i have 8 sp shimano and need a new rear der. i may want to go 9 sp later tho. can i get a 9 sp XT rear der. and have it work with my 8 sp shifters?

i have 8 sp shimano and need a new rear der. i may want to go 9 sp later tho. can i get a 9 sp XT rear der. and have it work with my 8 sp shifters?

The rear derailleurs are dumb (they only move as much as the shifters tell them to), so you can use a 9 speed on an otherwise 8 speed drivetrain without a problem.

An 8 speed cog set is the same width as a 9 speed. So an 8 spd. shifter on an 8 spd cog set with a 9 spd der. should work. You can't however, use an 8 spd shifter with a 9 spd cog set, as the shifter moves the der too far and thus won't line up with the teeth on the cog set correctly. Also a 9 spd shifter won't work with an 8 spd cog set for the same reason, too little movement across the distance.