Commuting - road derailleur on a mountain 11-34 casstte?

got a tiagra derailleur laying around and i need a derialleur for my 1x9 project. was wondering if it would work. the largest cog the tiagra was designed for was 27t but the mtn cassette had a 34 big cog. i am not worried about the cage capacity because it is a 1x9 drive train. should i just try it?

I replaced the 12-25 on my cross check (Tiagra RD) with a 11-32, and it works just fine. If I remember right, the guy at the LBS seemed to think 11-34 would be too big, but they didn't have one lying around to try, so I'm not sure.

Edit: looking at the tech documents, you are right that the small sprocket on the list is 11T, and max 27T. But the total capacity is 37 teeth on the long cage and 31 on the short cage. I've got the long cage. For my bike thats 12 (48 - 36, teeth range on the front double) plus 21 (32 - 11), or 33, well below the total capacity. If I had a 24 tooth granny gear for a triple front, I would exceed the total capacity though. If I had a 11-34 cassette, I'd still be okay probably.

Not a bike mechanic, so not sure if this is 100% accurate. But the answer apparently is: depends on the teeth difference in your crank and if you have the short cage or medium/long cage version.

Go ahead and try it, but don't expect it to work. I have a muttley road bike with a 42/53 Campy crank and 8spd Shimano 600 shifters. The cranks are swanky but everything here is steep, so I scored a ghetto Nashbar 8spd mtn cassette. I think it's a 12/32, maybe only a 30 but it looks slightly ridiculous. I bought a 105 derailleur and tuned everything the best I could, but the upper pulley rides against the biggest cog, and nearly in the second-biggest cog. What I ended up doing, and I would NOT recommend this, is shorten the chain a few links. This keeps the interference to a minimum because the cage is stretched enough in the lowest cog that the upper pulley gets pulled down a bit. Of course, if I were to put it in the biggest cog/big ring combo, it would result in bloody carnage as the drivetrain would instantly seize.

I just reserve the lowest gear for 20% and up climbs and it otherwise works great. After I put together this low-budget experiment, I learned that mtn derailleurs follow a different path that follows wide-range cassette profiles better. I wish I woulda known this and bought a Deore derailleur instead.

You never know, though, it might just work. As for me, I'm on the constant lookout for a screaming deal on a mtn derailleur.

When I swapped my Cross Check to an 11-32, it was a tight fit. You can give it a shot, but I think a 34t cog might bump the upper pulley on the Tiagra rd.
I recently swapped out to a Deore rd and I'm much happier with the pulley clearance.