Springs

The springs in the suspension have two important functions. Springs support the vehicle weight and absorb the bumps and movements that occur when driving. There are four types of springs used in suspension systems.

Coil springs—are a length of steel wound into a coil shape. Used on most front and many rear suspensions, coil springs, such as those shown in Figure 6-13, are large pieces of round steel formed into a coil. The spring absorbs energy as the coils are forced closer together. This is called compression. The stored energy is released when the coil extends back out. The energy continues to dissipate as the spring bounces. Eventually, the energy is exhausted and the spring stops bouncing. Coil springs are found in front and rear suspension systems, have a compact design, and do not need maintenance. When the spring becomes fatigued or weak, ride height will drop, and the spring will need to be replaced.

Coil springs are often sandwiched between the lower control arm and the vehicle frame. In this position, the weight of the vehicle is pushing down against the spring, which is supported by the lower control arm. This configuration allows movement of the suspension while the spring carries the weight and dampens out road shock. Coil springs often use rubber insulators between the spring and the frame to reduce noise.

The coil springs used in strut suspensions appear similar to those used in other applications, but are not interchangeable. Most strut coil springs are made of smaller diameter steel but are larger in total outside diameter than those in other applications. Strut coil springs are usually painted or coated with rustresistant coverings.

Coil springs are categorized as either standard or variable-rate springs. A standard-rate spring has evenly spaced coils and requires a specific amount of force to compress the spring a given amount. Further compression requires an additional force, equal to the original force. A variable-rate spring has unequally spaced coils and requires an increasing amount of force to achieve further compression. For example, a standard-rate spring may require 300 lbs. of force to compress one inch and an additional 300 lbs. to compress the second inch (600 lbs. equals two inches). A variable-rate spring requires the same 300 lbs. of force to compress one inch but requires 500 lbs. to compress the next inch (800 lbs. equals two inches). Coil springs used in passenger car rear suspensions are usually lighter duty than those found at the front. This is because the majority of the vehicle’s weight is often toward the front. Coil springs on the rear of larger passenger cars, trucks, and SUVs are often variable-rate springs.

Leaf springs—are long semi-elliptical pieces of flattened steel and are used on the rear of many vehicles. Leaf springs are typically mounted as shown in Figure 6-14. Leaf springs have been in use since the horse-and-buggy days. A leaf spring is a long, flat piece of spring steel, shaped into a semicircle. The spring is attached to the frame through a shackle or bracket assembly that permits changes in the effective length of the spring as it is compressed. To carry heavier loads, additional leaves can be stacked below the master leaf. Increasing the number of leaves increases load carrying capacity but makes the ride stiffer. Some suspensions use transverse leaf springs that are mounted perpendicular to the frame. In a transverse arrangement, one leaf spring supports both sides of the suspension. This style was used for many years on the Corvette and on some FWD vehicles with independent rear suspensions.

Service Warning: Vehicles with air springs often require special lifting and jacking procedures. Do not attempt to raise a vehicle with air springs until you have read and followed all applicable warnings and procedures.

Air springs—are thick, tough bags filled with air that act as springs. Air springs are used on some larger sedans and most large commercial semi trucks and trailers. Air springs are typically located in the rear, though some manufacturers use air springs at both the front and the rear, as shown in Figure 6-15. Air springs, like torsion bars, are adjustable. On many vehicles, the on-board computer system uses a ride height sensor to determine suspension load. As additional weight is added to the trunk, the suspension will drop. When the computer senses this drop, it can turn on an on-board air compressor to supply more air to the air springs. The increased pressure in the springs will restore the ride height to the desired position. Some systems may use the adjustment of air pressure to the air springs to control ride height based on the vehicle’s speed or driver input.

Torsion bars—are coil springs that are not coiled. Torsion bars are lengths of round steel bar fastened to a control arm on one end and the frame on the other end. Movement of the control arm causes the torsion bar to twist. The absorption of the twist is similar to compression of a coil spring. As the torsion bar untwists, the control arm returns to its normal position. Torsion bars are used in many 4WD vehicles where a front drive axle occupies the space where the coil spring normally sits. The torsion bar shown in Figure 6-16 is mounted to the lower control arm and the transmission crossmember. Torsion bars can be mounted in either the upper or lower control arms. The control arm acts as a lever against the torsion bar, twisting the bar. The bar twists since it is rigidly mounted in a crossmember. As it releases energy and untwists, the torsion bar returns to its original shape, forcing the control arm back into position.

An advantage of torsion bars is that they are adjustable. At the rear torsion bar mount is an adjustment mechanism. If a torsion bar-equipped vehicle is sagging, the torsion bar may be able to be adjusted to bring the vehicle back into specification. When a torsion bar is replaced, it must be tightened to provide the necessary lift to support the vehicle.

Spring Ratings

Automotive springs are rated for their frequency and their load rate. Springs, when either compressed or extended are under tension. When the tension is released, the springs will attempt to return to their original condition. When the springs are compressed or twisted, they store energy. Upward movement of the wheel that compresses the spring is called jounce. The stored energy is released when the spring rebounds. This downward movement of the tire, as the spring extends out, is called rebound. As you probably know, a compressed spring will rebound many times before all of the energy is dissipated. The number of times a spring oscillates or bounces before returning to its rest point is called the spring frequency. An example illustrating this is shown in Figure 6-17. The size of the spring and the spring material contribute to the spring frequency. Ideally, a spring should dampen out its oscillations quickly enough to provide a smooth ride but not so fast that it causes a harsh, jarring ride. If left to bounce or oscillate on its own, the spring will cause the vehicle to bounce excessively, probably to the discomfort of the passengers.

The amount of force it takes to compress or twist a spring a certain amount is called spring rate. Springs can have either linear or variable rates. Figure 6-18 illustrates the differences in spring rate. When a vehicle is designed, the engineers will factor spring size, rate, and frequency based on the intended vehicle use, tire size and type, suspension style, and many other factors. The goal is to have the best compromise between component weight, vehicle cost, and the ride and handling qualities desired for the vehicle.