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