GS) derailleur will be all you need to run an 11-46t as it has a capacity of 41t. If your bike currently has road shifters you'll also need to grab a Wolf Tooth Tan Pan Negotiator as the cable pull is different between Shimano's road and MTB groups. The Tan Pan Negotiator amplifies the road shifter cable pull to allow for seamless communication with mountain derailleurs. Without one, you won't be able to access the full range of your cassette. Feel free to send any other questions you have through to us at help@mountainbikesdirect.com.au Cheers, MTBD

1x11 question

By: on 22 November 2018
Hi, I'm confused as you say in one of the other 1x11 comments that for a 11-46 x 36t you only need a medium cage but based on the formula you issued, this would equate to a long cage! 36+ [46-11] 35 = 71 Am I missing something or is my maths just really bad ;) ?

Mountain Bikes Direct Response
Thanks for checking in! The formula is for 9 and 10-speed drivetrains and 11-speed is slightly different, and there isn't really one standard 11 speed formula, because every brand is different now. Also, when taking in to account a single front chainring, the 'difference' is 0, because there's no change in ring size to account for. Basically, a medium Shimano will take up to a 46t, but you need a long SRAM to go to 46t. SRAM only make long cage 11 speed anyway, so you're all good on that front. Cheers, MTB Direct.

Need information

By: on 12 November 2018
I'm planning on shifting to 1x 11 speed 11-46t with 36t chain ring (shimano). What cage should I get? Thanks a lot.

Mountain Bikes Direct Response
Hello, With a 1x 11 speed 11-46t you can use the Shimano Medium Cage. We have SLX and XT options available for sale. Hope this helps. Cheers,

Need advice

By: on 3 November 2018
Hi there, I'm planing to put 22t-32t at front and 11spd 11t-50t at rear. On my hardtail fatbike. Do I also need a 'goat link' extention? Need your advice. Thanks

Mountain Bikes Direct Response
Hi mate. It's not possible to have that kind of range cassette with a 2x system. Even with an extended cage and a goat link, it won't work. Sorry.

Cruiser bike on paved paths

By: on 27 October 2018
Have SRAM 3.0 (long cage?) 50/39 130mm BCD triple and Shimano MF-TZ21 22/11 freewheel. Mostly ride flat and gentle slopes, rarely on trails. Do I need to change something?

Mountain Bikes Direct Response
Hi mate, if it's shifting fine as it is, there's no need to worry.

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