|When you stray from the norm, you're on your own. My father and I did this completely on our own without advise or help of any kind. Luckily he's been building hot rods and race cars for 40 years, so he had spring experience.
As this was his idea, he more or less took the lead on it and I simply followed direction. Springs were flattened, if you read my previous posts, I explain that pretty good there. A simple shop press was used, I beleive the pressure it took was somewhere between 300 and 400 psi to get the spring to retain the desired shape. Dad made a wedge to place between the springs and the axle to give it caster (or camber, can't ever remember).
I had to drill out the old bushings to make room for the new bushings that came with the shackles. This was a royal pain with the shackle mounts on the frame. Those were some hard SOB's!
I used a rack and pinion for the steering. It works REALLY well! But it was a pain to set up. I might do this again, I might not. The mounting tabs are welded to the axle, and the stock steering arms have been altered to take the tapered ends of the rack.
Shocks are short travel hydraulics. I think they're actually motorcycle shocks. Something my father came up with anyway... The mounts for them are welded to the axle, and to the frame, all custom made stuff. The shocks are short and have rubber bumpers to prevent the axle from rebounding back into the frame. So far its never come close to doing that, and I've driven on some BAD roads.
The frame is boxed by the way...
Brakes are Mustang II disk up front and stock Ford Ranger drum in rear.
The driver's side water pump has been re-done to point the water DOWN instead of foward. This was done to clear the steering shaft on the rack. That detail usually gets some people scratching their heads.
Model A rear springs SUCK on lighter cars! That thing has SO little flex! If I had the funds, I'd replace it with coil-overs. Its frustrating to hit a bump in the road and have the front take it with ease, and have the rear bounce up so bad.
This is the lowest car I've ever owned, but its not the lowest hot rod I've ever seen. During mock-up, the oil pain was the lowest point on the car; that wasn't going to work for me. During reassembly we raised the engine an inch, and the new engine going in will have a new oil pan that raises it even higher, now the frame itself will be the lowest.
PAY ATTENTION to the scrubline! With an underlung its REALLY easy to get your frame so low that it'll rub the ground if you get a flat tire. That is one of the reasons my axle is under my springs (that and it just looks better that way IMHO). I'm running normal 16" wire wheels, though I have slightly larger than normal 600's for tires. It works though.
Number of leaves seemed to more affect ride height than spring rate. I realize that doesn't make a lot of sense, but the only difference I could ever tell was how high the car rode, it never really produced a smoother or rougher ride. The car corners really well. Far better than the traditional transverse leafe setup that a friend of mine is using. Attribute that to lower center of gravity and more spring at the corners. Also that with the rack, I can turn faster than him.
The absolute WORST thing I've run into with an underslung suspension is public opinion. There are just some people out there that can't handle "different". I'm more than happy to prove them wrong though. There are some though that insist its dangerous, its like "welding the axle to the frame" etc. 90% though think its a great idea. An interesting side note, is that many of the people I've seen making the "dangerous" comments are the very ones driving rods with suicide suspensions, which one could probably argue as more dangerous (its just been around longer), that's pure speculation/opinion on my part.
One thing I would recommend, is a new main leaf. I haven't yet, but I will be doing that. 80 yr old springs will work, but I'd feel better about it if the main leaves didn't have gouging scars on them.