Run the medium cage. Don't cross over from your big ring to your big cog. You'll be fine.

Mountain bikes with two (or even one) chainrings, which have become more popular in recent years. These drivetrains don't have nearly as much chain wrap requirement as drivetrains with triples. Long cage is for triples.

The first medium cage derailleur I recall is the 98 XTR. That was years before anyone started running two rings and a bash. I rode that 98 medium cage with 3 front rings for a few seasons. Still have it, still works fine, nothing exploded, world has not ended, &c. Sometimes empirical data trumps the owners manual.

I run a medium...I have a big ring...4 years...no problems...

I assume by recent years mentioned above you mean the last 7 or 8 or so. haha....at least out here, most people do not ride 3 rings unless they race. I only tried it well...to try it. I will be switching back to a bashgaurd soon

You guys that are advocating a medium cage derailleur (with a triple) seem to be saying the OP should purposefully run his chain a bit short for the shift to large cog/large chainring to keep from having chain slack in the small ring/smaller cogs. Correct? No? Or are you saying he should size the chain long enough for large/large and make sure and avoid small ring/smaller cogs so that excessive chain slack isn't a problem?

on my xc I run my chain long enough to go big ring big ring...

on my DH...with a medium ring, I run as tight as freaking possible...big ring middle ring. Chain slap sucks at high speeds...I commonly (when I am tired) through the big into big ring middle ring to keep the deraileur quiet while I am limping off of a mountain

I'd run it barely long enough to make the big/big combo. By barely I mean at the maximum capacity of the derailleur, jockey wheels close to horizontal.

OP- since you already have the medium, this is what you should do^^^^.

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.

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

Well, I guess it doesn't matter any more. As stated I ordered the medium cage. The seller sent me a long cage. This is a rare moment when a sellers mistake worked to my benifit.
Thanks to all that responded..............

That's a classic.

You should email them back, tell them you're pissed, but are willing to keep the derailleur and avoid having them pay for return x2 shipping costs if they credit you a small amount