I’m building err, something 😉 , and I’m now thinking about the gearing. I’d like 3 x something (ok, maybe 2x something) – either way it’ll need to sit on a (normal everyday) XT M756 hub, and it’ll all be tugged about by bar-end shifters, but other than that it’s kinda all up for grabs

The starting point is a couple of assumptions …

I’m assuming that the Shimano 10spd and 9spd cassettes have the same overall carrier width (so the 10spd block will sit on my hub fine), but just have different cog spacing, so I could use my current 9spd bar-end shifters on friction-mode with a 10spd cassette, the relevant mechs, M780 10spd chaninset and 10spd chain.

I’m also assuming that road 10spd and MTB 10spd is the same spacing, so if I really, really miss indexed shifting, I can just chuck on a set of Dura-Ace 7900 10spd bar-end shifters down the line.

… but, are those assumptions right?
😕

I’m pretty sure that 10 speed shimano mtb and road are slightly different. If I remember correctly 9 speed mtb has the same cable pull as 10 speed road, so you won’t be able to use your dura-ace shifters.

Sram 10 speed is the same, but shimano 10 speed is different.

Edit: http://forums.mtbr.com/drivetrain-shifters-derailleurs-cranks/dyna-sys-10-speed-compatability-9-speed-shimano-systems-646014.html

I believe you are correct, but you will need a road 10 speed rear mech if you end up going with 10spd DA 7900 shifters as the road 10 spd mech cable pull is different to the MTB 10 spd cable pull.

If you’re running 10-speed STIs and a 10 speed MTB cassette, you need a 9 speed MTB rear mech.