On my first teardrop, I had 3/4" walls bolted directly to the outside of the HF trailer frame. Had about 3/4"- 1" of clearance if I recall. The one tire rubbed slightly every now and then, but never had any issues through 2 years of camping and thousands of miles.
Best pic I could find.... the plywood side was notched right at the leaf spring bracket, so the tire only had an inclination to occasionally rub when the spring was compressed a lot (very rough dirt roads)

This pic below is the trailer I built last winter. Kind of addresses the 'tire clearance' question. When I was building it, a few different people PM'd me telling me that I didn't have enough clearance, and I was crazy, and blah blah blah. I did cut the wheel openings with a 45 degree bevel cut to allow a little more clearance, but its still pretty close. Radius for the wheel well arc was 12", centered on the axle.

Well, after a solid year of camping and state forest boondocking, the tires haven't rubbed. And when I say boondocking, I mean dirt forest service roads that have plenty of rocks and ditches that have put the trailer through its paces. I am not gentle on my trailer, and it has held up incredibly well. Would I build another? Heck yeah! I'm actually working on a design with a friend right now, and his wheel well arches will be just as close as mine.
If you don't push the limits from time to time, you'll never know whats possible. Just don't step over the edge.
