Will a 9spd front mech work on an 8 speed set up?

I recall once being told it won’t, but the cage looks pretty much the same width.

Any advice?

Short answer: yes.

Long answer: yes, but it might rub a bit.

World-weary answer: yes, about as well as any two pieces of metal designed to hit a chain sideways.

So the cage IS narrower?

Just got a front and rear Tiagra mech in a deal at a good price and was going to sell the front as I didn’t think it would work – should I now use it? I’m running a triple up front on 8 speed. Is there roadie front mechs that don’t do triple?

Front derailers + triple/double = **** minefield.

Just try what you’ve got, it’ll probably work. See the ‘Long answer’ above.

Good luck!