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Old 01-19-2013, 10:37 AM
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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.
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