Anti-roll bar
8.9.5 Anti-roll bar
the anti-roll bar has no effect because it pivots on The main purpose of an anti-roll bar (Figure 8.29)
its mountings.
is to reduce body roll on corners. The anti-roll As the suspension is compressed on just one bar can be thought of as a torsion bar. The centre is
side, a twisting force is exerted on the anti-roll pivoted on the body and each end bends to make
bar. The anti-roll bar is now under torsional load. connection with the suspension/wheel assembly.
Part of this load is transmitted to the opposite When the suspension is compressed on both sides,
wheel, pulling it upwards. This reduces the amount
Chassis systems 193
● ensure good tyre adhesion; ● support the weight of the vehicle; ● transmit gravity forces to the wheels.
There are a number of different types of spring in use on modern light vehicles. The following table lists these together with their main features.
Coil springs
The most common Can be progressive if spring currently in
the diameter of the use on light vehicles.
spring is tapered The coil spring is
conically
Cannot transmit wound into a spiral
a torsion bar
lateral or longitudinal forces, hence the need for links or arms Little internal damping Little or no maintenance High travel
Leaf springs
These springs can Can transmit
be single or
longitudinal and
multiple leaf.
lateral forces
Short travel Figure 8.28 Suspension system with front struts and rear
They are most
often mounted
High internal damping
High load capacity trailing arms
longitudinally.
Nowadays they are Maintenance may only ever used on
be required commercial vehicles
Low height but high weight
Torsion bar
A torsion bar is a Maintenance-free but
springs
spring where
can be adjusted twisting loads are
Transmit longitudinal
applied to a
and lateral forces steel bar.They can
Limited self-damping be round or square
Linear rate
section, solid or
Low weight
hollow.Their
May have limited
surface must be
fatigue life finished accurately to eliminate pressure points, which may cause cracking and fatigue failure.They can
be fitted
Figure 8.29 Anti-roll bar and mountings
longitudinally or laterally
of body roll on corners. The disadvantages are Progressive rate
Rubber springs Nowadays rubber
that some of the ‘independence’ is lost and the Transmit longitudinal
springs are only
used as a
and lateral forces
overall ride is harsher. Anti-roll bars can be fitted
supplement to
Short travel
to both front and rear axles.
other forms of
Low weight and springs.They are,
low cost however, popular on
Their springing and
8.9.6 Springs
trailers and caravans damping properties can change with
The requirements of the springs can be sum-
temperature
marised as follows:
Air springs
Air springs can be Expensive thought of as being
Good quality ride
● absorb road shocks from uneven surfaces;
like a balloon or Electronic control
● control ground clearance and ride height;
football on which can be used
194 Advanced automotive fault diagnosis
the car is supported. Progressive spring space behind a separator piston is The system involves
to compensate for the changes in compressors and
rate
cylinder volume caused as the pis- air tanks.They are
High production cost
ton rod moves. It is at a pressure not normally used on
of about 25 bar light vehicles
The twin tube gas damper is an Hydro-
Twin tube gas damper
improvement on the well-used pneumatic
A hydro-pneumatic
Progressive rate
(Figure 8.30)
twin tube system.The gas cushion springs
spring is a gas
Ride height control
spring with
is used in this case to prevent oil hydraulic force
Damping built in
foaming.The gas pressure on the transmission.
Pressurised oil supply
oil prevents foaming which in turn Nitrogen is usually
is required
ensures constant operation under used as the gas.
Expensive and
all operating conditions. Gas pres- The damper can be
complicated
sure is lower than for a single tube built in as part of
damper at about 5 bar the hydraulic system.
This is a special variation of the The springs can be
Variable rate damper
twin tube gas damper.The damping hydraulically
characteristics vary depending on connected together
the load on the vehicle. Bypass to reduce pitch or
grooves are machined in the upper roll. Ride height
half of the working chamber.With control can be
light loads the damper works in achieved by pumping
this area with a soft damping oil into or out of
effect.When the load is increased the working
the piston moves lower down the chamber
working chamber away from the grooves resulting in full damping effect
8.9.7 Dampers (shock absorbers)
Electronically controlled
These are dampers where the
The functions of a damper can be summarised as
dampers
damping rate can be controlled by
follows: solenoid valves inside the units.
With suitable electronic control,
● ensure directional stability;
the characteristics can be changed ●
ensure good contact between the tyres and the within milliseconds to react to
driving and/or load conditions
road; ● prevent buildup of vertical movements;
● reduce oscillations; ● reduce wear on tyres and chassis components.
There are a number of different types of damper.
Friction damper Not used on cars today but you will find this system used as part of caravan or trailer stabilisers
Lever type damper Used on earlier vehicles, the lever operates a piston which forces oil into a chamber
Twin tube telescopic This is a commonly used type of damper
damper consisting of two tubes.An outer tube forms a reservoir space and contains the oil displaced from an inner tube. Oil is forced through a valve by the action of a piston as the damper moves up or down.The reservoir space is essential to make up for the changes in volume as the piston rod moves in and out
Single tube telescopic This is often referred to as a gas damper
damper. However, the damping action is still achieved by forcing oil through a restriction.The gas
Figure 8.30 Twin tube gas damper
Chassis systems 195
8.10 Diagnostics –
The operating principle is shown as Figure 8.31 which indicates that the damper is not operating
suspension
correctly in this case.