Springs and Suspension: Basic Setup
A compressible spring is coupled with a flexible suspension system to smooth out the potentially damaging irregularities in road surfaces. Unfortunately, the springs weaken after repeated use causing a sagging suspension system or an uneven ride height. Sometimes the wear can get bad enough to cause the spring to break. In other cases, springs are replaced to enhance the load-carrying or handling characteristics of a vehicle. A good understanding of how springs are rated and how they operate is indispensable for achieving the anticipated outcomes in spring modifications.
The spring rate is the amount of pressure required to compress the spring one inch. Spring rate for coil springs is affected mostly by wire and spring diameter, and the number of active coils. While spring rate on torsion bars is affected by bar diameter and length. The length, thickness, number and position of the individual spring leaves affect spring rate on leaf springs.
Coil Springs, Torsion Bars, and Leaf Springs
Coil springs are rated by free length, compressed length, outside diameter, coil count, and wire size. Most import passenger vehicles use coil springs because of their size and versatility. Most are linear, but coil springs can be progressively wound so that as the load increases, the spring rate also increases.
Torsion bars are found on most import trucks, because they can easily be adjusted to fit the ride height. The downside is that torsion bars can only be produce a linear rate.
Leaf springs not only support the weight of the vehicle, but also attach the drive axles to the frame. Leaf springs are most commonly found in the rear suspension of trucks. In most modern applications, a leaf spring is built with one or more main leaves that are usually connected to the frame by a fixed mount at the front and a flexible shackle mount at the rear. The pivoting shackle allows the arched main spring leaves to change length as the spring compresses. Additional leaves of varying thickness and length are added to increase the load-bearing capacity of the spring. All of the leaves are held in position by a center bolt that also locates the spring assembly onto the axle housing spring perch. When leaf springs wear out, they lose the arch originally built into the springs. Wear in leaf springs can generally be detected by observing the amount of travel left in the rear spring shackle with the vehicle at rest and at normal load. If the rear shackle is fully extended, the spring should be replaced.
Suspension systems are designed to operate at a specific suspension height. If the suspension height is altered beyond specifications, the geometry of the suspension will change and cause bump steer and premature tire wear. Altering the height of the rear suspension system can also have adverse effects on the drivetrain, and suspension.
Suspension modifications are better left to shops with the experience and tooling. As for working with the street performance enthusiast who wants to increase the overall handling or appearance of his vehicle, it’s important to understand some spring performance basics. It’s important to understand the relationship between the spring, shock absorber and anti-sway bar. In general, stiffer springs require softer shock absorbers, while softer springs require stiffer shock absorbers to maintain rebound control. The same goes for sway bars.
Off-road driving will benefit from softer springs that absorb more of the road’s irregularities with more suspension travel. Pavement performance better utilizes harder springs that keep the tires on the ground for better cornering. In any case, it’s always a more efficient approach to buy a suspension performance package that matches individual components to their intended use. Most performance spring manufacturers have technical help lines that guide technicians and retail customers toward spring packages suited to their individual needs. Whatever the situation, always remember to replace springs in pairs and remember also that springs and correct suspension height go hand-in-hand to create maximum suspension and steering performance.