[Closed] will 10 spd sti's work with 9spd mech & cassette?

yes

No, they won't work.

err...

I tbink Steve Austin is replying to the "will I have problems?" question. Yes you will have problems. It won't work...

As crazy legs says...
They wont work...

But to give you a reason why... 9 and 10 speed cassettes are the same depth when sitting on the freehub. So a ten speed shifter moves less in one click than a 9 speed shifter would.
Hence if you had a 10 speed shifter on a 9 speed cassette it may work for some gears (the closer together ones) but as you get up high, around the 20 tooth mark, there wont be enough 'pull' to change the gear.

Also i have tried it, and it doesn't work...

Hope that helps
Josh..

ahh, thanks, was thinking it'd be something like that.

Cassette and shifter must be the same number of speeds, 8/9/10. Mech can be any of the above.
7 speed mech won't work with 8/9/10 speed cassette but 7 speed cassette is fine with 8/9/10 speed mech. Hope that makes sense.

You'll need a 10 speed cassette and chain.

on a similar note, will a 10 speed chainset be ok if run with 9 speed cassette and shifters? or is the chain the wrong width?
Also, if i change my chainset, does it matter what brand i get, e.g. will a campy chainset work with shimano shifters and cassette?

Sorry for hijack

10 speed chainset will run fine with 9 speed and yes a campag chainset will work with shimano mech and shifters

What Blazin said.

My road bike has
10 speed rear shifter (105)
10 speed cassette (Ultegra)
10 speed chain (SRAM 1070)
8/9/10 speed rear hub (105)
9 speed rear mech (XTR shadow)
3 speed front mech (Tiagra)
Double chainset (Tune)
3 speed front shifter (Tiagra)
8 speed granny ring (Sora)
9 speed big ring (Stronglight)
MTB BB (square taper XTR, UN91)

Works fine.