Originally Posted by 1971BB427
Spring twist is more about length, and height of the vehicle, as the height gives the vehicle more leverage on the spring. In some cases the greater arc is used to raise the vehicle's height, so it will then put more leverage on the spring, but not if the spring is mounted in such a way as to keep the weight lower. That's why many makers mount springs alongside the frame when large stacks of higher arc springs are used. It lowers the center of gravity, and keeps the vehicle's weight from working against the spring stack.
Some arc will give the spring more lateral support, but too much will indeed cause more twist, as there's just no way to keep the force against it low enough. Plus the spring needs to get longer as it's arc'd if it still has the same mounting points. Keep the length the same, and put some curve in it, and the spring will have a bit more resistance to twisting due to the curved shape.
You can test this with small spring material and a tension gauge. A flat piece of spring stock is easily twisted when gripped at the ends, but bend it and then try to twist it, and the tension will be greater. I've seen test sheets from the local spring shop where they use something similar to a big torque wrench to do these tests, and accurately measure spring twist.
I am not saying that the spring can't be twisted within its length when not installed in a vehicle. If you anchor it solidly at one end and twist the opposite, it most certainly will twist. And what you say about the arc increasing that effect is also very true.
What I am saying is that when installed in a vehicle and the spring is mounted at the frame on one end (in a rubber bushing), solidly tied to the axle near the center, and then tied to a shackle (in rubber or poly bushings) at the opposite end, you would have to rotate quite a distance before any spring twist would come into effect. All bushings would have to be fully compressed before the spring itself would begin to twist.
Then tie that same axle with one leaf spring mounted to another leaf spring on the other side of the vehicle, there is no amount of twist between those two springs (which are now tied together) that will be a factor in allowing the body to roll about the suspension. The rate of twist in that configuration would be massive and the shocks would not be able to control it. That is why I say that the ability for the body to roll about the suspension of a leaf spring suspended car is due largely to the arc of the spring and the bushings. The angle of the springs while mounted in the car (in side view) is the last major determining factor in allowing the body to roll.
An interesting way to test this would be to set the car on jackstands with the suspension loaded on three corners. Remove the leaf spring on one side of the rear axle, and with a floor jack under the end of the axle with no spring and move it up and down. That way you could easily see where the deflection is and also determine how much travel you have before the axle binds in rotation. If you were real ambitious, you could test with rubber bushings, then poly and then aluminum or brass, and see the results. My guess is that it would be a significant difference in axle travel between compliant and non-compliant bushings. If you did all aluminum bushings in both the front eye and the shackle, I would bet that you could hang the rear end nearly level with no support on the opposite side. I might try that the next time I have the opportunity.
(Warning!! I am now going a little off topic) To take this to the next level, try the same test with a parallel four link suspension with a track locator vs a longer and more lateral Panhard bar and see what happens. Or even a parallel 4 link with no lateral locator attached and see how much travel you have. After doing that you will likely see why I and many other folks that design and build suspensions prefer at least a triangulated 4 link for a car that has to turn, or my personal favorite, a 3 link rear suspension with a panhard for a live axle car. (Sorry, no more drifting off topic, I promise)