Originally Posted by
well biked
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.
i disagree with your disagreement of dminor's semi-disagreement.
he is here asking the question (and building up his own bike from the sound of it), so he is obviously not one of those morans* that runs big cog + big ring. he has been warned that he won't be able to run that gear, so i think he can run the medium cage if he wants to. he's a big boy. if the "no big-big for less chain slap" compromise is one he is willing to make, let him. it seems like less chain slap is what he is after, so he is probably willing to give up that moranic gear combination to get it.
the real question is: what if he wants to go to a 34t rear cassette? then he would have to change to a long cage. that by itself would make me go with a long cage.