Suspension System1

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Introduction to How Car Suspensions Work

Photo courtesy Honda Motor Co., Ltd.Double-wishbone suspension on Honda Accord 2005 Coupe.

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 providesteering stability with good handling and to ensure the comfort of the passengers. In this article, we'llexplore 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 were perfectly flat, with no irregularities, suspensions wouldn't be necessary. But roads are farfrom 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 downperpendicular to the road surface. The magnitude, of course, depends on whether the wheel is striking agiant 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 inthe same direction. In such a situation, the wheels can lose contact with the road completely. Then, underthe 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.


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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:

y Ride - a car's ability to smooth out a bumpy roady Handling - a car's ability to safely accelerate, brake and cornery 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

Road

Isolation

The vehicle's ability to

absorb or isolate roadshock from the passenger

compartment

Allow the vehiclebody to ride

undisturbed while

traveling over

rough roads.

Absorb energy

from roadbumps and

dissipate itwithout

causing undue

oscillation inthe vehicle.

Road

Holding

The degree to which a car

maintains contact with theroad surface in various

types of directional

changes and in a straightline (Example: Theweight of a car will shift

from the rear tires to the

front tires during braking.Because the nose of the

car dips toward the road,this type of motion isknown as "dive." The

opposite effect -- "squat" -

- occurs duringacceleration, which shiftsthe weight of the car from

the front tires to the back.)

Keep the tires incontact with theground, because

it is the friction

between the tiresand the road that

affects a vehicle'sability to steer,

brake and

accelerate.

Minimize thetransfer ofvehicle weight

from side to

side and frontto back, as this

transfer ofweight reducesthe tire's grip

on the road.

CorneringThe ability of a vehicle to

travel a curved path

Minimize bodyroll, which occurs

as centrifugal

force pushesoutward on a car'scenter of gravity

while cornering,

raising one sideof the vehicle andlowering the

opposite side.

Transfer theweight of the

car duringcornering from

the high sideof the vehicle

to the lowside.


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A car's suspension, with its various components, provides all of the solutions described.

The suspensionsystem functions are as follows:

y Support the weight of the frame, body, engine, transmission, drive train, passengers,and cargo.

y Provide a smooth, comfortable ride by allowingthe wheels and tires to move up and

down withminimum movement of the vehicle.

y Work with the steering system to help keep thewheels in correct alignment.y Keep the tires in firm contact with the road, evenafter striking bumps or holes in the

road.

y Allow rapid cornering without extreme body roll (vehicle leans to one side).y Prevent excessive body squat (body tilts down inrear) when accelerating or with heavy

loads.

y Prevent excessive body dive (body tilts down inthe front) when braking.

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.

Chassis


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Car Suspension Parts

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:

y The frame - structural, load-carrying component that supports the car's engine and body, whichare in turn supported by the suspension

y The suspension system - setup that supports weight, absorbs and dampens shock and helpsmaintain tire contact

y The steering system - mechanism that enables the driver to guide and direct the vehicle.y 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 anysuspension: springs, dampers and anti-sway bars.

Springs

Today's springing systems are based on one of four basic designs:

y Coil springs - This is the most common type of spring and is, in essence, a heavy-duty torsionbar coiled around an axis. Coil springs compress and expand to absorb the motion of the wheels.


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y Leaf springs - This type of spring consists of several layers of metal (called "leaves") boundtogether 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

heavy-duty vehicles.

y Torsion bars - Torsion bars use the twisting properties of a steel bar to provide coil-spring-likeperformance.

This is how they work: One end of a bar is anchored to the vehicle frame. The other end isattached 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. The torsion bar replaces both Coil spring and Leaf springs in some suspension systems.

The main advantage of the torsion bar over the Coil spring in the Front suspension is the ease of

adjusting front suspension height. Some are mounted longitudinally (i.e., front of car to back ofcar) or transversely (i.e., from the left side to right side of the car).

European carmakers used this system extensively, as did Packard and Chrysler in the United

States, through the 1950s and 1960s.

Torsion bar


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y Air springs - Air springs, which consist of a cylindrical chamber of air positioned between thewheel 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. Airsprings 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 oftenfind 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 looselydefined as the mass between the road and the suspension springs. The stiffness of the springs affects howthe 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 todive 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 aroundcorners.

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, springsalone can't provide a perfectly smooth ride. Why? Because springs are great at absorbing energy, but notso good at dissipating it. Other structures, known as dampers, are required to do this.


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Dampers: ShockAbsorbers

Unless a dampening structure is present, a car spring will extend and release the energy it absorbs from abump at an uncontrolled rate. The spring will continue to bounce at its natural frequency until all of theenergy 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 processknown 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 throughhydraulic fluid. To understand how this works, it's best to look inside a shock absorber to see its structure

and function.

A shock absorber is basically an oil pump placed between the frame of the car and the wheels. The uppermount 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 ofshock 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.


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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 andinto 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 compressioncycle 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 motionof 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.

Dampers: Struts and Anti-sway Bars

Common strut design

Another common dampening structure is the strut -- basically a shock absorber mounted inside a coilspring. 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


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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 criticalsafety 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 BarsAnti-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 andeffectively 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 otherwheel. 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.


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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 thefront 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 adependent 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

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. The

dependent suspension has both left and right wheels attached to the same solid axle. When one tire hits abump in the road, its upward movement causes a slight tilt in the other wheel. With a solid axle setup, thesteering knuckle and wheel spindle assemblies are connected to the axle beam by bronze-bushed

kingpins, or spindle bolts, which provide pivot points for each wheel.


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Independent Front Suspensions

The independent suspension allows one wheel to move up and down with a minimum effect on

the other wheels. Since each wheel is attached to its own suspension unit, movement of one wheel doesNOT cause direct movement of the wheel on the opposite side of the vehicle. With the independent frontsuspension the use of ball joints provides pivot points for each wheel. In operation, the swiveling action

of the ball joints allows the wheel and spindle assemblies to be turned left and right and to move up and

down with changes in road surfaces. This type of suspension is most widely used on modern vehicles.

In this setup, the front wheels are allowed to move independently. The MacPherson strut, developed byEarle 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.


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SUSPENSION SYSTEM COMPONENTSThe basic components of a suspension system are as follows:

y CONTROLARM (a movable lever that fastens the steering knuckle to the vehicleframe or body)

y CONTROL ARM BUSHING (a sleeve, which allows the control arm to move upand down on the frame)

y STRUT ROD (prevents the control arm from swinging to the front or rear of thevehicle)

y BALL JOINTS (a swivel joint that allows the control arm and steering knuckle tomove up and down, as well as side to side)

y SHOCK ABSORBER or STRUT (keeps the suspension from continuing tobounce after spring compression and extension)

y STABILIZER BAR (limits body roll of the vehicle during cornering)y SPRING (supports the weight of the vehicle; permits the control arm and wheel to

move up and down)

ControlArms and Bushings

The control arm, as shown in figure 8-6, holds the steering knuckle, bearing support, or axlehousing in position, as the wheel moves up and down. The outer end of the control arm has a ball joint

and the inner end has bushings. Vehicles, having control arms on the rear suspension, may have bushingson both ends.

The control arm bushings act as bearings, which allow the control arm to move up and down on a

shaft bolted to the frame or suspension unit. These bushings may be either pressed or screwed into theopenings of the control arm.

Strut RodsThe strut rod. as shown in figure 8-6, fastens to the outer end of the lower control arm and to

the frame.This prevents the control arm from swinging toward the rear or front of the vehicle. The frontof the strut rod has rubber bushings that soften the action of the strut rod. These bushings allow a

controlled amount of lower control arm movement while allowing full suspension travel.

Ball JointsThe ball joints are connections that allow limited rotation in every direction and support the

weight of the vehicle. They are used at the outer ends of the control arms where the arms attach to thesteering knuckle. In operation, the swiveling action of the ball joints allows the wheel and steering

knuckle to be turned left or right and to move up and down with changes in road surface.

Since the ball joint must be filled with grease, a grease fitting and grease seal are

normally placed on the joint. The end of the stud on the ball joint is threaded for a large nut. When thenut is tightened, it force fits the tapered stud in the steering knuckle or bearing support.


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ShockAbsorbers and Struts

Shock absorbers are necessary because springs do not "settle down" fast enough. After a spring

has been compressed and released, it continues to shorten and lengthen for a time. Such spring

action on a vehicle would produce a very bumpy and uncomfortable ride. It would also be dangerous

because a bouncing wheel makes the vehicle difficult to control; therefore, a dampening device is

needed to control the spring oscillations. This device is the shock absorber. The most common typeof shock absorber (fig. 8-8) used on modern vehicles is the double-acting, direct-action type, because it

allows the use of more flexible springs.One end of the shock absorber connects to a suspension component, usually a control arm. The

other end fastens to the frame. In this way, the shock absorber piston rod is pulled in and out andresists these movements.

MacPherson strut, is similar to a conventional shock absorber. However, it is longer and has

provisions (brackets and connections) for mounting and holding the steering knuckle (front ofvehicle) or bearing support (rear of vehicle) and spring. The strut assembly consists of a shockabsorber, coil spring (in most cases), and an upper damper unit. The strut assembly replaces the

upper control arm. Only the lower control arm and strut are required to support the front-wheel assembly.In a MacPherson strut type suspension, only one control arm and a strut is used to support each

wheel assembly. A conventional lower control arm attaches to the frame and to the lower ball joint. The ball joint holds the control arm to the steering knuckle or bearing support. The top of the steering

knuckle or bearing support is bolted to the strut. The top of the strut is bolted to the frame or

reinforced body structure. This type of suspension is the most common type used on late model passengervehicles. The advantages are a reduced number of parts in the suspension system, lower unsprung

weight, and a smoother ride.On some vehicles you may find a MODIFIED STRUT SUSPENSION that has the coil

springs mounted on the top of the control arm, not around the strut.


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The double-wishbone suspension (British name), also known as an A-arm (shape of an A or V)

suspension, is another common type of front independent suspension.

While there are several different possible configurations, this design typically uses two wishbone-shapedarms 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 tiltin 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.

Suspension Types: Rear

Historical Suspensions

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 anupturned 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-bodysuspension was not a true springing system, but it did enable the body and the wheels of the carriage tomove 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 ofgravity.

By the time powered vehicles hit the road, other, more efficient springing systems were being developed

to smooth out rides for passengers.

Photo courtesy Honda Motor Co., Ltd.

Double-wishbone suspension on Honda Accord 2005 Coupe


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Dependent 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 atthe 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 andshock 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.

Independent Rear Suspensions

If both the front and back suspensions are independent, then all of the wheels aremounted 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 onthe 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 thatincludes 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 thebasic principles remain the same.


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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 additionalbenefits unique to the driving conditions they must navigate. What follows is a brief overview ofhow suspensions are designed for three types of specialty cars -- Formula One races.

Specialized Suspensions: Formula One Racer

Formula One racecar

The Formula One racing car represents the pinnacle of automobile innovation and evolution.

Lightweight, composite bodies, powerful V10 engines and advanced aerodynamics have led tofaster, safer and more reliable cars.

To elevate driver skill as the key differentiating factor in a race, stringent rules and requirements

govern Formula One racecar design. For example, the rules regulating suspension design say thatall Formula One racers must be conventionally sprung, but they don't allow computer-controlled,

active suspensions. To accommodate this, the cars feature multi-link suspensions, which use amulti-rod mechanism equivalent to a double-wishbone system.

Recall that a double-wishbone design uses two wishbone-shaped control arms to guide eachwheel's up-and-down motion. Each arm has three mounting positions -- two at the frame and one

at the wheel hub -- and each joint is hinged to guide the wheel's motion. In all cars, the primarybenefit of a double-wishbone suspension is control. The geometry of the arms and the elasticity

of the joints give engineers ultimate control over the angle of the wheel and other vehicledynamics, such as lift, squat and dive. Unlike road cars, however, the shock absorbers and coil

springs of a Formula One racecar don't mount directly to the control arms. Instead, they areoriented along the length of the car and are controlled remotely through a series of pushrods and

bell cranks. In such an arrangement, the pushrods and bell cranks translate the up-and-downmotions of the wheel to the back-and-forth movement of the spring-and-damper apparatus.


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The Bose Suspension System

Photo courtesy BOSEBose Suspension Front Module

While there have been enhancements and improvements to both springs and shock absorbers, the

basic design of car suspensions has not undergone a significant evolution over the years. But allof that's about to change with the introduction of a brand-new suspension design conceived by

Bose -- the same Bose known for its innovations in acoustic technologies. Some experts aregoing so far as to say that the Bose suspension is the biggest advance in automobile suspensions

since the introduction of an all-independent design.

How does it work? The Bose system uses a linear electromagnetic motor (LEM) at each wheelin lieu of a conventional shock-and-spring setup. Amplifiers provide electricity to the motors in

such a way that their power is regenerated with each compression of the system. The mainbenefit of the motors is that they are not limited by the inertia inherent in conventional fluid-

based dampers. As a result, an LEM can extend and compress at a much greater speed, virtuallyeliminating all vibrations in the passenger cabin. The wheel's motion can be so finely controlled

that the body of the car remains level regardless of what's happening at the wheel. The LEM canalso counteract the body motion of the car while accelerating, braking and cornering, giving the

driver a greater sense of control.

Unfortunately, this paradigm-shifting suspension won't be available until 2009, when it will be

offered on one or more high-end luxury cars. Until then, drivers will have to rely on the tried-and-true suspension methods that have smoothed out bumpy rides for centuries.

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