Hello there,
I am aware of fragility of ordinary bicycle’s wheels for multi-wheel vehicles, especially if they are 26" as my MTB are. I just have a hope that my HPV-qaudricycle shouldn’t collapse under weight at corners and curves...
I wanted to build it entire my life and almost finished it. It wasn't as I planed, but I wanted to build it simple without "destroying" good new frames, without welding... So, it appeared too short, too narrow (but good for bicycle's lines here in Bern) and too high (but comfortable for seating position)... If I ever built one new, I will try to correct all that, making something as classic velocar, with engine...
However, problems of wheels shall stay. I suppose that with fixing axles on both sides in a style of bicycle, I would avoid problems with standard axles and bearings. But, wire-spokes and rims should be problematic. Some people suggested BMX 20" wheels, maybe with moped's tyres of 16"... Not quite in my style, and couldn't find any of them here. Or to build wheels by myself as was suggested in AtomicZombi Forum?
In Belgrade, I could by relatively cheap moped wheels of 18" complete with axles, bearings, and tyres... However, they had axles of under 1/2" so must be fixed on both sides...
Still have time to think about all that...