The coil spring is the most commonly used spring for suspension systems. A coil spring is actually a coiled-spring steel bar. When a wheel strikes a road irregularity, the coil spring compresses to absorb shock and then recoils back to its original installed height. Many coil springs are made of a steel alloy that contains other elements such as silicon or chromium. Coil springs may be manufactured by a cold or hot coiling process. The hot coiling process includes procedures for tempering and hardening the steel alloy. Coil springs are designed to carry heavy loads, but they must be light in weight. Many coil springs have a vinyl coating that increases corrosion resistance and reduces nouse.

Coil spring failures may be caused by these conditions:

1. Constant overloading

2. Continual jounce and rebound action

3. Metal fatigue

4. A crack or nick on the surface layer or coating

Coil springs do not resist lateral movement. However, when coil springs are used on the drive wheels, the suspension usually has special bars to prevent lateral movement.

Linear-Rate Coil Springs

Linear-rate springs characteristically have equal spacing between the coils and one basic shape with a consistent wire diameter. All linear springs are wound from a steel rod into a cylindrical shape. When the load is increased on a linear-rate spring, the spring compresses and the coils twist or deflect. As the load is removed from the spring, the coils unwind - or flex - back to their original position. The amount of load necessary to deflect the spring one inch (25.4mm) is the spring rate. Linear-rate coil springs have a constant spring rate regardless of the load or the amount the spring is compressed. For example, if 200 pounds deflect the spring 1 inch, 400 pounds will deflect the spring 2 inches. The spring rate on linear springs is usually calculated between 20 percent and 60 percent of the total spring deflection.

Heavy-Duty Coil Springs

Since heavy-duty springs are designed to carry 3 to 5 percent greater loads than regular duty springs, they are somewhat different. The first difference that can be noticed is wire diameter. On heavy-duty springs, wire diameter can be up to 0.100 inch (2.5mm) greater than the regular-duty spring for the same application. This larger diameter wire increases the load-carrying capacity of the spring.

The other difference is free length. A heavy-duty spring is up to 2-1/2 inches (64mm) shorter than a regular-duty spring for the same application.

Variable-Rate Coil Springs

Variable-rate spring designs are characterized by a combination of wire sizes and shapes. The most commonly used variable-rate springs have a constant wire diameter, are wound in a cylindrical shape, have unequally spaced coils. This type of spring is called a progressive rate coil spring.

The design of the coil spacing gives the spring three functional ranges of coils: inactive, transitional, and active. The inactive coils at the end of the spring introduce force into the spring when the wheel strikes a road irregularity. When the transitional coils are compressed to their point of maximum load-carrying capacity, these coils become inactive. Active coils work throughout the entire range of spring loading.

Theoretically, when a stationary load is applied to a variable-rate coil spring, the inactive coils support the vehicle's weight. If the load is increased, the transitional coils support the load until they reach their maximum load-carrying capacity, and the active coils carry the remaining overload. This action provides automatic load adjustment while maintaining vehicle height.

Some variable-rate coil springs have a tapered wire in which the active coils have the larger diameter, and the inactive coils have the smaller diameter.

The latest variable-rate spring designs deviate from the old cylindrical shape. These include the truncated cone, the double cone, and the barrel spring. The major advantage of these designs is the ability of the coils to nest or bottom out within each other without touching, which lessens the amount of space needed to store the springs in the vehicle.

Unlike a linear-rate spring, a variable-rate spring has no predictable standard spring rate. Instead, it has an average spring rate based on the load of a predetermined spring deflection. This makes it impossible to compare a linear-rate spring to a variable-rate spring. Variable-rate coil springs usually have more load-carrying capacity than linear-rate springs in the same application.