8 & 9 spd mix....

9 speed chains are thinner, but Ive seen what your referring to done...wouldn't recommend it, but it can be done. Total chain capacity is your biggest issue with your rear deraileur.

Quoted post said:

Originally posted by Cyclesmith
9 speed chains are thinner, but Ive seen what your referring to done...wouldn't recommend it, but it can be done. Total chain capacity is your biggest issue with your rear deraileur.

hmmm...thats a fair point. are the spacers between the cogs the same size on both atb and road cassettes?

If your running a standard Shimano HG 9 speed system, then the only difference is the length of the chain and the size of the cogs and chainrings. The chain pin width is 6.5mm for HG, which includes Mtn systems as well as Ultegra and Dura-Ace chains. HG73 (comes in a blue box) and HG93 (comes in a grey box) can be used on both Mtn and Road HG systems. 8 speed pin width is 7.1mm which requires larger spacing between the cogs to fit the thicker chain, but again can be used on both Mtn or Road 8 speed systems.

Quoted post said:

Originally posted by mattmatt
maybe a dumbass question but will a 9 spd rear mech work fine with a 8spd road cassette and 8 spd stx rc shifters??? they use the same freehub for both 8 and 9 spd so the total distance travelled by the mech is the same, so why are the mechs defined as either 8 spd or 9 spd, not both??? I cant get mine to set up correct. surely the only problem wud be if i was using 9 spd shifters with 8 spd cassette n mech???? cheers all

Most likely they will not work together.

9 speed setup uses a narrower chain, and chainring profile. SO putting a 9 speed chain on 8 speed cogs may not work out well. Vice versa as well, i think that an 8 speed chain on a thinner 9 speed cog would be sloppy.

Yes you should be able to get a 9 speed rear derailleur to set up on 9 speed shifters and cassette. What is the problem exactly that you are having?