chap87 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 87, the reader should
be able to:

•
•
•

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

Describe the various types and styles of rear
suspension.
Explain the difference among the different
types of rear axles.

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OBJECTIVES:
After studying Chapter 87, the reader should
be able to:

•

•

List the steps necessary to replace rear shock
absorbers.
Explain how to replace rear leaf and coil
springs.

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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KEY TERMS:
axle windup • chapman strut • hotchkiss drive
IRS • live axle • panhard rod
semi-independent suspension • semi-trailing arm • solid

axle
torque arm • track rod • trailing arm
watts linkage

Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
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All suspensions have two basic jobs: keeping the tires
on the ground and providing a smooth ride.

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SOLID REAR AXLES
A solid axle can be used on 
either a rear­wheel­drive or 
front­wheel­drive vehicle. 
Figure 87–1 Solid axles are used on rearwheel-drive vehicles as well as front-wheel
drive vehicles.

On a rear­wheel­drive vehicle, 
a solid rear axle consists of the 
differential gears and axle 
shafts inside a solid housing. 
On a front­wheel­drive, a 
solid axle is a U­shaped or 
tubular beam that may contain 
a torsion bar, rod, or tube.

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Solid axles have some handling characteristics that are inferior to 
those of an independent suspension. Disadvantages of a solid axle 
include the following:

Increased proportion of unsprung weight
Side­to­side road shock transference
Poorer tire adhesion
Increasing the proportion of unsprung weight decreases ride 
quality. Transferring road shock from side­to­side causes wheel 
tramp and shimmy, and poor tire adhesion accelerates tire wear.

See Figure 87–2.

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Figure 87–2 A solid axle supports the springs, so the axle and suspension components are
unsprung weight. When one wheel rides over a bump, the shock transfers through the solid
axle to the opposite side, leading to unstable handling.

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The sprung weight of the frame and body must be heavy to oppose 
the unsprung weight of a solid axle, especially on a driven axle.
Unsprung weight, wheel tramp, and shimmy all reduce tire 
adhesion. The effects of these problems are more noticeable in 
rear­wheel­drive vehicles due to the weight of the rear axle and 
differential assembly.
On a typical front­wheel­drive vehicle, the simple axle beam is not 
heavy enough to decrease ride quality or cause extreme tire wear. 
The rear tires of a front­wheel­drive model always wear more 
slowly than the front tires because the rear axle is so much lighter 
than the power train.
If engine torque is applied to the rear axle to drive the vehicle, the 
axle is referred to as a live axle.
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What is Axle Windup?
Axle windup is a product of the law of physics, which states that every
action produces an equal and opposite reaction. As the axle shafts rotate in
one direction to drive the wheels, the axle housing attempts to rotate in the
opposite direction. The force of this reaction tends to lift the front end of the
vehicle during acceleration.

Axle windup is a particular problem with a solid, driven rear axle because
the axle housing concentrates reacting force. Under extreme acceleration,
the reacting force can actually tilt the drive shaft upward and lift the front
wheels off the ground.
Leaf springs, control arms,

pinion snubbers, and torque
arms all are means of
controlling axle windup.
Figure 87–3 When the axle housing
reacts against the force of axle shaft
rotation, the front of the differential
tilts upward, creating axle windup.
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LEAF SPRING REAR SUSPENSIONS
A leaf spring suspension is a simple system because it does not 
require control arms to brace and position the axle. The leaf 
springs link the axle to the frame and serve two purposes:

Absorbing road shock
Locating the axle under the vehicle
Most rear­wheel­drive trucks use a solid rear axle with leaf springs 
in an arrangement called a Hotchkiss drive.
Leaf springs on a driven axle control axle windup by transferring 
force from the axle housing to the frame. The front portion of the 
leaf spring, from the axle housing to the frame mount, acts like a 
trailing control arm. See Figure 87–4. 
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Figure 87–4 A typical rear-wheel-drive pickup truck rear suspension equipped with leaf
springs. This type of arrangement is called a Hotchkiss drive and the drive train forces are

controlled by the rear suspension components.

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Leaf springs are used on the rear of many light­ and medium­duty 
trucks and vans. The wheel spindles bolt to the flanges, the centers 
of the leaf springs rest on the seats, and U­bolts secure the springs 
to the axle.
A shackle attaches the rear of each spring to the unit­body frame, 
while the front of each spring pivots through a bolt and bushing 
connected to a hanger that bolts to the body.
See Figure 87–5. 

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Figure 87–5 An exploded view of a beam axle with multi-leaf springs.

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TRAILING ARM REAR SUSPENSIONS
A trailing arm extends from a frame crossmember located ahead 
of the rear axle back to the axle housing or a wheel knuckle. 
Trailing arms run parallel to the centerline of the chassis.
 
A trailing arm mounts to the frame with bushings, which allows 
the arm to pivot as the wheel rides over bumps. Some rear 
suspensions use two sets of trailing arms, one set positioned 
higher in the chassis than the other.
The arms in this arrangement are commonly referred to as the 
upper and lower control arms, they are called trailing arms. The 
word trailing applies to any link where the supported member 
trails the arm. See Figure 87–6.
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Figure 87–6 A trailing arm rear suspension with a solid axle used on a front-wheel-drive
vehicle.

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Trailing arms may be used to brace either a driven or nondriven 
solid rear axle against front­to­rear forces, but they do not provide 
much resistance to side­to­side, or lateral, forces. 
The axle itself is one means of locating the wheels side­to­side, 
and solid rear suspensions frequently use another rod to provide 
additional support. Trailing arms transfer axle windup force to the 
frame and control front­to­rear axle movement. 
On a few models, especially those with a high­performance 
suspension, a torque arm provides additional resistance to axle 
windup.
See Figure 87–7. 
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Figure 87–7 The Camaro and
Firebird rear suspension
systems use a torque arm to
control axle windup.

The torque arm extends 
parallel to the drive shaft 
between the rear axle and 
transmission.

One end of the torque 
arm is rigidly bolted to 
the differential housing, 
while the other end 
attaches to the 
transmission through a 
cushioned bracket to 
allow some flex.
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A trailing arm rear suspension on a nondriven solid axle virtually 
always includes a track rod, also called a Panhard rod.
A track rod is a rod attached to the body or frame on one end and the 
rear axle on the other. The purpose of the track rod is to keep the 
rear axle centered under the vehicle. The suspension may use either 
coil springs or struts.
The rear axle is a U­shaped steel beam that is open on the bottom. 
Flat metal axle end plates, to which the wheel spindles bolt, are 
attached to each end of the beam. A torsion tube or rod fits inside the 
beam and is welded to the axle end plates
See Figure 87–8. 
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The trailing arms, welded 
to the outboard ends of the 
axle, extend forward and 
attach to the frame with 
pivot bushing mounts. 
Figure 87–8 A typical beam axle
rear suspension uses trailing arms
and coil springs along with a track
rod, also called a Panhard rod, to
control side-to-side axle
movement.

A bracket on top of the 
beam axle locates the ring­
type lower strut mount,
and the upper strut mount 
attaches to a reinforced
area of the wheel well.
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SEMI-TRAILING ARM REAR SUSPENSIONS
A semi­trailing arm is similar to a trailing arm in that it extends 
back from a frame member to the axle.
A trailing arm is parallel to the vehicle centerline whereas a 

semi­trailing arm pivots at an angle to the vehicle centerline. 
Semi­trailing arms have an advantage over trailing arms as they 
control both side­to­side and front­to­rear motion. Typically, a 
semi­trailing arm suspension uses coil springs, air springs, or 
pivot­base struts.
A semi­trailing arm suspension may be used with either a solid 
axle or an independent suspension.

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Ford suspension uses two pairs, upper and lower, of semi­trailing 
arms and a center pivot arrangement to locate the driven rear axle. 

This axle centering 
pivot bracket and 
linkage is called a 
Watts linkage.

Figure 87–9
This Ford rear suspension uses upper and lower
semi-trailing arms to mount the rear axle and a
Watts linkage to control side-to-side movement.
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INDEPENDENT REAR SUSPENSIONS

The use of independent rear suspension, called IRS, has grown 
dramatically over the past several decades to the point where 
such systems are now fairly common, especially on front­wheel­
drive vehicles and some rear­wheel­drive vehicles. 
The reduction in unsprung weight is particularly noticeable for 
driven axles, which are constructed to transfer the weight of the 
differential and axles to the frame.
A vehicle with an independent rear suspension rides and holds 
better than a similar vehicle equipped with a solid rear axle. An 
SLA­type of independent suspension may be used at the rear of a 
rear­wheel­drive vehicle.
See Figures 87–10 and 87–11.
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Figure 87–10 An independent rear suspension
provides a better ride because less weight is
unsprung and the suspension is able to react
quickly to bumps in the road without affecting
the opposite side.

Figure 87–11 A typical short/long-arm
independent rear suspension.
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The main benefit of an SLA suspension is that it reduces tire
scrub and improves traction. The positioning of the control
arms determines the suspension roll center location. The
lower the roll center, the less body roll.
Unit­body front­wheel­drive vehicles frequently use strut 
suspensions at the rear axle. The strut mounts to the knuckle
and replaces the upper control arm. 
The first designer to put MacPherson­type struts in a rear 
suspension was an Englishman, Colin Chapman. For this reason, a 
rear strut suspension is often referred to as a Chapman strut 
suspension by European manufacturers and engineers.

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