A steep shackle angle will...
A steep shackle angle will result in an effectively higher spring rate than with a shackle angle that leans closer to horizontal.
For instance, note that the number of leaves in the spring pack affect the spring rate. Also, note that the percentage of full length leaves affect the result. A higher number of full length leaves will increase the overall pack spring rate due to the fact that the ends of the spring pack become less flexible with the added longer leaves.
Another interesting fact is that while leaf width affects the spring rate, leaf thickness and length have a much greater affect on the stiffness of the pack. When making suspension modifications, leaf width and length changes may be hard to accommodate, but leaf thickness can be changed easily. A thick spring pack with many thin leaves may offer a softer ride than a pack with fewer leaves of greater thickness.
Leaf packs, by their nature,...
Leaf packs, by their nature, have friction between the leaves. This can cause the springs to ride roughly over small bumps because the spring must overcome the pack friction before it starts to flex. Teflon, or similar anti-friction pads, are now commonly used between the spring leaves to help reduce the friction and allow the leaves to slide smoother and with less force required.
Other factors can also raise the effective spring rate of your springs. Dry (ungreased) bushings can inhibit smooth movement of your spring and shackle assembly and the resulting bind can stiffen your suspension. Greasable shackles and spring pins can help cure this problem.
Shackles also play a part in determining spring rate in a leaf spring system. The closer the shackle is to vertical, the higher the effective spring rate. When the shackle lies back at an angle, it effectively becomes a moving link with the spring and the spring ride feels softer.
| DIMENSION INCREASED | EFFECT ON RATE |
| Number of Leaves | Higher |
| Leaf Length | Lower |
| Leaf Width | Higher |
| Leaf Thickness | Higher |
Coil Springs
There are three main physical dimensions that can affect the spring rate of a coil spring: coil wire diameter, coil spring mean diameter, and number of active coils. The table below shows the effect that increasing each of these dimensions has on the spring rate of the coil.
| DIMENSION INCREASED | EFFECT ON RATE |
| Wire Diameter | Higher |
| Mean Diameter | Lower |
| Number of Active Coils | Lower |
The equation below can be used to estimate the spring rate of a coil spring:
| Spring Rate | = | G x d4 |
| 8 x n x D³ |
| G | = | Shear modulus (11,385,500) |
| d | = | coil wire diameter (inches) |
| n | = | number of active coils |
| D | = | mean coil diameter (inches) |
For example, we can calculate the spring rate for a coil spring with a coil diameter of 0.625 inches, a mean diameter of 5 inches, and 10 active coils. Using the equation above gives us a spring rate of about 174 lb-ft/in.
Shear modulus (G) is the same for all steel springs. It changes only when the spring material changes. Coil wire diameter (d) is a constant number unless the spring is designed as a progressive spring using a varying spring wire size. For that case, this equation is no longer valid. Number of active coils (n) is the number of free coils (complete circle) free to move with the spring action. Mean coil diameter (D) is the distance through the centerline of the coil as shown above.
Using the equation can help you compare coil springs or help you design a set of custom coil springs. Note that coil height specifically does not affect the spring rate significantly, only the active number of coils does. Note also that coil spring rates remain nearly constant even if the coil sags over time.
 Dimensions of a typical coil...  Dimensions of a typical coil spring. |  A taller or shorter coil may...  A taller or shorter coil may be stiffer or softer, based on the other dimensions of the spring. Taller coils can provide lift while still maintaining the stock spring rate, or provide a stiffer rate, depending on coil design. Usually lifts incorporate stiffer springs to compensate for larger, heavier tires and to stabilize the vehicle at the taller ride height. |  Note that small changes in...  Note that small changes in coil wire diameter can significantly affect the spring rate. A caliper or dial indicator should be used to measure coil wire diameter. Precise dimensions are necessary for accurate spring rate prediction. |