Suspension improved handling. Independent front suspension

8.9 Suspension improved handling. Independent front suspension

(IFS) was developed to meet this need. The main advantages of independent front suspension are

8.9.1 Introduction

as follows:

The purpose of a suspension system can best be summarised by the following requirements: ● when one wheel is lifted or drops, it does not

affect the opposite wheel; ● cushion the car, passengers and load from

● the unsprung mass is lower, therefore the road surface irregularities;

road wheel stays in better contact with the road; ● resist the effects of steering, braking and accel-

● problems with changing steering geometry are eration, even on hills and when loads are carried;

reduced;

Chassis systems 191

Figure 8.27 Front wheel drive vehicle showing the suspension layout

● there is more space for the engine at the front;

8.9.3 Front axle suspensions

● softer springing with larger wheel movement is possible.

As with most design aspects of the vehicle, com- promise often has to be reached between

There are a number of basic suspension systems performance, body styling and cost. The follow- in common use. Figure 8.27 shows a typical sus-

ing table compares the common front axle sus- pension layout.

pension systems.

Name Description

Disadvantages Double transverse arms

Advantages

A large number of pivot points is wheels located by two

Independently suspended

Low bonnet line

required arms perpendicular to

Only slight changes of track and

High production costs direction of travel.The

camber with suspension

movements

arms support stub axles Transverse arms with

Harsh response when lightly leaf spring

A transverse arm and a leaf

The spring can act as an anti-roll

spring locate the wheel

bar, hence low cost

loaded Major changes of camber as vehicle is loaded

Transverse arm with

The body must be strengthened McPherson strut

A combination of the spring,

Only slight changes in track and

damper, wheel hub, steering

around the upper mounting arm and axle joints in

camber with suspension

A low bonnet line is difficult to one unit

movement

Forces on the joints are

achieve

reduced because of the long strut

Double trailing arms Two trailing arms support

Lots of space is required at the the stub axle.These can

No change in castor, camber or

front of the vehicle act on torsion bars often

track with suspension movement

Expensive to produce formed as a single assembly

Can be assembled and adjusted

off the vehicle

Acceleration and braking cause pitching movements which in turn changes the wheel base

192 Advanced automotive fault diagnosis Figure 8.28 shows a suspension system with

8.9.4 Rear axle suspensions

front struts and rear trailing arms. The following table compares the common rear

axle suspension systems.

Name Description

Disadvantages Rigid axle with leaf springs

Advantages

The final drive, differential

High unsprung mass and axle shafts are all

Rear track remains constant

The interaction of the wheels one unit

reducing tyre wear

Good directional stability

causes lateral movement reducing

because no camber change

tyre adhesion when the

causes body roll on corners

suspension is compressed

Low cost

on one side

Strong design for load carrying

Rigid axle with A-bracket Solid axle with coil springs

High cost and a central joint supports

Rear of the vehicle pulls

Large unsprung mass the axle on the body

down on braking which

stabilises the vehicle

Rigid axle with Coil springs provide the

High loads on the welded joints compression/tension

Suspension extension is

High weight overall struts

springing and the axle is

reduced when braking or

located by struts

accelerating

Large unsprung mass

The springs are isolated from these forces

Torsion beam trailing Two links are used,

Torsion bar springing on this arm axle

Track and camber does not

connected by a ‘U’ section

system can be more expensive that has low torsional

change

than coil springs stiffness but high resistance

Low unsprung mass

Simple to produce

to bending

Space saving

Torsion beam axle with Two links are welded to an

Torsion bar springing on this Panhard rod

Track and camber does not

axle tube or ‘U’ section and

system can be more expensive lateral forces are taken by a

change

than coil springs Panhard rod

Simple flexible joints to the

bodywork

Trailing arms The pivot axis of the trailing

Slight change of wheel base when arms is at 90° to the direction

When braking the rear of

the suspension is compressed of vehicle travel

the vehicle pulls down giving

stable handling Track and camber does not change Space saving

Semi-trailing arms – The trailing arms are pivoted

Sharp changes in track when the fixed length drive shafts

Only very small dive when

suspension is compressed of travel. Only one UJ is

at an angle to the direction

braking

resulting in tyre wear required because the

Lower cost than when

Slight tendency to oversteer radius of the suspension arm

variable length shafts are

used

is the same as the driveshaft when the suspension is compressed

Semi-trailing arms – The final drive assembly is The two arms are independent Large camber changes variable length drive

High cost because of the drive shafts

mounted to the body and

of each other

two UJs are used on

shafts and joints each shaft

Only slight track changes