My current design is a clone of an Easy Racers Javelin: 700c rear, 451 front, LWB. I figure this is an easy way to get started -- later on, I can try more fancy designs with integrated seats. I'm trying not to "overmodel", so here is a sketch of just the parts I need to build a simple prototype of the rear wheel attachment. I would certainly not build such a long structure to the right, but I want to give an idea of how it would fit together:
The dropout is made from 1/4" 6061-T6 plate. The stays are 5/8" diameter, .035" wall 2024-T3 round drawn tubing, flattened at the ends and attached with two #8 MS27039 machine screws:
The stays are attached to the body by (roughly T-shaped) brackets made from 1/8" thick 6061-T6 material:
They are held together at the middle by a spreader piece made from 1/16" 6061-T6:
Removing the stay and spreader piece, we can peek in to see that the 1/8" bracket is built up on each side with a 1/16" thick spacer, such that the stay is flattened on both ends to an inside dimension of 1/4". This ensures that it has a nice radius without cracking:
The approach I show here, with a thicker bracket and a stay flattened onto the bracket, is a lot simpler and more symmetrical than either riveting the stay from the sides or inserting the stays into the structure. Note that I'm relying on the brackets themselves to resist side-to-side forces since the 1/8" plate has significant bending strength (though I haven't done the math on that part to know for sure, mainly because I am not sure what the design lateral loads should be).