Bicycle Mechanics - Using 9spd RD with 8spd cassette?

I imagine this should work fine -- can anyone comment on this? It would seem that simply adjusting the limit screws should prevent the RD from going where it shouldn't -- i.e. not trying to shift onto that 9th sprocket. Am I missing anything?

8 or 9 it doesn't matter. An 8 speeed cassette is the same width as a 9 its the shifter that makes it 8 or 9 not the derailieur

The rear derailler is "dumb". It only moves based on the shifter. So long as your limit screws are adjusted you're cool.

The rear derailler is "dumb". It only moves based on the shifter. So long as your limit screws are adjusted you're cool.No really in index systems.. The RD has to be compatible with the shifter.That's way you can't generally mix campy and shimano.

Guess I'm thinking specifically about shimano 8 to 9 speed. I'm no expert, but I've had no problem hooking up an 8 speed rear to 9 speed shifters.

Guess I'm thinking specifically about shimano 8 to 9 speed. I'm no expert, but I've had no problem hooking up an 8 speed rear to 9 speed shifters.Even with Shimano there is incomaptibility with pre 9 speed DA RDs. ...Point being, in index systems, the RD is NOT dumb.

I can confirm that an '8 speed' RD works fine with a 9 speed cassette - I just upgraded myself and everything's fine.

I've got a related question though. I've just upgraded from the square splined '8 speed' XT crank and chainrings to the new octalink XT crank.

It looks like the largest chain ring is a bit bigger than the one I've just taken off - should I still be able to use my existing FD???

I imagine this should work fine -- can anyone comment on this? It would seem that simply adjusting the limit screws should prevent the RD from going where it shouldn't -- i.e. not trying to shift onto that 9th sprocket. Am I missing anything?

If your shifters are Dura-ace 8-speed, you will need to use an alternate cable attachment at the rear derailer to get it to work. No problems if your shifters are any other Shimano 8-speed. See http://www.sheldonbrown.com/dura-ace.html#indexing

Just to add a little explanation to sydney's comment that indexed rear deraillers are not dumb - for smooth shifting with an indexed system, the rear derailleur has to move the correct distance sideways (the spacing of the rear cogs) on each "click" of the shifter. The distance it move is determined by BOTH the amount of cable moved by each click (determined by the mechanism in the shift/brake lever) and the geometry of the rear derailleur which translates that cable pull into a sideways motion.
For example, Campagnolo changed the design of the parallelogram on their rear derailleurs in 2001 making pre-2001 9 speed shifters and deraillers incompatible with later model 9 speed shifters.

For example, Campagnolo changed the design of the parallelogram on their rear derailleurs in 2001 making pre-2001 9 speed shifters and deraillers incompatible with later model 9 speed shifters.That's 'what Campy sez', but in real life it just ain't true.