Can you use 9 spd cassettes on a road bike?
Looking to do some big hills soon and my road gearing wont do at the moment.
Any advice?

if youre matching the same number of gears then yes, as long as your mech cage is long enough for the tooth spread.

If you have 9-speed Shimano shifters, then a 9-speed MTB cassette will work using a 9-speed MTB rear mech.

If it’s 10-speed shifters, then use a 10-speed MTB cassette and a 9-speed MTB mech.

ok its all 9 speed
I only want to run the mtb cassette for 1 race
so can I use existing road mech and shifter with mtb cassette as long as cage is long enough on mech?

Yes.

So long as the rear mech has the capacity to cope it won’t be an issue.

Also check the chain length

also worth buying a new chain and keeping it with the big cassette.. for each time you switch to it.

remember mech have two capacity issues, one is taking up chain slack and the other is maximum sprocket size they can deal with.

So long as the rear mech has the capacity to cope it won’t be an issue.

It’s not (just) about the capacity of the mech, it’s also about the parallelogram angle. A long cage road mech might have enough capacity, but it has the same angle as a short cage road mech, which is unlikely to be enough to clear a 32T large sprocket – for that you’ll need an MTB mech. This is one of those “it depends” things though, as depending on frame design a road mech might work with a 32 – some have reported getting this working OK if they screw in the B-screw enough – however on my crosser it didn’t. A 34T sprocket almost certainly won’t work with a road mech.

will be fine I have one on my Turbo wheeel as i had one lying around.
I changed nothing except the wheel and it works in all gears [ well I have never tried the granny tbh]

I run a 9 speed mtb cassette and mtb mech on road bike. No issues apart from piss taking.

I switched my Roubaix to use a 11:32 on the back with a 9-speed LX MTB mech and it works fine with the 9-speed Tiagra shifters. I initially tried with the original road rear mech but while it worked fine with either an 11:25 or 11:28 cassette it couldn’t cope with the 32.

Pretty sure in the mountains on the TdF this year some of the riders switched to MTB mechs and cassettes so should be fine.

Cheers guys- all for 1 road climb tt!! Nothing is ever easy! 🙂

I have an ultegra 9 speed mech running on a 11-32 cassette, I replaced the b tension screw with a slightly longer one and wound it right in, works fine