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Apologies if this is is obvious or has been answered before.

I'm looking at setting up a touring bike, using road STI levers (Shimano or equivalent, looking at Tiagra 3x9 or possibly the Microshift 3x9) with a MTB derailleur (probably just Deore) and 9 speed cassette, in order to get the gearing right for loaded riding.

Will there be any compatibility issues?

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For Shimano gear If you are going all 9-speed, you're fine using most STI shifters with 9-speed mountain derailleurs.

New 10 spd Dyna-Sys rear derailleurs won't work with 10 spd STI shifters if you're going Shimano. However, I've heard people have had success using a 9-speed Shimano Deore M592 Shadow Rear Derailleur with 10 speed STI shifters.

See thread on MTBR for a bit more info on someone's successful setup with a Shimano 10 speed road/mountain setup

I don't think there are any issues if you're going for a SRAM road shifter / mountain derailleur combination if you're going 10 speed and using the XX mountain group which has the same pull ratio as the road groups. It appears that 9 speed mountain gear and some of the other 10 speed mountain group sets, have a slightly different pull ratio and don't play well with road shifters.

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Relevant forum post on Sram road shifter/mtn derailleur compatibility: – WTHarper Sep 1 '12 at 2:22
Relevant blog post about SRAM XX mountain group… – Benzo Sep 1 '12 at 10:04
Interesting! I don't think that their setup will kill a triple (especially for loaded touring), but it certainly eliminates some of the kinks. – WTHarper Sep 1 '12 at 13:31

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