[Rob Stehlik]

Actually, my original concept called for 1/2" shoulder bolts in a slightly different configuration. I intended to have the shoulder portion of the bolt pass through the frame, and tighten the entire wheel assembly against the frame with very thick washers.

The thread of a 3/8" shoulder bolt is 5/16"-18, which in my opinion is too small to be supporting this kind of cantilevered load. Thicker washers would help some, but the thread length is only 1/2", so you don't have much to work with, probably not even enough for a heavy lock nut.

I think what you have shown could work with a larger shoulder bolt, but it looks like squirrel's method with 1/2" threaded rod would be simpler and stronger. And fine thread really helps to get things tight.

Another issue is bending of the frame itself. Putting a cross bar at the top of the angle next to each wheel would help a lot. It could be a thin wall tube with a small threaded rod running all the way through.