I recently bought a second hand ride with some of these rims fitted.

I like riding 1.8″ Panaracer Fire XC Pros at this time of year when things start getting muddy. These rims seem to be designed for fat rubber. What are the potential side effects of running 1.8″ (if any!).

Cheers

Side effects will be a fairly square tyre profile.

1.8’s will be ok, they’ll look slightly odd, but work fine. I ran a 1.9″ tyre on a 721 on the rear of one of my bikes for about a year, it was fine.

I’d worry more going the other way, fitting anything larger than about a 2.2″ on something like a 717 personally, as that would squirm a lot.

Mavic recommend running Tyres between 2.1-3.0 on their EX721’s. Have a look see here-

http://www.tech-mavic.com/tech-mavic/technical_manual/data/docs/products/2_102.pdf