After weighing and measuring our truck, Bill found out we had two separate problems. Our first problem was that we had too much spring rate in our leaves for the rear of our truck. The second problem was a bigger issue: Our springs did not have enough room to flatten out. The shackle was too short with the spring we were using, and there was not enough available length forcing the spring to bind up. This is the number one way springs get broken and bent. It is imperative to match your spring with your shackle.