How Car Suspensions Work Table of Contents:
› Introduction to How Car Suspensions Work
› Vehicle Dynamics
› The Chassis
› Springs: Sprung and Unsprung Mass
› Dampers: Shock Absorbers
› Dampers: Struts and Anti-sway Bars
› Suspension Types: Front
› Suspension Types: Rear
› Specialized Suspensions: The Baja Bug
› Specialized Suspensions: Formula One Racers
› Specialized Suspensions: Hot Rods
› The Future of Car Suspensions
› Lots More Information
› Compare Prices for Car Suspensions
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.
Photo courtesy Honda Motor Co., Ltd.
Double-wishbone suspension on Honda Accord 2005 Coupe
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.
Vehicle Dynamics If a road were 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 wheel's vertical energy is transferred to the frame, which moves in
the same direction. In such a situation, the wheels can lose contact with the road completely. Then,
under the downward force of gravity, the wheels 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 wheels 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:
? Ride - a car's ability to smooth out a bumpy road
? Handling - a car's ability to safely accelerate, brake and corner
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.
Principle Definition Goal Solution
from road Allow the vehicle bumps and The vehicle's ability to Road body to ride dissipate it absorb or isolate road Isolation undisturbed while without causing shock from the traveling over undue passenger compartment rough roads. oscillation in
The degree to which a
car maintains contact
with the road surface in
various types of
directional changes and Keep the tires in Minimize the in a straight line contact with the transfer of (Example: The weight of ground, because vehicle weight a car will shift from the it is the friction from side to rear tires to the front tires between the tires side and front during braking. Because and the road that to back, as this the nose of the car dips affects a vehicle's transfer of toward the road, this type ability to steer, weight reduces of motion is known as brake and the tire's grip "dive." The opposite Road Holding accelerate.on the road. effect -- "squat" -- occurs
which shifts the weight of
the car from the front tires
to the back.)
roll, which occurs
as centrifugal Transfer the force pushes weight of the outward on a Cornering car during car's center of The ability of a vehicle to cornering from gravity while travel a curved path the high side of cornering, raising the vehicle to one side of the the low side. vehicle and
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:
? The frame - structural, load-carrying component that supports the car's engine and body, which
are in turn supported by the suspension
? The suspension system - setup that supports weight, absorbs and dampens shock and helps
maintain tire contact
? The steering system - mechanism that enables the driver to guide and direct the vehicle
? The tires and wheels - components that make vehicle motion possible by way of grip and/or
friction with the road
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:
? 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
Photo courtesy Car Domain
? 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 Photo courtesy HowStuffWorks Shopper
1985. They are still used today on most trucks Leaf spring
and heavy-duty vehicles.
? 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.
Photo courtesy HowStuffWorks Shopper
? 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.
Photo courtesy HSW Shopper
Air springs 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. Springs: 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
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.
Sixteenth-century wagons and carriages tried to solve the problem of
"feeling every bump in the road" by slinging the carriage body from leather
straps attached to four posts of a chassis that looked like an upturned table.
Because the carriage body was suspended from the chassis, the system
came to be known as a "suspension" -- a term still used today to describe
the entire class of solutions. The slung-body suspension was not a true
springing system, but it did enable the body and the wheels of the carriage
to move independently.
Semi-elliptical spring designs, also known as cart springs, quickly replaced
the leather-strap suspension. Popular on wagons, buggies and carriages,
the semi-elliptical springs were often used on both the front and rear axles.
They did, however, tend to allow forward and backward sway and had a
high center of gravity.
By the time powered vehicles hit the road, other, more efficient springing
systems were being developed to smooth out rides for passengers.
Dampers: Shock Absorbers 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 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
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
Dampers: Struts 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.
Common strut design 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.
Photo courtesy HSW Shopper
Anti-sway bars 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.
Suspension Types: Front
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
Front Suspension - Dependent Systems Dependent front suspensions have a rigid front axle that connects the front wheels. Basically, this looks