Shimano Tech doc says, that a medium cage can take 34T in the back. Since this is the max (9s 970XTR) I dont understand why I should go longcage. I ride a standard XTR 970 44/32/22 setup. I have always used longcage RD, but a salesguy claims, that I can just use medium. Is he right?

You need to match the total capacity as well, check the tech docs.

(capacity=(largest front-smallest front)+(largest rear-smallest rear) so for a typical 42/32/22 with 11-34 you need a capacity of (42-22)+(34-11)=43

Rule of thumb (only) single chainring - short, double - medium and triple - long.

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Impoverished weight weenie wanna-be!
Budget 26" HT build viewtopic.php?f=10&t=110956

Perfect explanation! None of the other guys I asked understood the Tech Doc correctly

I have a medium on a bike with an eccentric bottom bracket so I can optimize the chainstay effective length it has 22-32-44 front and 12-34 rear. Adjusting so that it barely makes the shift to big/big when in small/small the chain is rubbing on the chain on the top/forward roller of the rear derraileur so I never use small/small. It will not work very well at all since if you don't have an adjustable bottom bracket, you will likely have an even worse situation than I do. Getting enough chain to make the big/big will probably leave your chain rubbing in at least the small/small and small/2nd smallest and maybe up to 3rd smallest.

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For certain parts stiffer is more important than lighter.

No idea how you think having an EBB effects total chain growth from small/small to big/big.....however yes if you don't mind the chain being slackish in small/small (which you shouldn't really use anyway) then on some triples a medium is fine, I would've use it with the wider spacer Touring chainsets though!

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Impoverished weight weenie wanna-be!
Budget 26" HT build viewtopic.php?f=10&t=110956

Simple, if you can't move the bottom bracket to get the chain on big/big you have to go in discrete increments of 2 inches since you can only add 2 links at a time. With eccentric bb on a non-single speed bike (Sir9 set up geared) you can cut the chain to a usable length and then rotate the BB to get it so that it will just shift onto big/big. At that point you have the minimum amount of extra chain when you are on small small. Without the eccentric BB you are liable to have up to 2 inches more chain when you are on small/small since you can't optimize the effective chainstay length on big/big.

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For certain parts stiffer is more important than lighter.

A pair of links is 1" not 2" , 1/2" on wheelbase as it covers two runs of course.....

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Impoverished weight weenie wanna-be!
Budget 26" HT build viewtopic.php?f=10&t=110956

exactly, total capacity is the key.

I use a middle cage xtr rear der. and 44-32-22 front... but i use 12-27 cassette (9speed), so middle is fine for this. 11-34 + triple; long cage is the best option.