When people think of automobile performance, they normally think of horsepower, torque and zero-to-60 acceleration. But all of the power generated by a piston engine is useless if the driver can't control the car. That's why automobile engineers turned their attention to the suspension system almost Contact online >>
When people think of automobile performance, they normally think of horsepower, torque and zero-to-60 acceleration. But all of the power generated by a piston engine is useless if the driver can''t control the car. That''s why automobile engineers turned their attention to the suspension system almost as soon as they had mastered the four-stroke internal combustion engine.
The job of a car suspension is to maximize the friction between the tires and the road surface, to provide steering stability with good handling and to ensure the comfort of the passengers. In this article, we''ll explore how car suspensions work, how they''ve evolved over the years and where the design of suspensions is headed in the future.
If a road was perfectly flat, with no irregularities, suspensions wouldn''t be necessary. But roads are far from flat. Even freshly paved highways have subtle imperfections that can interact with the wheels of a car. It''s these imperfections that apply forces to the wheels. According to Newton''s Laws of Motion, all forces have both magnitude and direction. A bump in the road causes the wheel to move up and down perpendicular to the road surface. The magnitude, of course, depends on whether the wheel is striking a giant bump or a tiny speck. Either way, the car wheel experiences a vertical acceleration as it passes over an imperfection.
Without an intervening structure, all of the wheel''s vertical energy is transferred to the frame, which moves in the same direction. In such a situation, the tires can lose contact with the road completely. Then, under the downward force of gravity, the tires can slam back into the road surface. What you need is a system that will absorb the energy of the vertically accelerated wheel, allowing the frame and body to ride undisturbed while the tires follow bumps in the road.
The study of the forces at work on a moving car is called vehicle dynamics, and you need to understand some of these concepts in order to appreciate why a suspension is necessary in the first place. Most automobile engineers consider the dynamics of a moving car from two perspectives:
These two characteristics can be further described in three important principles — road isolation, road holding and cornering. The table below describes these principles and how engineers attempt to solve the challenges unique to each.
A car''s suspension, with its various components, provides all of the solutions described.
Let''s look at the parts of a typical suspension, working from the bigger picture of the chassis down to the individual components that make up the suspension proper.
The suspension of a car is actually part of the chassis, which comprises all of the important systems located beneath the car''s body. These systems include:
So the suspension is just one of the major systems in any vehicle.
With this big-picture overview in mind, it''s time to look at the three fundamental components of any suspension: springs, dampers and sway bars.
Today''s springing systems are based on one of four basic designs:
The sprung mass is the mass of the vehicle supported on the springs, while the unsprung mass is loosely defined as the mass between the road and the suspension springs. The stiffness of the springs affects how the sprung mass responds while the car is being driven. Loosely sprung cars, such as luxury cars (think Mercedes-Benz C-Class), can swallow bumps and provide a super-smooth ride; however, such a car is prone to dive and squat during braking and acceleration and tends to experience body sway or roll during cornering. Tightly sprung cars, such as sports cars (think Mazda Miata MX-5), are less forgiving on bumpy roads, but they minimize body motion well, which means they can be driven aggressively, even around corners.
So, while springs by themselves seem like simple devices, designing and implementing them on a car to balance passenger comfort with handling is a complex task. And to make matters more complex, springs alone can''t provide a perfectly smooth ride. Why? Because springs are great at absorbing energy, but not so good at dissipating it. Other structures, known as dampers, are required to do this.
Unless a dampening structure is present, a car spring will extend and release the energy it absorbs from a bump at an uncontrolled rate. The spring will continue to bounce at its natural frequency until all of the energy originally put into it is used up. A suspension built on springs alone would make for an extremely bouncy ride and, depending on the terrain, an uncontrollable car.
Enter the shock absorber, or snubber, a device that controls unwanted spring motion through a process known as dampening. Shock absorbers slow down and reduce the magnitude of vibratory motions by turning the kinetic energy of suspension movement into heat energy that can be dissipated through hydraulic fluid. To understand how this works, it''s best to look inside a shock absorber to see its structure and function.
When the car wheel encounters a bump in the road and causes the spring to coil and uncoil, the energy of the spring is transferred to the shock absorber through the upper mount, down through the piston rod and into the piston. Holes perforate the piston and allow fluid to leak through as the piston moves up and down in the pressure tube. Because the holes are relatively tiny, only a small amount of fluid, under great pressure, passes through. This slows down the piston, which in turn slows down the spring.
Shock absorbers work in two cycles — the compression cycle and the extension cycle. The compression cycle occurs as the piston moves downward, compressing the hydraulic fluid in the chamber below the piston. The extension cycle occurs as the piston moves toward the top of the pressure tube, compressing the fluid in the chamber above the piston. A typical car or light truck will have more resistance during its extension cycle than its compression cycle. With that in mind, the compression cycle controls the motion of the vehicle''s unsprung weight, while extension controls the heavier, sprung weight.
All modern shock absorbers are velocity-sensitive — the faster the suspension moves, the more resistance the shock absorber provides. This enables shocks to adjust to road conditions and to control all of the unwanted motions that can occur in a moving vehicle, including bounce, sway, brake dive and acceleration squat.
Another common dampening structure is the strut — basically a shock absorber mounted inside a coil spring. Struts perform two jobs: They provide a dampening function like shock absorbers, and they provide structural support for the vehicle suspension. That means struts deliver a bit more than shock absorbers, which don''t support vehicle weight — they only control the speed at which weight is transferred in a car, not the weight itself.
Because shocks and struts have so much to do with the handling of a car, they can be considered critical safety features. Worn shocks and struts can allow excessive vehicle-weight transfer from side to side and front to back. This reduces the tire''s ability to grip the road, as well as handling and braking performance.
Sway bars (also known as anti-roll bars) are used along with shock absorbers or struts to give a moving automobile additional stability. A sway bar is a metal rod that spans the entire axle and effectively joins each side of the suspension together.
When the suspension at one wheel moves up and down, the sway bar transfers movement to the other wheel. This creates a more level ride and reduces vehicle sway. In particular, it combats the roll of a car on its suspension as it corners. For this reason, almost all cars today are fitted with sway bars as standard equipment, although if they''re not, kits make it easy to install the bars at any time.
So far, our discussions have focused on how springs and dampers function on any given wheel. But the four wheels of a car work together in two independent systems — the two wheels connected by the front axle and the two wheels connected by the rear axle. That means that a car can and usually does have a different type of suspension on the front and back.
Much is determined by whether a rigid axle binds the wheels or if the wheels are permitted to move independently. The former arrangement is known as a dependent system, while the latter arrangement is known as an independent system. In the following sections, we''ll look at some of the common types of front and back suspensions typically used on mainstream cars.
Dependent front suspensions have a rigid front axle that connects the front wheels. Basically, this looks like a solid bar under the front of the car, kept in place by leaf springs and shock absorbers. Common on trucks, dependent front suspensions haven''t been used in mainstream cars for years.
In this setup, the front wheels are allowed to move independently. The MacPherson strut, developed by Earle S. MacPherson of General Motors in 1947, is the most widely used front suspension system, especially in cars of European origin.
The MacPherson strut combines a shock absorber and a coil spring into a single unit. This provides a more compact and lighter suspension system that can be used for front-wheel drive vehicles.
The double-wishbone suspension, also known as an A-arm suspension or control-arm suspension, is another common type of front independent suspension.
While there are several different possible configurations, this design typically uses two wishbone-shaped arms to locate the wheel. Each wishbone, which has two mounting positions to the frame and one at the wheel, bears a shock absorber and a coil spring to absorb vibrations. Double-wishbone suspensions allow for more control over the camber angle of the wheel, which describes the degree to which the wheels tilt in and out. They also help minimize roll or sway and provide for a more consistent steering feel. Because of these characteristics, the double-wishbone suspension is common on the front wheels of larger cars.
Now let''s look at some common rear suspensions.
If a solid axle connects the rear wheels of a car, then the suspension is usually quite simple — based either on a leaf spring or a coil spring. In the former design, the leaf springs clamp directly to the drive axle. The ends of the leaf springs attach directly to the frame, and the shock absorber is attached at the clamp that holds the spring to the axle. For many years, American car manufacturers preferred this design because of its simplicity.
The same basic design can be achieved with coil springs replacing the leaves. In this case, the spring and shock absorber can be mounted as a single unit or as separate components. When they''re separate, the springs can be much smaller, which reduces the amount of space the suspension takes up.
If both the front and back suspensions are independent, then all of the wheels are mounted and sprung individually, resulting in what car advertisements tout as "four-wheel independent suspension." Any suspension that can be used on the front of the car can be used on the rear, and versions of the front independent systems described in the previous section can be found on the rear axles. Of course, in the rear of the car, the steering rack — the assembly that includes the pinion gear wheel and enables the wheels to turn from side to side — is absent. This means that rear independent suspensions can be simplified versions of front ones, although the basic principles remain the same.
Next, we''ll look at the suspensions of specialty cars.
For the most part, this article has focused on the suspensions of mainstream front- and rear-wheel-drive cars — cars that drive on normal roads in normal driving conditions. But what about the suspensions of specialty cars, such as hot rods, racers or extreme off-road vehicles? Although the suspensions of specialty autos obey the same basic principles, they do provide additional benefits unique to the driving conditions they must navigate. What follows is a brief overview of how suspensions are designed for three types of specialty cars — Baja Bugs, Formula One racers and American-style hot rods.
The Volkswagen Beetle, or Bug, was destined to become a favorite among off-road enthusiasts. With a low center of gravity and engine placement over the rear axle, the two-wheel-drive Bug handles off-road conditions as well as some four-wheel-drive vehicles. Of course, the VW Bug isn''t ready for off-road conditions with its factory equipment. Most Bugs require some modifications, or conversions, to get them ready for racing in harsh conditions like the deserts of Baja California.
The Formula One racing car represents the pinnacle of automobile innovation and evolution. Lightweight, composite bodies, powerful V10 engines and advanced aerodynamics have led to faster, safer and more reliable cars.
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