As mentioned previously, one way to construct a progressive rate coil spring is to use a spring wire size that tapers over its length. Another method is to wind the coil in a cone shape so that the mean diameter varies over the coil length.
For example, suppose we're...
For example, suppose we're using a 200-lb ft/in upper coil and a 300 lb-ft/in lower coil. The initial effective spring rate would be 120 lb-ft/in. Once the upper coil is stopped, the final spring rate would jump to 300 lb-ft/in for the remaining distance of shock travel.
Coilover Shocks
Another form of spring assembly is that of a coilover shock, which is actually one or more stacked coil springs with a dampener shock inside. The spring function of this acts in the same way as a regular coil spring. However, coilovers typically provide more than one spring rate as the shock moves through its range of travel.
For example, a dual rate coilover is one that has two springs stacked one atop the other. During the initial portion of the compressive shock travel, both springs compress. The effective spring rate is a combination of the two springs and is less then the rated value of either coil, based on this formula:
| Spring Rate | = | (Upper coil rate) | x | (Lower coil rate) |
| (Upper coil rate) | + | (Lower coil rate) |
The lighter rated coil will compress more as the suspension compresses and a coilover has an adjustable stop ring that can be set to stop the compression movement of the upper (lighter rate) spring. Once the upper coil is stopped (when the dual rate slider hits the stop ring), the spring rate of the shock is simply the spring rate of the lower (higher rated) coil.
By varying the rate of the two coils and adjusting the stop point of the upper coil, the shock can be dialed in to provide a soft ride over small bumps, but then use the higher final spring rate to soak up the bigger hits and prevent bottoming. Additionally, nitrogen pressure is used in the shock body to provide another compressible medium.
The amount of nitrogen charge...
The amount of nitrogen charge determines the effective spring rate, plus the internal valving uses the oil to dampen the movement. The mixture is confined inside the shock. The oil is incompressible but the nitrogen can be compressed as the shock is compressed. This nitrogen compression provides a fairly constant spring rate over perhaps the first two thrids of travel but then the rate rises almost exponentially as the shock is compressed towards the end of its travel.
Air Shocks
As with a coilover shock, a nitrogen charged ,"air shock," can be used to provide both spring and dampening functions. These units look much like a fat hydraulic shock but incorporate beefier shafts and are charged with high pressure nitrogen. Inside the shock, the oil and nitrogen are mixed together and move through internal orifices.
Behavior can be tuned by changing the internal oil passage valving, the amount of oil in use, plus the nitrogen charge pressure. More oil in the shock will cause the spring rate to rise faster and help prevent bottoming. Less oil will allow the shock to compress further before the shock starts to go to a hydraulic lock condition. Nitrogen pressure can be adjusted to dial in the base spring force.
Torsion Bars
The equation below can be used to determine the spring rate of a torsion bar:
| Spring Rate | = | G x d4 |
| 584 x l |
| G | = | Shear modulus (11,385,500) |
| d | = | bar diameter (inches) |
| l | = | bar length (inches) |
Dimensions of a typical torsion...
Dimensions of a typical torsion bar.
One point to note about torsion...
One point to note about torsion bars as used on IFS; the force from the tire acts through the lever distance of the A-arm to twist the torsion bar. Thus a torsion bar spring rate is often relatively high to resist the force applied through this lever. When longer A-arms are installed to widen the track (greater lever distance), the torsion bar rate must be raised to retain the same effective front spring rate.
For example, we can calculate the spring rate for a torsion bar with a diameter of 0.90 inches, and an active length of 40 inches. Using the equation above gives us a spring rate of about 320 lb-ft/in. While this may seem somewhat high; torsion bar systems typically work in a leverage system such that the effective spring rate at the tire is substantially less.
Similar to coil spring measurements, small changes in bar diameter can significantly affect the spring rate. Bar size should be precisely measured for accurate spring rate calculations.
Using this knowledge of spring types and the equations given, you can make some estimates about spring rates for your vehicle. The effects of dimensional changes and spring modifications can be seen so that you can predict your results with some degree of accuracy.