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If I went 1x 10, is there any reason why my 9 speed rear mech wouldn't work in the new set up as it's movement is controlled by the shifter?
Doesn't work, the cable pull is different so (for mtb parts) there's no direct compatability unfortunately. You can run a 9-speed Shimano rear mtb mech with 10-speed Shimano road front, though, so I guess you could get a flatbar road shifter? But that gets silly.
There's another way to do it- you could, if you've got SRAM shifters, go 1x9 with a 10-speed rear Shimano mech and a wee spacer to tweak the cable ratio, to get the clutch. But takes away the good 11-36 cassette options so it's not ideal.
For the same reason that you can't mix shimano and sram kit (except with mods as above), you can't mix mtb 9 and 10.
Ho hum, I'll stick with 2x9 then, stuff 'em