Seeing as a branch kindly wrenched my rear derailleur off I need to buy a new one. Excuse my dunderheadedness but what is the difference with the cage sizes ie Large/Med or Small. Is it preference/frame size or some other wonderful calculation that I dont know about?
It depends on gear ratios. A road bike geta away with a short cage becuse the rear cassette might have 11-21, whereas MTB runs 11-32 or 11-34, needing a longer leverage to allow the chain be long enough. Add a 22/32/44 to the front, and the chain has to be even longer. So normally MTB has long cage, unless you only have a single ring up front. DH riders sometimes use a road cassette, and single ring, so get away with short cage.
Obviously, short cage is less likely to get damaged.
Hope this helps.
It doesn't really matter what you got going on at the back.. as in ratios.. Its more to do with the front end of the bike.. as in what kind of chainset your running.. ie 3x9, 2x9 or 1x9...
If your running 3x9 then a long cage mech is recommended.. you'll need it to take up the slack in the chain.. you'll need a long chain to run in the big ring and big gears at the back at the same time..
If your running 2x9 or 1x9 then a small or medium cage will do.. you won't need such a long chain for this kind of set up.. most road bike run 2x9..
But.... you can run a short or medium cage with a 3x9 if you want.. you will have to be careful about your gear choice though.. if you run it in big big then you may possibly rip the derailler off the frame.. if you run it in small small then there will be too much slack on the chain.. it will flop around all over the place & may catch in your spokes or get dragged across the ground and the likes..
At the moment I'm running a 2x9 setup with an 11-34 rear cassette, I'm using a super short cage XO mech and have no issues with it whatsoever..
If your unsure what size to choose then just run a long cage mech and you can't go wrong..