Hmmm, we don't run 24" rims. 24" rims are used on kids' bikes in the mountain biking world. Our wheels are typically 26" or 29" with some 650B thrown in there. I suppose some rims are made available in 24" sizes but I'm not sure if all rims are available in 24".

Also, the XC weight weenies might aim for light wheels but you have to understand that a mountain bike's wheels will generally be heavier than a road bike's wheels by a huge margin as in mountain biking we're looking for a little more robustness. Do you ride your wheelchair in downhill or are you only rolling on the road?

I believe the recumbent guys (they have a sub-forum) and the folding bike guys (under commuter forum) run smaller wheels down to 20" so you can ask them.

Sorry I can't be of much help but I'm not sure what kind of guidance you're looking for from mountain bikers unless you're riding one of these!