chap86 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 86, the reader should
be able to:

•
•
•

Prepare for ASE Suspension and Steering (A4) certification
test content area “B” (Suspension System Diagnosis and
Repair).

Explain how to perform a road test, a dry park test, a visual
inspection, and a bounce test.
Discuss the procedures for testing load-carrying and
follower-type ball joints.

Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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OBJECTIVES:
After studying Chapter 86, the reader should
be able to:

•

•
•

Describe ball joint replacement procedures.
List the steps required to replace control arm and stabilizer
bar bushings.
Explain routine service procedures of the suspension
system.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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KEY TERMS:
a-arm

cow catcher • cuppy tire wear
dry park test • durometer
indicator ball joints
kingpin • pinch bolt • radius rod
SLA • steering knuckle • strut suspension
twin I-beam

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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FRONT SUSPENSION TYPES
Solid Axles Early cars and trucks used a solid (or straight) front
axle to support the front wheels. A solid-axle front suspension is
very strong and is still being used in the manufacture of medium

and heavy trucks.
The main disadvantage of solid-axle design is its lack of ride
quality. When one wheel hits a bump or dip in the road, the forces
are transferred through the axle to the opposite wheel.
Solid axles are currently used in the rear of most vehicles.
See Figure 86–1.
Continued
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Figure 86–1 Most early vehicles used single straight axles.

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Kingpins To allow the front wheels to rotate for steering the
end of many solid I-beam or tube axles have kingpins. These
are hardened steel pins that attach the steering knuckle to the
front axle, allowing the front wheels to move for steering.
Kingpins usually have grease fittings to lubricate the kingpin
bushings. Failure to keep these bushings lubricated with
chassis grease can cause wear and freeplay or can cause
the pins to become galled (seized or frozen), resulting
in hard steering and/or loud noise while turning.
See Figure 86–2.

Continued

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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Figure 86–2
Typical kingpin used with a solid axle.

Continued
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Twin I-Beams A twin I-beam front suspension was used for
over 30 years on Ford pickup trucks and vans. Strong steel twin
beams that cross provide independent front suspension operation
with the strength of a solid front axle.
Early versions used kingpins; later models used ball joints to
support the steering knuckle and spindle. Coil springs are
usually used on twin I-beam suspensions, even though the
original design and patent used leaf springs.
A radius rod is attached to each beam and is anchored to the
frame of the truck using rubber bushings, which allow the front
axle to move up and down while still insulating road noise and
vibration from the frame and body.
See Figure 86–3.
Continued
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Figure 86–3 Twin I-beam front suspension.

Continued
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Radius Rod Bushing Noise
When the radius rod bushing on a Ford truck or van deteriorates, the most common
complaint from the driver is noise. Besides causing tire wear, worn or defective
radius rod bushing deterioration can cause:

1. A clicking sound when braking (sounds as if the brake caliper may
be loose).
2. A clunking noise when hitting bumps. When the bushing
deteriorates, the axles can move forward and backward with less
control. Noise is the first sign that something is wrong. Without
proper axle support, handling and cornering can also be affected.

Figure 86–4 The rubber radius rod bushing absorbs
road shocks and helps isolate road noise.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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SHORT/LONG-ARM SUSPENSIONS
The short/long-arm suspension uses a short upper control arm and
a longer lower control arm. It is referred to as the SLA-type
suspension.
The two main links are the upper and lower control arms. The upper
control arm is the shorter one. This suspension goes by a variety of
names, including unequal-arm suspension, double-wishbone
suspension, or A-arm suspension.
When an SLA suspension uses a straight, two-point lower control
arm, there is almost always a strut rod that braces the lower arm
against the frame. The strut rod can attach to the frame at a point
either forward or to the rear of the control arm. See Figure 86–5.
Continued
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Figure 86–5 The upper control arm is shorter than the lower control arm on a short/ long-arm
(SLA) suspension.

Continued
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A strut rod, also called a reaction rod, provides support to the axle
during braking and acceleration forces.
Using a strut rod provides triangulation between the wheel and the
frame. Triangulation means that the front suspension has a threepoint brace that resists forces from every direction, while still
allowing the wheels to pivot on the steering axis.
Locating the coil spring on the lower control arm is the most
common SLA-type suspension configuration.
See Figure 86–6.

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Figure 86–6 A typical SLA front suspension using coil springs.

Continued
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The upper control arm is A-shaped, with two mounting points at the
frame side member and one at the steering knuckle.
The inboard ends of the arm attach to a pivot bar that is rigidly bolted to

the frame, and the outboard end connects to the steering knuckle with a
ball joint. Bushings between the inner arm mounts and the pivot bar allow
the arm to swing vertically.
The lower control arm is also A-shaped with two inboard pivot mounts
and a ball joint connection at the knuckle. Bolts attach the inboard ends of
the lower arm to brackets that extend from the bottom of the frame
crossmember. Bushings on the inboard mounts allow the arm to pivot
vertically.
See Figure 86–7.

Continued
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The coil springs seat between the
lower control arm and a bracket
below the frame side member.

Figure 86–7 An SLA-type suspension with
the coil spring placed on top of the upper
control arm.

The shock absorber attaches to
the lower control arm and the
frame side member.
In a typical coil spring on the
upper control arm design, a
portion of the unit-body wheel
well is reinforced to withstand
spring force and act as the upper
spring seats.

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The forward end of the torsion bar has external splines and shares
the splined socket of the torque arm with the control arm through a
bolt. Because the torque arm is rigidly bolted to the control arm,
motion transfers to the torsion bar whenever the control arm pivots.
The torsion bar extends rearward from the control arm and connects
to an anchor arm that attaches to a frame crossmember.
The torsion bar splines to the anchor arm and the anchor arm is
secured to the frame by a bolt and adjusting nut.
See Figure 86–8.

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Short/long-arm suspensions, especially in trucks, use longitudinal
torsion bars. The adjusting nut varies the spring force to establish
the ride height of the vehicle.
Figure 86–8 A torsion bar SLA suspension can use either the lower or the upper control arm.

Continued
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STRUT SUSPENSION
Strut suspension can be of
several types.
Figure 86–9 A typical MacPherson strut
showing all of the components of the
assembly.

A MacPherson strut includes
the main, load-carrying
suspension spring that
transfers the weight of
the body to the wheel.

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A MacPherson strut typically incorporates an upper and a lower
spring seat, a shock absorber mount and dust cap, a dust cover for
the piston rod, and a bump stop.
The upper mount secures the upper spring seat to the strut tower. A
rubber bushing at the top of the strut absorbs vibrations. In most
applications, a bearing on a front-wheel strut allows it to rotate on
the vertical steering axis without rubbing against the strut tower
when the steering knuckle turns.
The lower spring seat is attached to the strut casing. The piston rod
dust cover is similar to the dust cover on a conventional shock
absorber, and a bump stop at the top of the piston rod keeps the
strut from bottoming out during suspension jounce.
See Figure 86–10.
Continued
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Figure 86–10 The modified strut front suspension is similar to a MacPherson strut suspension
except that the coil spring is seated on the lower control arm and is not part of the strut
assembly.

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A modified strut is used on some GM vehicles. Except for the lack
of a spring and spring seats, modified struts are basically the same
as a MacPherson strut. When used on front suspension, a modified
strut also rotates on the steering axis. A multilink front suspension
uses two control arms as well as a structural strut assembly. This is
also called a strut/SLA or long spindle, short/long-arm suspension.

Figure 86–11 Multilink front suspension design varies depending on the vehicle manufacturer.
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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SERVICING THE SUSPENSION SYSTEM
Suspension systems are manufactured to provide years of troublefree service with a minimum amount of maintenance. The
suspension system is often “invisible” or “transparent” to the
driver because the vehicle rides and handles as expected.
The smart technician should always road test any vehicle before
and after servicing.
The purpose of any diagnosis is to
eliminate known good components.
See the suspension diagnostic chart for a list of components that
can cause the problem or customer complaint.
Continued
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See the chart on Page 1076 of your textbook.
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