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I'm not aware of a Shimano 9sp rear mech with a clutch
There isn't one.
- I think that came in with 10 speed - but my knowledge of Shimano derailleurs is far from exhaustive so I may be wrong.
They released ten speed as shadow - a lower profile, and shadow +, low profile with the clutch. As you say, neither are compatible with road shifters - they use the 'Dyna-Sys' pull ratio, whatever that means.
The only advantage I can think of for using a 9sp MTB mech is that they have a longer cage so you can run a bigger cassette.
As you know, only if you need the extra cage to pick up the chain slack. At Howard towers we ran 1x10 with 11-32 on short cage mechs - cage length does not dictate maximum tooth capacity. I think I run an 11-36 on a short cage Zee mech on the jokes long travel bike.
I'm really surprised by how shite the options are for running 1x10 shimano beyond MTBs.
- I think that came in with 10 speed - but my knowledge of Shimano derailleurs is far from exhaustive so I may be wrong.
Howard-
I think I run an 11-36 on a short cage Zee mech on the jokes long travel bike.
Seems likely, I have a short cage X7 on my SRAM 1×10 (12-36). It's only 24T total capacity, well within the capability of a short cage. Shimano road RDs can usually be pushed to about 30T maximum sprocket, so you only actually need to contemplate MTB RDs if you're going bigger than that.
gbj_tester
I'm not aware of a Shimano 9sp rear mech with a clutch - I think that came in with 10 speed - but my knowledge of Shimano derailleurs is far from exhaustive so I may be wrong. The only advantage I can think of for using a 9sp MTB mech is that they have a longer cage so you can run a bigger cassette. That's why I used one - I wanted to be able to run an 11-34 cassette with a 30-39-53 triple at the front, and needed the long cage to do so.