Josh, I've thought about your comments aoubt the wheel base shifting in a leaning situation, and I think this is a problem with my car. It did not show up until I stiffened up the front end however. I drove it for years with a high arch rear spring set, and sway bars front and rear with no ill effects. Now it does exhibit firghtening oversteer in curves that comes on rather unexpectedly. I thought it was just because it sits too high, resulting in a cneter of gravity that is too high. Your comment has got me to thinking. What is the ideal rear axle behavior? It is possible to design things several different ways, with different outcomes.
Here are my thoughts on the situation: Forgive the length, but it requires 1000 words since I don;t have a single picture.
The change in wheel base is as much because the vertical relationship of the axle center to the front hanger bolt changes relative to the center line of all four wheels, which would be level to the ground, if all wheels and tires are the same size. The front half of the spring is considerably shorter than the rear, which means the front is stiffer, and the last to change it's shape, but it does rotate continually around the front hanger bolt. This means the axle is traveling in a circle around the front hanger bolt as it moves up and down, and the wheel base is constantly changing by small amounts. If the axle sits at rest higher or lower than the front hanger bolt, then the wheel base for that side will get longer when it goes wither up or down.
The rear of the spring does most of the shape changing, since it is longer, and actually more flexible by virtue or where the lower support springs end in ralationship to the shackle. The shackle is free to move front to back to accomodate the changing arch of the rear spring as the axle moves up and down.
Obviously, the longest wheel base would be when the front of the axle arch was flat, and when the wheel center was level with the front spring hanger for the rear leaf. If the design of the spring arch was such that the center of the axle was level with the front hanger bolt, and the arch was flat in front, then the wheel base would only get shorter as the car leaned either way, on both sides. The rear of the spring would not have any effect on this dynamic. True it supports everything in its way, but since the shackle is free to move front and back, it does not force the axle in this movement. Only the front of the spring has this effect.
When a car with the front hanger bolt level to the axle center tilts, the wheel on the down side, gets pulled shorter by virtue of entering the lower part of the circle described by the axle around the front hanger bolt. The wheel on the up side also get shorter,because it is entering the top side of the circle. There would be no shift in the squareness of the wheel base. It would only get slightly shorter. This would be true only as long as the front arch did not change shape. You could guarantee this by strapping a really strong piece of channel, or other stiff shape to the front section.
So it seems if the goal is to keep the axle always square, then get the axle center level with the front hanger, and stiffen up the front section. The easiest way to do this would be with lowering blocks. If the rear axle center is low in relationship to the front hanger, simply put blocks of the correct thickness in there. You would have the concern of drive shaft vibration if you get the pinion angle wrong.You would be changing the front u joint angle more than the rear one. They sell shims at 4x4 shops to correct for that.
When my car was factory, the right rear wheel was 1/4" further back than the left one. It handled quite strangely until I put a 1/4" shim between the front hanger and the body mounting for the pad. Everyone who followed me down the road remarked at how the car crabbed it's way down the road. Even my wife made note of it. I did not make any fancy measurements, I just guessed, and bought a 1/4" plate, drilled the holes, and installed it. When I had the wheels aligned they said the rear toe was perfect, so I just lucked out.
So Josh, I ask, is the ideal to have the rear axle remain perfecty square in curves, or is it desirable to have some shift one way or the other. What do these new 4 wheel steering vehicles do? Do they steer to the left in a right hand curve, or do they stear to the right in a right hand curve?
One thing is clear, the design of your rear spring mounting, and spring shape is very important.
Sam
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