Spring selection is next. Generally, the spring manufacturers have a good idea which rate to recommend based on experience. If you have a custom application or want to be very precise, QA1 has a formula to measure the lever arm and shock angle that will determine an ideal spring rate. Be sure to take these measurements while the car is on level ground and set to the ride height you want. You’ll be measuring the distance between the lower control arm pivot and the lower shock mount. Then divide that number by the distance between the pivot and the ball joint. The result is the force ratio—the leverage the control arm puts on the spring and shock. Figures for my car are 10.5 inches to the shock mount and 15 inches to the ball joint, for a force ratio of 0.7:1.