I'm going to stop now before itbecomes a career. A historical view:
https://occhiolungo.wordpress.com/2012/03/29/wicker-sidecars/
Before you go Ludwig, I'd been planning an air/shock combination setup to spring the car wheel, but both Curtis & yourself have mentioned using trailer hub suspension. I'm seriously thinking a right hand (passenger side U.S) half axle torsion spring setup would be pretty trick, especially if it were mounted on a sturdy pedestal & upside down. This would permit the low chassis height I'm looking for. I've a supplier who will supply me with whatever rating required over 250Lbs. with hub less swing arm. I won't require the hub since I'm running moped hub & brake assembly.
A clearer explanation of mounting the torsion spring assembly in the manner previously described might clarify things a bit. Normally a torsion suspension is mounted under frame with the torsion swing arm under no load at an angle of about 45 degrees. This setup increases the ride height which I don't want. By using a 4" square steel tube for a riser, cut to whatever length is required I can obtain the desired ride height of the frame. The torsion bar has to retain the same attitude as it would be in mounted under the frame in order to function, therefore the square tube riser will need to be notched to fit up the torsion spring half axle. A 3/16" plate, also notched, welded on top of the riser & drilled to match the 3" center, 4 bolt pattern of the spring assembly locks it all in place. Pretty simple to add a slip plate setup for adjusting toe in at the base of the riser.
My plan is to spring the rear of the car using 2 half length utility leaf springs. Buy one and cut in half, like I did with the leaf fork setup my Simplex is running. I'll simply pivot the front of the shell on Oilite bushings. With the leaf springs located outboard of the body shell, the body's ride height can be kept minimal.
Thanks for all the input & please continue with more. Rick C.