AUDI A6 ALLROAD 1999 C5 / 2.G Pneumatic Suspension System

21
-s +s± 0
Air spring parameters
Resilience/spring rate
The resilience (supporting force) F of an air
spring is determined by the effective surface
A
w and the excess pressure in the air
spring p
i.
F = p
i x Aw
The effective surface Aw is deÞned by the
effective diameter d
w.
In the case of a rigid structure, such as piston
and cylinder, the effective diameter
corresponds to the piston diameter.
In the case of air springs with U-bellows, the
effective diameter is determined by the
lowest point of the fold.
As the formula shows, the supporting force of
an air spring is in direct relation to the
internal pressure and the effective surface. It
is very easy to alter the supporting strength
(resilience) statically (no movement of the
bodywork) by varying the pressure in the air
spring.
The various pressures, depending on the
load, result in the relevant characteristic
curves of the springs and/or spring rates.
The spring rate alters at the same rate as the
bodywork weight, while the natural frequency
of the bodywork which determines the
handling characteristics remains constant.
The air suspension is adapted to a natural
frequency of the bodywork of 1.1 Hz.
242_023
242_025 Supporting force
dW
Supporting force
dW
Piston and cylinder
U-bellows
Spring travel
Supporting force
242_078 6 bar 7 bar 8 bar 9 bar
pi
pi
laden
un-laden

22
-s+s 0
Characteristic curve of springs
Owing to the functional principle, the
characteristic curve of an air spring is
progressive (in the case of cylindrical
pistons).
The progress of the characteristic curve of the
spring (ßat/steep inclination) is determined
by the spring volume.
A large spring volume produces a ßat
progression of the characteristic curve (soft
springs), a small spring volume produces a
steep progression of the characteristic curve
(hard springs).
The progression of the characteristic curve of
a spring can be inßuenced by the contour of
the piston.
Changing the contour of the piston alters the
effective diameter and thereby the resilience.
Result
The following options are available for
matching the air springs using U-bellows:
¥ Size of the effective surface
¥ Size of spring volume
¥ Contour of the piston
Principles of air suspension
242_026242_027
Spring volume
Piston volume
Small spring volume
Large spring volume
(+ piston volume)
Spring travel
Supporting weight = weight of the sprung masses
6 bar 7 bar
8 bar 9 bar
242_084 Spring volume

23
Example of the contour of a piston
(suspension strut in the Audi allroad quattro)
Vibration dampers are available in different
designs but their basic function and purpose
are the same.
Hydraulic/mechanical damping has found
widespread application in modern vehicle
design. The telescopic shock absorber is now
particularly favoured due to its small
dimensions, minimum friction, precise
damping and simple design.
Vibration damping
Without vibration damping, the vibration of
the masses during driving operation would
be increased to such an extent by repeated
road irregularities, that bodywork vibration
would build up increasingly and the wheels
would lose contact with the road surface.
The purpose of the vibration damping system
is to eliminate vibrations (energy) as quickly
as possible via the suspension.
For this purpose, hydraulic vibration dampers
(shock absorbers) are located parallel to the
springs.
242_079
U-bellows
Piston
Compressed

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Principles of air suspension
As previously mentioned, vibration damping
has a fundamental effect on driving safety
and comfort.
However, the requirements of driving safety
(driving dynamics) and driving comfort are
conßicting.
Within certain limits, the following applies in
principle:
¥ A higher rate of damping improves driving
dynamics and reduces driving comfort.
¥ A lower rate of damping lessens driving
dynamics and improves driving comfort.The term Òshock absorbersÓ is
misleading as it does not precisely
describe the function.
For this reason we shall use the term
Òvibration damperÓ instead.
242_022 Damped vibration
Un-damped vibrationUneven groundDirection of
travel Sprung mass
Unsprung mass

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Shock absorbers (vibration
dampers).
Dual pipe gas-pressure shock absorber
The dual pipe gas-pressure shock absorber
has become established as the standard
damper.
In the dual pipe gas-pressure shock absorber,
the working cylinder and the housing form
two chambers. The piston and piston rod
move inside the working chamber, which is
completely Þlled with hydraulic oil. The ring-
shaped oil reservoir between the working
cylinder and the housing serves to
compensate volumetric changes caused by
the piston rods and temperature changes in
the hydraulic oil.
The oil reservoir is only partially Þlled with oil
and is under a pressure of 6 - 8 bar, which
reduces the tendency towards cavitation.
Two damping valve units are used for
damping; the piston valve and the bottom
valve. These comprise a system of spring
washers, coil springs and valve bodies with
throttle bores.
242_080
Cavitation is the formation cavities and
the creation of a vacuum in a rapid
liquid ßow.
Working cylinder
Gas Þlling
Damping valve unit
(piston valve)
Damping valve unit
(bottom valve)
Oil reservoir
Damper valve
Non-return valve

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Principles of air suspension
During rebound, the piston valve alone
carries out the damping action and exerts a
predetermined resistance against the oil
ßowing downwards.
The oil required in the working chamber can
ßow back unhindered via the non-return valve
in the bottom valve. Function
During compression, damping is determined
by the bottom valve and to a certain extent by
the return ßow resistance of the piston.
The oil displaced by the piston rod ßows into
the oil reservoir. The bottom valve exerts a
deÞned resistance against this ßow, thereby
braking the movement.
242_081
Rebound Compression
Bottom valve
Oil reservoir
Piston valve
Damper valve
Non-return valve

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Single pipe gas-pressure shock absorber
With the single pipe gas-pressure shock
absorber, the working chamber and the oil
reservoir are located in a single cylinder.
Volumetric changes caused by the piston rod
and the temperature changes in the oil are
compensated by another gas chamber which
is separated from the working cylinder by a
dividing piston. The level of pressure in the
gas chamber is approx. 25 - 30 bar and must
be able to sustain the damping forces during
compression.
The damping valves for compression and
rebound are integrated into the piston.
Comparison of single/dual pipe gas-pressure shock absorbers
Dual pipe gas-pressure shock
absorberSingle pipe gas-pressure shock
absorber
Valve function The tendency towards cavitation
is reduced by the gas pressure in
the oil reservoirMinimal tendency towards
cavitation thanks to high gas
pressure and separation of oil and
gas
Characteristic
curvesAny, due to separate valves for
compression and reboundDependant on the gas pressure
during compression
Short damping
strokesGood Better
Friction Low Higher due to seal under pressure
Design Greater diameter Longer due to gas chamber in the
cylinder
Installation
positionApproximately vertical Any
Weight Heavier Lighter
242_082
Piston with damping
valves
Dividing piston
Gas chamber
Damper valves

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Principles of air suspension
During rebound, oil is forced out of the upper
chamber through the suction valve integrated
into the piston which exerts a deÞned
resistance against the oil. The gas cushion
thereby expands by the amount of the
emerging piston rod volume. Function
During compression, oil is forced out of the
lower chamber through the discharge valve
integrated into the piston which exerts a
deÞned resistance against the oil. The gas
cushion thereby compresses by the amount
of the piston rod volume inserted.
242_083
Rebound Compression
Gas cushion
Reboundvalve
Compression
valve
Gas cushion
Damper valves

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0,13 0 0 200 400 600 800 1000
1200 1400 1600
0,26
0,390,520,650,78 0,91 1,04
Advantage of this matching:
Good response of the vehicle suspension
ensures greater driving comfort.
The disadvantage of this matching occurs in
the case of a quick succession of irregularities
in the road. If the time between the individual
impacts is no longer sufÞcient for rebound,
the suspension can ÒhardenÓ signiÞcantly in
extreme cases, impairing driver comfort and
driver safety. Damping matching
We can basically distinguish between
compression and rebound in the damping
process.
The damping force during compression is
generally smaller than during rebound.
Consequently, irregularities in the road are
transmitted to the vehicle bodywork with
diminished force. The spring absorbs the
energy which is quickly dissipated during
rebound by the more efÞcient action of the
shock absorber.
242_084
Piston speed in m/s
Damping force in N
Compression Rebound

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Principles of air suspension
The degree of damping
... (the factor which determines how quickly
the vibrations are eliminated)
of the vehicle body is dependant on the
damping force of the shock absorber and the
sprung masses.
If the damping force is unchanged, the
following applies:
An increase of the sprung masses reduces the
degree of damping. This means that the
vibrations are eliminated more slowly.
A reduction of the sprung masses increases
the degree of damping. This means that the
vibrations are eliminated more rapidly.The degree of damping describes how
much kinetic energy a vibration system
been dissipated between two vibration
cycles as a result of damping.
The damping coefÞcient is just another
term for degree of damping.
242_068
Increased sprung mass
Reduced sprung mass
Spring travel Spring travel
Low degree of damping
Higher degree of damping