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What length rear derailleur cage do I need for a double up front 40 / 28 and a 36-11 cassette
I found a general rule of thumb that says a double up front needs a medium, yet working out the formula gives a figure of 37 which is more that the 33 the Shimano specify for a medium mech.
Would I get away with a medium or will it get destroyed if I change into the wrong gear?
Sounds like you need a long for that capacity - although with a major caveat that I've no experience of 10sp.
It can save the hassle on the trail - and the embarrasment in a weeks time of posting something like I did after forgetting to check before changing or buying a mech. Also note after this post I realised the mech was also twisted and for the bin. What I'd done was change the wheels (and therefore its cassette to a 11-34) that was past the mech's capacity with the original chainset:
http://singletrackworld.com/forum/topic/wtf-would-make-my-slx-jockey-wheels-look-like-this
If you learn to use your gears then you can get away with a medium. A medium cage just won't take the chain slack up when you're in the 28 front and last few rings on the cassette.
You just need to make sure you change to the 40 up front once you get past the middle of the cassette.
I have done this on mine and it works fine.
I have 26-39 on and 11-36 and med cage (SRAM) which would be OK on hard tail but on full-sus the chain growth means I have to have the chain one pair of links too long to stop the chain snapping in 39/36 which makes the chain loose in 26/11.