Yep. There were several earlier posts saying things like "just avoid the big/big combination and it will be all right," and that sounded to me like folks were recommending just sizing the chain a bit short for big/big to avoid excessive slack in the small ring/smaller cogs. That would be bad advice, but I don't think that was what was meant by the posters after all.

The chain shouldn't necessarily be any longer or shorter no matter what the cage length, it HAS to be long enough to cover big/big or you're inviting disaster. No, you shouldn't ever shift to big/big, but that doesn't mean a momentary brain lapse can't allow it to happen, and the chain had better be long enough to do it, believe me.

With a long cage derailleur, you could even use a different method to size the chain, using the small/small combination to size the chain as LONG as possible for the derailleur's max chain wrap. Doing it this way, you could, for example, switch from a 30t largest cog to a 34t largest cog without lengthening the chain, because it would already be long enough. But that method assumes your drivetrain is within the limits of the derailleurs chain wrap capacity; with a medium cage derailleur and a mountain bike triple, you can't do it that way because the drivetrain is beyond the derailleur's capacity, the chain will be too short for big/big. You have to use the "big/big plus one inch" method for chain sizing with the medium cage and a triple, and realize that in the small chainring and smallest cogs the derailleur won't take up all the slack. Not nearly as big a problem as a chain that's too short, but still not ideal. That's why they make long cage derailleurs.

Sheldon, as usual, wrote a nice set of instructions explaining the big/big plus one inch method for sizing a chain, with excellent photos. He also mentions the importance of the chain being at least long enough to safely cover big/big.

http://www.sheldonbrown.com/derailer...ent.html#chain