chap85 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 85, the reader should
be able to:

• Prepare for ASE Suspension and Steering (A4)
•
•

certification test content area “B” (Suspension
System Diagnosis and Repair).
List various types of suspensions and their

component parts.
Explain how coil, leaf, and torsion bar springs
work.

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OBJECTIVES:
After studying Chapter 85, the reader should
be able to:

•
•

Describe how suspension components
function to allow wheel movement up and
down and provide for turning.
Describe how shock absorbers control spring
forces.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
air spring • anti-dive • anti-squat
ball joints • bulkhead • bump stop
center bolt • coil springs • composite leaf spring • control

arms • cradle
full frame • gVW • hooke’s law
independent suspension • insulators
kingpin • ladder frame • lateral links
leaf springs • load-carrying ball joint
Continued
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KEY TERMS:
mono leaf • non-load-carrying ball joint
perimeter frame • platforms • rebound clips
shackles • shock absorbers • space frame • spring pocket
• spring rate • springs • sprung weight • stabilizer bars •

steering knuckles • stress riser • strut rod • struts • stubtype frame
torsion bar
unit-body • unsprung weight
wheel rate
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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Street-driven cars and trucks use a suspension system to
keep the tires on the road and to provide acceptable riding
comfort.
A vehicle with a solid suspension, or no suspension, would
bounce off the ground when the tires hit a bump.
If the tires are off the ground, even for a fraction of a second,
loss of control is possible.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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FRAME CONSTRUCTION
Frame construction consists of channel­shaped steel beams 
welded and/or fastened together. The frame supports all the 
“running gear”, including the engine, transmission, rear axle 
assembly (if rear­wheel drive), and all suspension components.
Referred to as full frame, it is so complete that most vehicles can 
usually be driven without the body. Most trucks and larger rear­
wheel­drive cars use a full frame.
Ladder Frame  A common name for a type of perimeter frame 
where the transverse (lateral) connecting members are straight 

across is ladder frame. When viewed with the body removed, the 
frame resembles a ladder. Most pickup trucks are constructed with 
a ladder­type frame. See Figures 85–1 and 85–2
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Figure 85–1 A typical truck frame is an excellent example of a ladder-type frame. The two side =
members are connected by a crossmember.

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Perimeter Frame
A perimeter frame consists 
of welded or riveted frame 
members around the entire 
perimeter of the body. 

Figure 85–2 Rubber cushions used in body or
frame construction isolate noise and vibration
from traveling to the passenger compartment.

This means frame members 
provide support underneath 
the sides as well as for the 
suspension and suspension 

components.

Continued
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Stub­Type Frames
A stub­type frame is a partial 
frame often used on unit­body 
vehicles to support power train 
and suspension components. 

(a)

It is also called a cradle on 
many front­wheel­drive 
vehicles. 
Figure 85–3 (a) Separate body and frame
construction; (b) unitized construction: the
small frame members are for support of the
engine and suspension components. Many
vehicles attach the suspension components
directly to the reinforced sections of the body
and do not require the rear frame section.

(b)

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Unit­Body Construction Unit­body  (called unibody) combines 
the body with the structure of the frame. The body supports the 
engine and drive line components, as well as the suspension and 
steering components. The body is composed of many individual 
stamped­steel panels welded together.
The strength of this type of construction lies in the shape of the 
assembly. The typical vehicle uses 300 separate, different stamped 
steel panels that are spot­welded to form a vehicle’s body. 
See Figure 85–4.

NOTE: A typical vehicle contains about 10,000 individual parts.
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Figure 85–4 Welded metal sections create a platform that combines the body with the frame
using unit-body construction.

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Space Frame Construction  Formed sheet steel used to construct a 
framework for the entire vehicle is called space frame. The vehicle 
is drivable without the body, which uses plastic or steel panels to 

cover the steel framework.

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What Does GVM Mean?
GVW, gross vehicle weight is the weight of the vehicle plus the weight of
all passengers the vehicle is designed to carry (150 lb [68 kg] each), plus
the maximum allowable payload or luggage load.
Curb weight is the weight of a vehicle when wet, meaning with a full tank of
fuel and all fluids filled, but without passengers or cargo (luggage). Model
weight is the weight of a vehicle wet and with passengers.
The GVW is found stamped on a plate fastened to the doorjamb of most

vehicles. A high GVW rating does not mean that the vehicle itself weighs a
lot more than other vehicles. For example, a light truck with a GVW of
6,000 lbs (2,700 kg) will not ride like an old 6,000-lb luxury car. In fact, a
high GVW rating usually requires stiff springs to support the payload; these
stiff springs result in a harsh ride.
Often techs are asked to correct a harsh-riding truck that has a high GVW
rating. The tech can only check that everything in the suspension is
satisfactory and then try to convince the owner that a harsher-than-normal
ride is the result of a higher GVW rating.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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PLATFORMS
The platform of any vehicle is the basic size and shape. Various 

vehicles of different makes can share the same platform, and 
therefore many of the same drive train (engine, transmission, and 
final drive components) and suspension and steering components.
A platform of a unit­body vehicle includes all major sheet­metal 
components that form the load­bearing structure of the vehicle, 
including the front suspension and engine­supporting sections.
The area separating the engine compartment from the passenger 
compartment is called the bulkhead. The height and location of 
the bulkhead panel determines the shape of the rest of the vehicle.
Other components of vehicle platform design that affect hand­ling 
and ride are the track and wheelbase of the vehicle.
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Wheelbase of a vehicle is distance between the center of the front 
wheel and the center of the rear wheel, as viewed from the side.
A vehicle with a long wheelbase tends to ride smoother than a 
vehicle with a short one.
Track of a vehicle is distance between
the wheels, as viewed from the front or rear.
A wide­track vehicle is a vehicle with a wide wheel stance; this 
increases the stability of the vehicle, especially when cornering.
Hollander Interchange Manual
Most salvage businesses that deal with wrecked vehicles use a reference book called
the Hollander Interchange Manual. In this yearly publication, every vehicle part is given
a number. If a part from another vehicle has the same Hollander number, then the parts
are interchangeable.
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UNSPRUNG WEIGHT
A suspension system has allow the wheels to move up and down 
quickly over bumps and dips without affecting the entire weight of 
the car or truck. The lighter the total weight of the components, the 
better the handling and ride. This is called unsprung weight.
The idea of very light weight resulted in magnesium wheels for 
racing cars, very light yet strong. Aftermarket wheels that  
resemble racing wheels are often referred to as mag wheels. 
Unsprung weight should be kept as low as possible.
Sprung weight is the term used to identify the weight of the car or 
truck that does not move up and down and is supported or sprung 
by the suspension.

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TYPES OF SUSPENSIONS
Early suspension systems on
old horse wagons, buggies,
and older vehicles used a solid 
axle for front and rear wheels. 

Figure 85–6 When one wheel hits a bump or
drops into a hole, both left and right wheels
are moved. Because both wheels are affected,
the ride is often harsh and feels stiff.

Figure 85–5 Solid I-beam axle with leaf springs.

If one wheel hit a bump, the 
other wheel was affected, as 
shown above.

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Most vehicles today use a separate control­arm­type of suspension 
for each front wheel, which allows for movement of one front 
wheel without affecting the other front wheel. This type of front 
suspension is called independent suspension. 
Figure 85–7 A typical independent front suspension used on a rear-wheel-drive vehicle. Each
wheel can hit a bump or hole in the road independently without affecting the opposite wheel.

Continued
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Springs  A suspension spring serves two purposes. It acts as a 
buffer between the suspension and frame to absorb vertical wheel 
and suspension movement without passing it on to the frame. Each 
spring transfers part of the vehicle weight to the suspension 
component it rests on, which transfers it to the wheels.
All springs give way to absorb the vertical force of the moving 
wheel during jounce, then release that force during rebound as they 
return to their original shape and position. Leaf springs flatten, coil 
and air springs compress, and torsion bars twist. 
Spring Materials  Most are made of a tempered steel alloy known 
as spring steel, usually chrome silicon or chrome­vanadium alloy. 
Tempering is controlled heating and cooling metal to increase the 
ability of the metal to return to, or spring back to, its original shape 
after being twisted or compressed.
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HOOKE’S LAW
All suspensions use springs with a common characteristic 
described Robert Hooke (1635–1703). An English physicist, he 
discovered force the characteristics of springs 
Hooke’s Law  
Deflection (movement or 
deformation) of a spring
is directly proportional
to the applied force.

Figure 85–8 This spring was depressed 4 inches due to a weight of 2,000 Ib. This means that
this spring has a spring rate (K) of 500 Ib per inch (2000 4 in.500 Ib./in.)
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COIL SPRINGS
Coil springs are made of special 
round spring steel wrapped in a 
helix shape. 
Figure 85–9 The spring rate of a coil spring is
determined by the diameter of the spring and
diameter of the steel used in its construction
plus number of coils and free length (height).

Characteristics of a coil spring 
(strength, etc.) depend on:
1.
2.
3.
4.

Coil diameter
Number of coils
Height of spring
Diameter of the steel coil
that forms the spring 

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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When a coil spring (for example) is depressed 1 in., it pushes back 
with a certain force (in pounds), such as 400 pounds. If the spring 
is depressed another inch, force exerted by the spring is increased 
by another 400 pounds.

The spring rate or force 
constant for this spring is
“400 lb per inch,” usually 
symbolized by the letter K.
The spring rate (K) for 
coil springs is expressed 
by the formula at right.
Since the force constant is the force per unit of displacement 
(movement), it is a measure of the stiffness of the spring. The 
higher the spring rate (K), the stiffer the spring.
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The larger the diameter of the steel, the “stiffer” the spring.
The shorter the height of the spring, the stiffer the spring.
The fewer the coils, the stiffer the spring.

Springs are designed to provide desired ride and handling and 
come in a variety of spring ends, as shown here:

Figure 85–10
Coil spring ends
are shaped to fit
the needs of a
variety of
suspension
designs.

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Spring Rate  called deflection rate, 
is the weight in pounds it takes to 
compress the spring 1 inch. 
If a 100­lb weight causes a spring
to compress 1 inch, the spring has
a spring rate of 100 lb
A constant­rate spring continues 
to compress at the same rate 
throughout its complete range of 
deflection. 
If a constant­rate spring will 
compress one inch under a 100­
pound load, it will compress two 
inches under a 200­pound load.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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