In the world of off-road, suspension handling is always a compromise between good ride and good performance. Today's systems can provide both, but a little understanding of what factors affect spring rate can help you optimize spring performance for your particular application, be it a high speed desert racer or slow speed rough-terrain crawler.
A softer spring will allow the suspension to move more with less force applied. This is great for rockcrawling, but may be too flexy for mud bogging, or too light if you're into jumping your rig. A stiff spring may help the truck corner flat on the street, but may be really choppy on speed bumps or whoops.
Spring rate is specified in pounds per inch. This number defines the weight or force required to compress the spring through its first inch of travel. While this may seem like a fairly straightforward way of defining spring specs, not all suspension companies publish their spring rates.
Coils differ from leaf packs in several ways. Leaf packs always have some amount of inter-leaf friction that provides some self-dampening to the suspension. Coils have no such significant friction, and heavier damped shocks are required to prevent oscillation of a coil spring suspension. Coils, by themselves, are also considerably lighter than equivalent leaf springs, but they require more suspension components (links) for axle location and control.
Leaf packs are often progressive to some extent. In other words, as the spring is compressed, the spring rate increases. This serves to help prevent bottoming of the suspension. Coils, however, generally have the same spring rate throughout their range of travel. There are some progressive-rate coil springs, but they are not as common as constant-rate spring coils.
We'll look at leaf springs, coil springs, coilovers, air shocks, and torsion bars and discuss the factors that affect the spring rate of each.
Leaf Springs
There are four physical dimensions that can affect the spring rate of a leaf pack: number of leaves, leaf length, leaf width, and leaf thickness. The table below shows the effect that increasing each of these dimensions has on the spring rate of the pack.
The equation below can be used to estimate the spring rate of a given leaf spring pack:
| Spring Rate | = | E x (2 + nfull/ntot) x ntot x w x t³ |
| 0.75 x l³ |
| ntot | = | total number of leaves |
| nfull | = | number of full length leaves |
| E | = | Young's modulus (29,900,000) |
| w | = | leaf spring width (inches) |
| t | = | leaf spring thickness per leaf (inches) |
| l | = | spring pack length (inches) |
For example, let's calculate the spring rate for a leaf pack with five leaves, two of which are full-length leaves. Assume we have a 2.5-inch wide spring, with leaves that are each 0.230-inch thick. The eye-to-eye length is 45 inches.
Plugging these values into the equation above gives us a spring rate of 160 lb-ft/in. That means it would take approximately 160 pounds of force to compress this spring through its first inch of travel. From there, the spring rate will typically rise as more springs in the pack come into use.
Using the above equation can help you compare leaf packs or help you decide the best way to modify a leaf pack to better suit your needs. With some understanding of the factors affecting spring rate, you can more accurately predict the final results.