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 (15)

Using a shimano mtb derailleur on a road bike

By: on 21 October 2017
For health reasons I now need to have very low gears. Is it possible to run a 50 -34 compact with a 11-42 cassette using ultegra with a shimano mtb rear derailleur? If so what derailleur do you recommend?

Mountain Bikes Direct Response
<p>Hey mate!</p> <p>&nbsp;</p> <p>Mountain derailleurs and road shifters don't usually blend together, however, Wolftooth have a fantastic adapter device that allows this to work.&nbsp;https://www.mountainbikesdirect.com.au/wolf-tooth-tanpan-negotiator<br /> <br /> I personally would recommend an&nbsp;<a href="https://www.mountainbikesdirect.com.au/shimano-xt-rd-m8000-11sp-rear-derailleur">XT</a>&nbsp;derailleur, with XT&nbsp;<a href="https://www.mountainbikesdirect.com.au/shimano-xt-ultegra-cn-701-11-speed-chain">chain</a>&nbsp;and SLX <a href="https://www.mountainbikesdirect.com.au/shimano-slx-cs-m7000-11-speed-cassette">cassette</a>.</p> <p>&nbsp;</p> <p>Hope this helps.</p>

Sram x9

By: on 3 October 2017
So I have a 36t chainring up front and an 11-34 in the rear what size derailleur will I need to run a x9 derailleur?

Mountain Bikes Direct Response
<p>For an 11-34t cassette, a medium cage will be the one to go for.</p>

Could you please verify my selection ?

By: on 13 August 2017
Hi there ! First of all I have to tell you THANK YOU for the explanation about short, medium or long Derailleurs!! So useful ! I have a Scott full suspension bike with 150mm travel and I want to be sure I'm selecting the correct Derailleurs if buy the fallowing combination: New - XT M8000 Cassette 11-46T - Chain HG701 11 SPD - Race Face Single Narrow Wide Chainring 32 Tooth - XT RD-M8000 11SP Rear Derailleur SGS (long) ? Have an excellent ride day!!!!

Mountain Bikes Direct Response
Hi there! The above formula doesn't quite work with the new 11-speed Shimano line. From their site, the GS (medium cage) is compatible with the 11-42T cassette when used on a 2x setup. The GS cage will also work with an 11-46T cassette when used as a 1x.

Changing 11-46t cogs

By: on 9 August 2017
I'm thinking of changing my xt m8000 11-40t cogs with 11-46t cogs and using 34t nw chainring. Does medium cage derailleur work for this set up? Thank you

Mountain Bikes Direct Response
Hi there! Going off the formula above to work this out for a 1x setup, you don't need to worry about the chainring part of the equation so let's have a look at the numbers to work out what you'd need to run an 11-46t cassette: 46 - 11 = 35. This puts you a couple of teeth above the maximum to run with a medium cage Shimano derailleur so you'd want to go with a long cage. Cheers! Rob

Another X1 cage tech length question...

By: on 1 June 2017
After doing some reading and math homework, I've decided to upgrade my 3 x 9 | 44-32-22 & 11-34 setup, and try a 1 x 11 | 38T & 11-40 setup. Basically I need to choose the correct cage length, and according to your write up I should go with a small cage, however I would like to know how "detrimental" would be if instead I decide to choose a medium cage, in case down the road I decide to swap the cassette to an 11-46 for better climbing. I really would prefer to be open for future changes without incurring into additional expenses, but at the same time would like to predict how "well" will a medium cage perform when a short cage should be the recommended one for this particular setup. Thank you very much, j

Mountain Bikes Direct Response
Hi there! With the new crop of 11 speed derailleurs, there isn't the same "range" of options. For SRAM 1x, there is only one cage size, while Shimano have 2 sizes available -essentially a medium and a long - so you'd want the medium if going for Shimano. Hope this helps! Tim

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