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:
1] The frame -
structural, load-carrying component that supports the car's engine and
body, which are in turn supported by the suspension
2] The suspension system
- setup that supports weight, absorbs and dampens shock and helps
maintain tire contact
3] The steering system
- mechanism that enables the driver to guide and direct the vehicle
4] The tires and wheels
- components that make vehicle motion possible by way of grip and/or
friction with the road
5] The front and rear swaybars
- components that make vehicle understeer or oversteer in a turn
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 anti-sway bars.
: Today's springing systems are based on one of four basic designs:
1] Coil springs
- This is the most common type of spring and is, in essence, a
heavy-duty torsion bar coiled around an axis. Coil springs compress and
expand to absorb the motion of the wheels.
2] Leaf springs
- This type of spring consists of several layers of metal (called
"leaves") bound together to act as a single unit. Leaf springs were
first used on horse-drawn carriages and were found on most American
automobiles until 1985. They are still used today on most trucks and
3] Torsion bars
- Torsion bars use the twisting properties of a steel bar to provide
coil-spring-like performance. This is how they work: One end of a bar
is anchored to the vehicle frame. The other end is attached to a
wishbone, which acts like a lever that moves perpendicular to the
torsion bar. When the wheel hits a bump, vertical motion is transferred
to the wishbone and then, through the levering action, to the torsion
bar. The torsion bar then twists along its axis to provide the spring
force. European carmakers used this system extensively, as did Packard
and Chrysler in the United States, through the 1950s and 1960s
4] Air springs
- Air springs, which consist of a cylindrical chamber of air positioned
between the wheel and the car's body, use the compressive qualities of
air to absorb wheel vibrations. The concept is actually more than a
century old and could be found on horse-drawn buggies. Air springs from
this era were made from air-filled, leather diaphragms, much like a
bellows; they were replaced with molded-rubber air springs in the 1930s.
Based on where springs
are located on a car -- i.e., between the wheels and the frame --
engineers often find it convenient to talk about the sprung
mass and the unsprung mass.
Sprung and Unsprung Mass. 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 Lincoln Town Car), 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), are less forgiving on bumpy roads, but they minimize body
motion well, which means they can be driven aggressively, even around
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
A shock absorber is
basically an oil pump placed between the frame of the
car and the wheels. The upper mount of the shock connects to the frame
(i.e., the sprung weight), while the lower mount connects to the axle,
near the wheel (i.e., the unsprung weight). In a twin-tube
design, one of the most common types of shock absorbers, the
upper mount is connected to a piston rod, which in turn is connected to
a piston, which in turn sits in a tube filled with hydraulic fluid. The
inner tube is known as the pressure tube, and the outer tube is known
as the reserve tube. The reserve tube stores excess hydraulic fluid.
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.
Orifices perforate the piston and allow fluid to leak through as the
piston moves up and down in the pressure tube. Because the orifices are
relatively tiny, only a small amount of fluid, under great pressure,
passes through. This slows down the piston, which in turn slows down
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.
and Anti-sway Bars
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.
Anti-sway bars (also known as anti-roll bars) are used along with shock
absorbers or struts to give a moving automobile additional stability.
An anti-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 anti-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 anti-sway bars as standard equipment, although if
they're not, kits make it easy to install the bars at any time.
So far, we 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. Now, 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.
suspension, also known as an A-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.
Suspension Types: Rear
If a solid axle connects the rear wheels of a car, and 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.
Suspensions - 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.