Is it fine to use road bike rims for a mtb? or is that just asking for trouble?

You’ll probably get your head kicked in by the trail crew.

is that just asking for trouble?

if you’re prepared to run road width/cross bike tyres then it’ll be fine.

If you want ‘proper’ mtb tyres then a wider rim is required.

Depends on the rim, your weight, and your bike handling finesse.

i used the front wheel off my roadie at strathpuffer as it was easier than taking the tire off the rim and mounting it on a stans arch.

road bike wheel was heavier than my stans arch 29er anyhoo

My Open Sports feel pretty dang sturdy. And they’re only about 40g lighter than a 29er 719.

width is an issue.

Your tires won’t fit 😛

Seriously, I ride Open pros on my 29er (rigid) no issues so far.

It’s a 29er, i think 2.1 width. 15 stone. no bike handling finesse. just dont want to be going back again in a month for yet another wheel build

15 stone, no handling finesse probably fine. Combine this with Rocks, Drops and the odd jump. Personally I would be getting some proper MTB rims – DT470’s, Mavics, Stans Arch or Flow (probably not Crests).

Stainless spokes and brass nipples.

If you need to save that much weight – there are some other ways that I am sure others may be happy to volunteer.

I’d be worried about tyres that wide rolling off the rims unless pumped up really hard too.

some of the first decent MTB rims was a re rolled MA40s.

you’ll be fine unless you want to run 3″ tyres