Coil springs have 2 common specifications, free length, also called spring height, and spring rate, although lowering and lifting springs also specify the amount they will raise or lower the vehicle. Free length is the length of the spring when in its unloaded state. Spring rate is the measure of spring stiffness. It greatly affects handling and must be soft enough to let the suspension move over bumps, yet stiff enough to keep from bottoming or topping out. There are linear rate, sometimes called constant rate, progressive, also known as variable rate , and dual rate springs.