Hi my rear mech has given up the ghost and I'm looking for a new one. Problem is I am running 9spd but upgrading to 10spd for new frame. So what I need to know is will a 10 spd mech work with my existing 9spd set up. I am using shimano gears.
the very short answer - yes.
The rear mech will only move as much as the STI/lever 'tells' it to, so will carry out the duties. Do ensure though (as normal) the H & L stops are set correctly to prevent over movement.
Hope it helps.