FWIW before I got my new axles I called every engineer I used to work with and not one took a torsion bar axle design course in college. The only thing left to do was call a torsion bar axle manufacturer and asked to speak to a torsion bar axle design engineer.
He gave me a crash course in torsion bar axle design parameters.
1. The capacity of the axles should have 50 to 66% rated load capacity placed on them.
2. When a torsion bar axle is loaded it arrives at the capacity of the axle when it rotates 22 1/2 degrees.
3. When it gets there the axle movement ability really stiffens up and if it moves another 11 1/4 degrees you have reached a point that is double the capacity and things get into a hard stop mode quickly.
4. Anything past that and it is like there were no springs at all.
I told him I estimated my road weight to be 7000 lbs and then I asked him, "If this were your trailer and cost was not a factor what would you put on?
He said he would install a pair of 5200 lb axles because if you hit a really bad spot on the road you have another 3000 lb axle capacity left to absorb load before you get into the 11 1/4 degree range. And two the 5200 lb axles come with 12" brakes.
My axles had taken a set at parallel to the road as can be see below:
When new axle arrived I made a template so I would know exactly where to cut the side plate coming off the trailer frame.
Placed the template on the side plate of the frame and scribed out where the new axle would be located and this told me what material needed to be removed.
To distribute the load I put in a piece of I believe it was 2"X3/8" bar stock between the frame and new axles to further distribute the load over the entire side mounting plate. It was tack welded in place.
Put it on the road for a 800 mile trip and was following a 18 wheeler with a car between us and all of a sudden on I 40 there was this big deteriorated section in the roadway and I was on it before I could react and the 2500HD went over and rattled us. I was looking at the trailer in west coast mirror when the wheels got to the bad spot it just felt as if it floated right over it. The 2500HD definitely show far more reaction to it than the AS.
I knew then the advice I got from the designer was good and the advice I got from this forum to run 10 ply rated Light Truck Tires was sound and the purchase of the Tredit Steel wheels with a heavier rating than Airstream used was the right choice.
I mounted the axles myself. First one took about 9 hours as I was working out how to do it and the first couple of ideas did not pan out and finally I figured out how to do it. The second one took about three more hours from removal to installation.
I placed the new axle on a floor jack at the balance point and gently raised it up and rotated the axle around till I got the bolts into the rear hole on each side. I then raised the jack more and the axle rotated and brought the front holes up even with their corresponding holes in the side plate.
And as Jackie Gleason was known to say: "And away we go!"