Fortunately, the outlook isn't nearly as bleak at a car's posterior. Since most muscle cars employ solid rear axles, the springs can be mounted directly to the rearend housing very easily. This applies to both coil and leaf-spring suspensions, and yields spring rates that are equal to the effective wheel rate. However, a solid rear axle doesn't always ensure a wheel-to-spring rate ratio of 1:1, as certain suspension designs position the springs on the control arms. The good news is that GM got things right by mounting the springs directly to the rearend housing in both its four- and three-link coil spring suspension designs, such as in the Chevelle and third/fourth-gen Camaros, respectively.
A sometimes overlooked factor in spring performance is its orientation in relation to the chassis. Springs absorb suspension loads most efficiently when mounted vertically. While mounting springs at a slight angle will have little consequence, anything greater than 25 degrees substantially reduces their effectiveness.
Calculating Spring Rate
The easiest way to measure spring stiffness is with a spring rate checker tool, but if you don't have one, the rate can be determined using simple math and a ruler. Calculating spring rate-whether for coils or leaves-is far from abstract, and directly related to a spring's physical proportions. In the equation below for coil springs, "G" represents the torsional modules for steel, which is 11,250,000. This figure is always the same for steel coil springs. Next, "d" is a spring's wire diameter in inches, "N" is the number of active coils, "D" is the mean coil diameter, and the number 8 is a constant for all coil springs.
Coil spring rate = Gd4/8ND
The formula may be simple, but what it illustrates about spring design is profound. One of the most obvious lessons is that cutting coils from a spring increases its rate due to the reduction in active coils. Furthermore, increasing the wire diameter by a few hundredths of an inch substantially increases the spring rate. Ultimately, spring rate is determined by the material it's made of and its dimensions.
A bit less apparent in examining the formula is the fact that a spring doesn't lose its rate over time. Although a spring may wear out and sag as it ages, losing its load capacity, its rate never changes. Consequently, if the 40-year-old springs in your muscle car are sagging, they don't need more spring rate. They need more spring load.
As with coil springs, calculating the rate of leaf springs is based on simple math. In the formula below, "W" represents the width of the leaves in inches, "N" is the number of leaves, "t" is the thickness of each leaf, "L" is the length of the spring, and the number 12 is a constant for all leaf springs.
Leaf spring rate = (WN 12) x (1000t L)
One of the most difficult aspects of setting up a suspension is selecting the ideal spring rate. Not only is rate selection contingent upon a car's intended application, any component that adds or removes weight changes the equation entirely. This is why GM used more than 100 different rear springs for first-gen Camaros and X-bodies.
"Whether you run a small- or big-block, a stick or automatic, cast wheels or billet wheels, A/C or no A/C, move the battery to the trunk or not-literally everything on your car affects what spring rate you should run," says Chris Alston. "Shocks determine how fast weight is transferred from side to side and front to back, but the springs determine how much weight is transferred. Therefore, if you have the wrong springs on your car you're never going to get the shock valving and wheel damping correct. In fact, 80 percent of the shock problems we see is because people have the wrong springs on their car."