Originally Posted by
dminor
I semi-disagree. As long as you don't seriously cross-chain (and really, how many people use the big ring on an MTB unless they're in an almost-spun-out situation anyway?), you should do fine with a medium cage.
Example: a medium cage derailleur with a capacity of 38. Front difference of a 44-32-22 is 22; rear difference of a 12-32 (typical 8-speed) is 20. Sum is 42 - - not far enough off 38 (IMO) to where you can't easily fudge it if you don't do silly things.
I disagree with your semi-disagreement. Number one rule of chain sizing is make sure the chain is long enough to safely shift to the largest cog/largest chainring. With that in mind, with a triple crank and medium cage derailleur, when in the smallest chainring there will be a few gear combinations (the smallest few cogs on the cassette) where the chain is too slack to function correctly. Should you severely cross chain? No, of course not; not in the big ring, not in the small ring. Do people do these "silly things?" Based on my experience working in my bike shop: yes, very often. The simple, straightforward answer to the OP's question is to get a long cage derailleur. His type of drivetrain is what they are designed for. A medium cage is not.