Originally Posted by
Darth_Firebolt
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.
First, a long and medium cage mtb derailleur both have the same largest cog compatibility. The only difference is cage length, which affects chain wrap capacity, not largest cog compatibility.
Second, it's irresponsible to advise someone to use a chain that's too short to safely shift to the largest cog/largest chainring combination. It is indeed moronic to run a chain that's too short to do this. One brain lapse and you'll likely snap your deraiiler, your hanger, your chain, maybe damage your rear wheel, and maybe even your frame. Risking this for what reason? The difference between using a medium and long cage deraiileur?
Bottom line: if you have a mountain bike with a triple crank, use a long cage deraiileur. That's why they make them. And make damned sure your chain is long enough to safely cover the largest cog/largest chainring, no matter what.