I'm confused about 10 speed.
A 10 speed cassette will fit any hub that will take a 9 speed hub yes?
yes, unless it's bodged from something else probably
I guess I mean a "normal" 9 speed hub..
So can I usea 9 speed mech?
no, maybe probably not
Why not?
If we're talking MTB, you can't use a 9 speed mech as the cable pull is different, but yes, the cassette is the same width.
If we're talking road the cassette is actually 1mm narrower than 9 speed (but enough as to not be an issue in the slightest), but 9 and 10 speed mechs are interchangeable.
Mtb, I dont understand about diff cable pull? Can you elaborate?
there is some good info on the 9/10 speed mech compatibility here...
[url= http://www.bikerumor.com/2011/10/19/question-can-i-use-9-speed-derailleurs-with-10-speed-shifters/ ]Bike Rumour[/url]
Sram and Shimano say they are not compatible, but reading between the lines they might be ok.
The mech moves a certain (fixed) amount for every mm of cable pulled at the shifter. This ratio varies. So 9 speed SRAM was 1:1 - ie 1mm of movement at the shifter = 1mm movement at the mech (in simplified terms). 9 speed Shimano was 2:1, it's why you can't mix and match Shimano and SRAM rear mechs.
With 10 speed things have been tweaked, such that they're not cross compatible. A few people pop up and say "x combo works", but they're not designed to be compatible, and I'd suggest you'll never get it working perfectly.