If you peer under the cars and light trucks on the road today, you will find a variety of suspension systems. These suspension systems have several distinct functions, including:


1. Attaching the wheels and tires to the vehicle

2. Maintaining the proper wheel alignment and location as the vehicle traverses bumps
potholes, and uneven road surfaces

3. Stabilizing the vehicle's attitude during acceleration braking, and cornering, and

4. Isolating the road's roughness from the passenger compartment.



As with most real-world systems, some of these functions require conflicting design characteristics. For instance the ride over rough roads is most comfortable when the suspension is soft and compliant. Handling stability, on the other hand, typically improves as suspension stiffness (and ride harshness) increases. Consequently, the design of any particular suspension system is always a compromise among its various performance characteristics.

Over the years, vehicle design engineers have developed a number of different suspension geometiries in a never-ending quest for a suspension system that provides superior performance in all of the categories mentioned above, while yielding an acceptable installed cost.

Suspension systems consist of several basic components, based on the functions that the system has to perform. All suspensions require springs of some kind, motion dampers (e.g., shock absorbers), and structural members that tie the springs and motion dampers to the vehicle, as well as locate the wheels properly.

Springs can be coil springs, leaf springs, air springs, or torsion bars. Every one of them, regardless of type, has as its primary duty to absorb the loads and motion induced by the wheels' encounter with bumps, holes, and uneven surfaces, and to help keep the tires in firm contact with the pavement under a variety of operating conditions.

Coil springs are exactly as you would expect --each one is made up of one piece of spring steel wrapped into a series of cylindrical coils. By varying the distance between adjacent coils, these springs can be made "progressive;" that is, they become progressively harder to compress as the amount of compression increases. With small movements, they compress quite easily, but they resist larger movements that might threaten to bottom out the suspension. Coil springs are relatively light and compact, but they have no ability to act as structural members. That task must be performed by other components in the suspension assembly.

Leaf springs consist of flat pieces of steel, held together with with bands. These springs can be made to handles just about any vehicle weight or suspension load, just by adding additional leaves. Because of their heavy duty nature, they are commonly used on truck suspensions --not only for pick-ups and sport utility vehicles, but for medium and heavy duty trucks as well. Progressive leaf springs are also possible, by adjusting the length and thickness of each leaf. While they're strong, leaf springs are also heavy and bulky, so they see less and less use in modern passenger cars.

Air springs actually consist of air chambers or "bags" filled with compressed air. The bags can be sealed, or supplied with varying amounts of air by an onbaord air compressor. Newer versions often use a computer-controlled compressor to adjust ride height as vehicle load changes. While they offer excellent performance characteristics and light weight, they are expensive, and so are usually reserved for luxury or sport models.

Torsion bars also act like springs, but they consist of a straight length of solid or tubular spring steel, withone end firmly fixed. When the other end is rotated about the long axis, the twisting energy is stored, and thesteel rod will return to its original state as soon as the twisting force is removed. Imagine holding one end of a rod firmly in one hand, and twisting the opposite end as if wringing out a washcloth. If you wee strong enough, or the rod thin enough, you could twist it. When you let go, the rod would immediately "untwist," behaving exactly as a spring.

Motion dampers are typically either conventional "shock absorbers," or similiar piston-type devices mounted integrally within a strut assembly. These dampers contain hydraulic fluid, and in some cases, pressurized nitrogen gas, or air.

While they are called "shock absorbers" by legions of people who should know better, that really doesn't describe their function. It is actually the springs in a suspension system that react to load changes, or "shocks" to the system, absorbing the force so that the vehicle body won't.

But it is in the nature of a spring to store the energy of the load or shock it absorbs, and then release that energy again by returning to its original shape and size. In practical application, in fact, the spring -- left on its own -- will not just return to its relaxed position, but will overshoot that, and continue to oscillate back and forth from a compressed condition to an extended contition through several diminishing cycles until the original energy it absorbed is finally dissipated.

It's the principle on which those bouncy-headed novelty figures that ride on the shelf behind the rear seat are based. One good jolt will keep the thing bouncing for minutes.

While that my be amusing for some folks when it's an icon of their favorite animal or political figure, they don't have the same appreciation for having their own heads bounced around in such a manner. And that is what lead early suspension designers to include motion dampers in the system.

A motion damper, by expending energy through its own movement, dampens the oscillating action of the spring. Properly tuned, the motion damper will allow a compressed spring, when released, to extend slightly beyond its natural length or shape, then return to rest, and that's all. In engineering terms, that's called critical damping.

Damping is typically accomplished by inserting a piston mounted on the end of a shaft inside a cylinder filled with hydraulic fluid. Small holes, or orifices, in the face of the piston allow the hydraulic fluid to pass from one side of the piston to the other as the piston moves along the inside of the cylinder. The friction caused by the hydraulic fluid's being forced through the small openings absorbs the energy of motion.

With one end of the cylinder attached to the car body, and the other attached to a suspension member, the motion damper can control the movement of the spring, and hence of the wheel and suspension assembly, preventing the spring from oscillating back and forth.

Many different kinds of suspension systems have been developed, using a variety of spring types and structural members, as well as a couple of different kinds of motion dampers.

A few of the most common are shown here though we haven't room to show the myriad of variations used by the world's car makers.

Front suspensions, of course, must deal with not only the motion of the suspension assembly caused by road irregularities, but also the steering motion. Front-wheel-drive complicates the suspension geometry even more, because drive shafts must adjust as wheels change angles during turns.

Rear suspensions can be much simpler by comparison, since in all but the most sophisticated rear-wheel-steering set-ups, the track of the rear wheels is a relative constant.

Independent rear suspensions on front-wheel-drive vehicles often use assemblies (MacPherson strut or modified strut) similar to those shown for front suspensions, except that no steering knuckle is required, and a variety of leading and trailing links are used to maintain wheel location.

Catalogs and service manuals are available from leading manufacturers that include parts descriptions, photos and drawings of suspension assemblies of various types, as well as part lists. Familiarize yourself with those, and make sure you know what vehicle information you need for proper parts selection.

Suspension system work also requires a number of special tools. Know what they are, and don't forget to purchase them at your local Bennett Auto Supply when you order the corresponding suspension parts.