Will a 9sp rear mech work with 10sp shifter and cassette?

I am talking of using a 10 speed road shifter with a 10 speed chain and cassette(MTB) but an MTB rear mech long cage to temporarily get some lower ratios on my road bike for a very hilly ride.

Works with Shimano – I run 10 speed Ultegra STI levers with a 10 speed cassette using an XT 9 speed mech.

I’d have thought you need a 10 speed rear mech, cassette and shifter.

Not with road levers and MTB mech – different pull ratio or something.

I will give it a try only my time to be lost thanks!

Go for it. It’s counter-intuitive, but trust me, it does work (for Shimano anyway, don’t know about SRAM)

Road 9/10 speed stuff is interchangeable, MTB is not.

So for the scenario you pose you need a 9 speed MTB mech.

Mech’s are interchangable despite what the manufacturers want you to believe. there is no indexing – thats in the shifters- and the pull ratio is the same- so all that is required is for the mech to cover the required width of the rear cassette which hasn’t changed since 8sp (maybe even 7sp ??).

Maybe slight differences in width of jockey wheels – but negligable between 9 and 10.

and the pull ratio is the same

which hasn’t changed since 8sp

not true. the pull ratio has changed

Njee is correct.

Road levers pull slightly less cable than mtb levers, so a 10 speed mtb mech slightly undershifts.
10x cassette + STI road levers + 10x mtb mech doesn’t work – I know I’ve tried it.
10x cassette + STI road levers + 9x mtb mech does work – I know I’ve tried it.

9 speed XT on 10-speed 105 here. Shifts fine but the mech seems to stick over wheel-wise a very long way, in the 28T ring it can touch the spokes. Not sure if that’s a mech thing, a wheel thing or a frame thing.

if its 10 speed Dyna (basically 10 speed MTB) then you need Dyna shifters and mech.
10 speed road is not Dyna so will work with 9 speed.
10 speed Dyna MTB rear mechs will not index with non Dyna 10 road or 9 speed rear mechs. it will work but you will only get above 6 or 7 speed.

North wind – haveyou adjusted the B screw in to push the mech further downa nd away from the spokes.

Yah, it’s already quite a lot further down than the recommended position. Need to take a better look at it really.

“
The Re-designed 10 speed Shimano 105 rear derailleur takes the basic architecture of the Ultegra and Dura-Ace models and makes it significantly more affordable without sacrificing much performance. The Advanced Wide Link design increases rigidity and shifting performance, which along with the super stiff construction allows fast and accurate changes across the cassette. The super light shift action is complemented by large 11 tooth pulleys which run on high quality bushings to offer reduced noise and extended service life.

The Shimano 105 rear mech is compatible with 8, 9, and 10-speed drivetrains and Shimano shifters with a 2:1 pull ratio.
”

How can the above be true if the pull ratios have changed?
My guess is the pull ratios of the shifters have changed

How can the above be true if the pull ratios have changed?

They’ve only changed for 10 speed mtb.

As plenty of people have said.

Listen to njee, he knows his stuff.