In terms of measurements are the rims any different?

I know generally a road 700c rim is flimsy and narrow in comparison to a 29er rim.

But in theory could you take a wide cyclo cross tyre and put it on your 29er?

“I know generally a road 700c rim is flimsy and narrow in comparison to a 29er rim”

Say what now?

They are the same

Yes, they are both the same diameter.
More useful to check the ETRTO size – both are 622mm.

A road rim is just narrower; it’s the MTB rim that is usually “flimsier”, on account of it not having to take extreme tyre pressures or perform braking duties.

Road rims are probably stiffer and stronger due to weighing quite a bit more

Brill, thanks. I didnt realise road rims were actually stronger. Learn something every day!

With 29ers now firmly on the dh circuit it’s becoming quite popular for riders to use a road rim, for the reasons stated above. They’ve usually been re-stickered though so you couldn’t tell at a glance.