A vehicle's suspension is essential not only for keeping the ride smooth, but also for keeping the tires of the car squarely on the ground while driving. The operation of the suspension can be looked at from either of two perspectives; from the ground or from the car. From either point of view, the idea is to have the wheel follow the terrain of the road, while reducing any up and down movement of the rest of the car. For the sake of this discussion, we should assume the perspective of the vehicle rather than the road. This will better explain how the suspension helps smooth the ride and force the tire to follow the road.
The most important components of any suspension system are the springs.These are the devices that will effectively absorb the shock (contrary to their title, shock absorbers do not actually serve this purpose) of driving over bumps and ruts in the road. They allow the wheel to travel up and down relative to the car's position, while allowing the car to travel its course relatively undisturbed. As a car drives over a bump, the bump will push upward on the spring, which will compress, absorbing the upward motion that would otherwise be felt by the car's occupants. This stored energy is then released as the spring rebounds, pushing the wheel back towards the pavement.
There are three types of springs commonly used in cars today. They are the coil spring, the leaf spring, and the torsion bar. Perhaps the most familiar type is the coil spring, which as its name implies, is simply a coil of heavy wire. Leaf springs are flat leaves of steel that are arched such that they will push the wheel down towards the road. Torsion bars are long, round bars that are attached to the suspension in such a manner as to twist when a wheel is pushed up or down. Regardless of the design of the spring, they are all made of specially heat treated steel that is designed to retain its shape. The heat treating process used makes the spring steel somewhat more brittle, but less maleable than steel used for doors or fenders. It is the tendency to retain their shape that allows springs to bounce back after a bump.
Shock absorbers are probably the most poorly named component in a car's suspension. In England, they are referred to as dampers, and aptly so. Their purpose is actually to dampen the action of the spring, not to absorb shock. A spring, like all things physical, have a resonant frequency or a rate at which they will tend to naturally oscillate. If a car with no shock absorbers drove over a bump in the road, the spring would first compress as the wheel rose over the bump, then the spring would rebound back to the road. However, uncontrolled, the spring would continue to compress and rebound indefinately, limited only by the friction of the car's suspension. The shock absorber serves to dampen these oscillations by providing controlled resistance to the movement of the spring. By design, they are usually engineered to provide low resistance to compression of the spring, but will provide increased resistance to spring rebound. This way, the spring can easily compress to absorb the bump, but will release its energy more gradually.
The physical construction of a shock absorber is really quite simple. It consists of an oil filled cylinder and a piston. The piston is connected to either the car's frame structure or the suspension. The cylinder is connected to the opposite part. Valves in the piston allow the oil to flow from one side to the other, in a controlled fashion. These valves are sized to permit a pre-engineered rate of flow, based on the needs of the particular vehicle that they are designed to work with.
There are several types of suspension systems, each catered to the type of vehicle that they are used on. Here are a few of the many types available:
A-Arms: This type of suspension uses two hinged A-shaped arms which are mounted to the car's frame at the inside end and to the wheel hub at the other. The upper arm is generally shorter than the lower arm, due to the need to allow room for the car's engine. This system is most often used on the front of cars, and provides independent movement of each wheel. Since the two arms are not equal in length, there will be a change in the angle of the wheel as the suspension is compressed upward. This can be put to beneficial use by helping to keep the tire square on the road when a vehicle is turning. Keep in mind that as a car turns a corner, it tends to lean in a manner that forces the side of the car on the outside of the turn lower than the side on the inside. Ordinarily, this would cause the tire to lean over along with the body of the car. However, by using unequal length A-Arms, the angle of the tire can be different from that of the car, and can be designed to work in the opposite direction of the lean of the car. This way, the tire will remain square to the pavement, providing the traction benefit of the entire tread of the tire. A-Arm type suspensions use either coil springs or torsion bars.
Solid Axle: Solid axles are most often used in rear suspensions. They are the simplest design, as they use a solid bar between the two wheels. The bar or axle can be connected to the vehicle either by leaf springs or by control arms. These arms are are hinged at one end to allow the axle to move up and down with bumps. Leaf springs, by design, serve the same purpose of control arms for locating the axle in its correct position under the vehicle. Additional control arms may be used to prevent side to side movement of the axle, such as might occur when cornering. If the axle contains drive train components, as is common with rear wheel drive vehicles, it is referred to as a "live axle." Axles located by control arms generally use coil springs.
I-Beam (also known as Swing Arm): The I-Beam type suspension isn't used often these days, but it does provide a rugged suspension for use in trucks. This system uses a long transversally mounted beam, hinged toward the opposite end of the vehicle to allow for the up and down movement of the wheel. A second beam is mounted nearly parallel to the vehicle and serves to prevent the I beam from moving forward and back. The long beam allows for longer up and down travel than the shorter A-Arm type suspension systems, which makes it desirable for driving over rough terrain.
MacPherson Strut: This type of suspension is similar to the A-Arm system, but only uses one arm. The bottom A-shaped arm locates the wheel relative to the car, while an upward-pointing shaft houses the spring and the shock absorber together. This type of system is simple, and is generally lighter than other systems. It also takes up less space, since there is no upper control arm to accomodate. It is popular on smaller vehicles, due to space considerations. The only hinging action is by the lower arm; the strut serves only to allow up and down motion.
Other Suspension components:
Anti-sway bars: These bars are designed to attach the two sides of the suspension on either the front or rear of a car together. Thier purpose is to prevent a car from leaning too much while cornering. Originally, they were only used in performance cars, but can be found on virtually all cars today. Their operation is similar to that of a torsion bar, but instead of being mounted between the car's frame and its suspension, they are mounted between the suspension parts on the two sides of a car. They work by partially equalizing the compression of the springs at both sides of either the front or rear of the car. For example, suppose a car is rounding a turn. Without the anti-sway bar, the outside spring will compress greatly while the inside spring will be unloaded and will extend. The anti-sway bar, due to its attachment scheme, will limit the amount of difference between the two springs, and will serve to allow the car to remain more flat while cornering. This helps keep the tires more square on the ground and increases driver comfort. By keeping the tire treads more square, higher cornering speeds can be acheived.
Panhard rod: This simple rod extends from one end of a solid axle to the opposite side of the vehicle where it is mounted to the frame. Its purpose is to keep the axle from moving from side to side while cornering. It is hinged at both ends so that the suspension can still move up and down, but side movement would require that the bar changes length, which it can't since it is solid. This part is vital for locating the axle if no other means is employed. It is helpful on vehicles with leaf springs as well.
Any vehicle with independent suspension must be correctly aligned to work properly. Alignments deal with how the tires are pointed and whether or not they are tilted to one side or the other. The following is a brief description of the various angles measured in an alignment:
Toe-in and Toe-out: This is the simplest concept to understand, since the name implies how the tires point under either condition. If tires are toed in, the front of the tires are closer together than the rear. When toed out, the front of the tires are farther apart than the rear of the tires. Either case will result in uneven tire wear and wandering when driven straight. Ideally, toe should be very close to zero, though a minute amount of toe-in may be desirable to account for tolerances in the suspension and flexing of the various rubber bushings which will tend to even out the toe when driving at speed.
Camber angle: This angle measures the distance between the tops of the tires and the bottom. In other words, it measures whether the tires are standing straight up or leaning inward towards the car or outward away from the car. This angle can be positive or negative, and is specified by the design of the vehicle. In positive camber, the tops of the wheels are moved inward slightly, while the opposite is true of negative camber. It should be noted that in any suspension other than the solid axle, the camber angle will change when the suspension is compressed.
Caster angle: This measurement can best be described by looking at the casters on a piece of furniture. The purpose of caster is to promote centering of the steering. Just as a caster on a cart or other movable furniture will tend to position itself, the caster angle on a car is designed to nudge the wheels to the straight-ahead position. This helps maintain a straight track when travelling at speed, and also allows the car to straighten out more easily after turning.
Steering Axis Inclination: This mouthful describes the angle that the steering pivot axis takes relative to the angle of the tire. When looking at a car from straight in front, the tires appear to be standing pretty much straight up. That is, they are mounted at a 90 degree angle to the ground, give or take a couple of degrees of camber angle. The steering axis can be thought of as an invisible shaft that runs through the upper and lower pivot points (called ball joints on an A-Arm suspension) that mount the control arms to the wheel knuckle/hub assembly. As the wheels of the car are turned through the steering wheel, this is the axis upon which the wheels pivot. This axis isn't exactly parallel to that of the wheels, but is at an angle such that if you drew a straight line to the ground, it would strike the ground at a point somewhere near the middle of where the tire tread meets the ground. The angle between perpendicular and that of the steering axis is called the steering axis inclination. This angle exists by design. If the steering axis were straight up and down, every bump a driver hit would pull the steering wheel in that direction. Conversely, if the axis were inclined too far, bumps would tend to pull the wheel in the opposite direction. So by design, the suspension is generally built such that this axis intersects the ground within the tire contact patch, minimizing "bump steer".