Sorry Dyna, I may have misinterpreted your initial post and assumed that you were going to run a 10 speed shifter and cassette with a 9 speed rear mech (which would work because the shifter determines how far the mech moves).
Using a 10 speed shifter as a 9 speed one won't work because the mech movement for one click won't be enough because the gap between each jump on the cassette is smaller on 10 speed than 9 speed.
OK ,to clarify
I have a 9spd pair of X.9 trigger shifters along with a 9spd XO long cage rear mech and an X.9 front mech.
Ive bought the XX 10spd shifters and WILL change to a 10spd cassette. The question is will the 9spd XO rear mech work for 10spd
Im more than prepared to also buy an XX rear mech but i really dont want to because they cost the friggin earth.
For it to work and because the distances are the same the 9spd mech would have to effectively overshift
Cool. I think it may just work...I've certainly used 8 speed mechs on 9 speed blocks.
If my understanding is right, a 10 speed and a 9 speed block are the same overall width the only thing different is the spacing of the cogs so the mech moves the same overall distance. The shifter determines how far the mech moves in one shift. All Sram stuff works on a 1:1 actuation...there website describes Exact Actuation as 1:1.
Therefore you should get away with it. I'd try it and then worry about a 10 speed mech if it doesn't.
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