I have been building my own bikes for a while and never put much thought into what length cage rear derailleur i am using.  I usually just get what ever is being sold cheap.  Well i shredded mine on Tuesday, so it is time to get me a new one.

My question is what size cage should i get?    I have read more information on this than i thought i ever would, and am still confused.  I am torn between the short and the mid cage.   

I know the formula,  (Big ring - little ring) + (big ring on cassette - little ring on cassette) = XXt   

then you look for the rear derailleur that is in that range.    Well if i do that math i come up with

32 (single ring up front) + 25 (11-36 10 speed in the rear) = 57t.

Now SRAM claims their rear derailleur ranges are as follows
Long 43T
Med 37T
Short 30T


By that chart i would not even be able to run a Long cage.  I have been running 1x10 for a few years now, and am pretty sure that i had a M cage rear derailleur.  It is however shredded now, and the whole lower cage is destroyed and part of it is out in the woods somewhere (i could not find it).


Sooo anyone know what i am going to need?

I think your formula is wrong. 

Here's how I understand it:

32 + 36 = 68 ("big ring" plus big cog on cassette)

32 + 11 = 43 ( "little ring" plus little cog on cassette)

68 - 43 = 25. so your tooth capacity is 25 = short cage. 

Here's an example of a 3x setup

42 + 36 = 78 (big ring plus big cog on cassette)

24 + 11 = 35  (little ring plus little cog on cassette)

78 - 35 = 43 = Long cage

actually your formula is correct, but in your c ase, big ring minus little ring = 0. so the # of teeth in your cassette is all the range you need = 25.

kawsakimx6 wrote on 08/02/13 at 1:14pm:


Emily Batty?

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drdirt55 wrote on 08/02/13 at 4:30pm:


Emily is the current Canadian mtn bike champion.
She races on the Trek factory mtn bike team.

Banjopickin wrote on 08/03/13 at 1:57am:

Yup. Emily is awesome.