Load Vs. Rate
Two of the most basic characteristics of a spring are load and rate. Spring rate is the amount of force required to compress a spring 1 inch, while spring load is the force required to compress a spring to a certain height. For example, a 350-lb/in spring requires 350 pounds of force to compress 1 inch. A spring that compresses 2 inches under 200 pounds of load will compress 4 inches under 400 pounds of load. As this scenario illustrates, load changes as a spring is compressed, but rate does not. This explains why aftermarket lowering springs, which reduce suspension travel, must be stiffer than their stock counterparts to prevent the suspension from bottoming out over bumps. Ultimately, spring rate affects a car's weight transfer characteristics while spring load determines how much weight a spring can support at a given height.
Just as the gross lobe lift of a camshaft is vastly different from net valve lift at the seat, spring rate and wheel rate are different animals entirely. To put it succinctly, wheel rate is the effective spring rate once you take into account the linkage ratios of the control arms. The only way to achieve a wheel rate that's in direct proportion to spring rate is by mounting the springs directly to a solid axle, which is possible in the rear suspension but not practical in the front. The formula for determining wheel rate is outlined below, where "a" is the distance from lower control arm's inner pivot point to the spring center, "b" is the distance from the lower control arm's inner pivot to the ball joint, "c" is the distance from the lower ball joint to the front instant center, and "d" is the distance from the tire's contact patch to the front instant center.
Wheel rate = Spring rate x (a b) x (c d)
Regardless of the numbers plugged into this equation, the bottom line is that the effective wheel rate will always be less than the spring rate. This relationship is important to keep in mind when upgrading the springs in your car, since adding 300 lb/in of spring rate may only increase the effective wheel rate 150 lb/in. Furthermore, comparing spring rates from different cars is almost useless. That 600-lb/in spring rate that works so well in your buddy's autocross Camaro might not be the best setup for your Chevelle.
With an independent suspension, the best way to minimize the disparity between wheel rate and spring rate is by mounting the spring as close to the lower ball joint as possible. Increasing the distance from the spring to the ball joint reduces wheel rate, which necessitates a stiffer spring rate to compensate. While aftermarket control arms can tweak the positioning of the springs, how much they can be moved is very limited.
"The packaging constraints of a front suspension are a disaster, and to a certain extent you're stuck with what the factory gave you. The spring, shock, sway bar, endlink, bumpstop, spindle, tie rod, and control arms are all crammed into a the same small space," Chris Alston explains. "A larger-diameter spring eats up precious real estate as well. By starting with a clean-sheet design, one of the big benefits of an aftermarket front clip assembly is that you have more flexibility of where to position each suspension component. However, when designing a suspension around the factory design and pick-up locations, you can't move things around much at all."