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

2
242_067
Pneumatic self-levelling suspension system
The 4-level air suspension of the Audi
allroad quattro is described in self-
study program 243.
You will Þnd further information on the
Audi allroad quattro in self-study
programme 241.
Principles of spring suspension, damping and
air suspension
Self-levelling suspension, A6
The rear axle air suspension system for the
Audi A6 Avant is described here.
242_046242_048
This self-study programme is divided into two
parts:

3
Contents
Principles
Vehicle suspension.................................................................. 4
The suspension system .......................................................... 6
Vibration ................................................................................... 8
Characteristic values of springs .......................................... 12
Conventional running gear without self-levelling ............ 14
The self-study programme is not intended as a workshop manual.
The self-study programme will provide you with
information on design and functions.
New
NoteImportant:
Note
Page
For maintenance and repairs please refer to the current
technical literature.
Principles of air suspension
Self-levelling air suspension ............................................... 16
Characteristic values of air spring ...................................... 21
Vibration damping................................................................. 23
Shock absorbers (vibration dampers) ................................ 25
PDC shock absorbers ........................................................... 33
System overview ................................................................... 38
Air springs .............................................................................. 40
Air supply unit ........................................................................ 42
Diagram of pneumatic system ............................................. 43
Compressor ........................................................................... 44
Air dryer ................................................................................. 47
Discharge valve N111 ........................................................... 48
Valve for suspension struts N150 and N151....................... 51
Self-levelling suspension sender G84 ................................ 52
Self-levelling suspension control unit J197 ....................... 54
Self-levelling suspension warning lamps K134 ................ 55
Function diagram ................................................................... 56
Interfaces................................................................................ 57
The control concept .............................................................. 58
Other features of the control concept ................................ 60
Self-levelling suspension, A6

6
Principles
The suspension system
As ÓsupportingÓ components of the
suspension system, the suspension elements
form the connection between the wheel
suspension and the bodywork. This system is
complemented by the spring action of the
tyres and vehicle seats.
The suspension elements include steel
springs, gas/air and rubber/elastomers or
combinations of the above.
Steel spring suspensions have become well
established in passenger vehicles. Steel
springs are available in a wide variety of
designs, of which the coil spring has become
the most widespread.
Air suspension, which has been used for
many years in heavy goods vehicles, is
Þnding increasing application in passenger
vehicles due to its system-related
advantages.
242_047
In the case of the passenger vehicle we can
differentiate between
sprung masses
(body
with drive train and parts of the running gear)
and
unsprung masses
(the wheels, brakes
and parts of the running gear and the axle
shafts).
As a result of the suspension system, the
vehicle forms an oscillatory unit with a
natural frequency of the bodywork
determined by the sprung masses and the
matching of the suspension system (see
ÓVibrationÓ chapter).
Sprung mass
Unsprung mass Suspension element
Suspension element

8
Principles
The natural frequency of the bodywork
The vibrations are deÞned by the degree of
amplitude and its frequency. The natural
frequency of the bodywork is particularly
important during matching of the
suspension.
The natural frequency of unsprung parts is
between 10 Hz and 16 Hz for a medium-size
vehicle. Appropriate matching of the
suspension system reduces the natural
frequency of the bodywork (sprung mass) to
between 1 Hz and 1.5 Hz.
Vibration
If a mass on a spring is deßected from its rest
position by a force, a restoring force develops
in the spring which allows the mass to
rebound. The mass
oscillates
beyond its rest
position which results in a further restoring
force being exerted. This process is repeated
until air resistance and the internal friction of
the spring causes the vibration to cease.
242_021
Rest position Mass
Spring
Vibration
Rebound
Compression
1 cycleAmplitude

16
Principles of air suspension
Self-levelling air
suspension
Air suspension is a controllable form of
vehicle suspension.
With air suspension, it is simple to achieve
self-levelling and it is therefore generally
integrated into the system.
The basic advantages of self-levelling are:
¥ Static compression remains the same,
irrespective of vehicle loads (see overleaf).
The space requirement in the wheel
arches for free wheel movement kept to a
minimum, which has beneÞts for the
overall use of available space.
¥ The vehicle body can be suspended more
softly, which improves driving comfort.
¥ Full compression and rebound travel is
maintained, whatever the load.
242_074
¥ Ground clearance is maintained, whatever
the load.
¥ There are no track or camber changes
when vehicle is laden.
¥ The c
w value is maintained, as is the visual
appearance.
¥ Less wear to ball joints due to reduced
working angle.
¥ Greater loads are possible if required.
= constant

17
In addition to the main advantages offered by
self-levelling, its realisation by means of air
suspension (Audi A6) offers another
signiÞcant advantage.
As the air pressure in the air springs is
adapted in accordance with the load, the
spring rate alters proportionally to the sprung
mass. The positive outcome is that the natural
frequency of the bodywork and thereby
driving comfort remain virtually constant,
irrespective of the load. With the aid of self-levelling, the vehicle
(sprung masses) remains at one level (design
position) because the air spring pressure is
adapted accordingly.
Static compression is thus the same at all
times thanks to the self-levelling system and
need not be accounted for when designing
the wheel clearances.
s
stat = 0
Another feature of self-levelling air
suspension is that the natural frequency of
the bodywork is kept virtually constant
between un-laden and full-load (see chapter
ÒAir spring characteristic valuesÓ page 21).
242_077H = constant
fully laden
Design position H
un-laden
sstat
0
Supporting force in kN.
10
8
6
+80 mm+40 mm-40 mm -80 mm4
2
Air suspension
dyn. rebound dyn. compression
Spring travel
Characteristic curves
of springs

18
Principles of air suspension
Another beneÞt is the principle-related
progressive characteristic curve of an air
spring.
With fully supporting air suspension on both
axles (Audi allroad quattro), different vehicle
levels can be set, e.g.:
¥ Normal driving position for city driving.
¥ Lowered driving position for high speeds
to improve driving dynamics and air
resistance.
¥ Raised driving position for travel off-road
and on poor road surfaces.
You can Þnd further details in SSP 243
Ò4-Level air suspension in the Audi allroad
quattroÓ.Fully supporting means:
Self-levelling systems are often
combined with steel or gas-Þlled spring
devices with hydraulic or pneumatic
control. The supporting force of these
systems results from the sum of both
systems. We therefore call them
Òpartially supportingÓ (Audi 100/
Audi A8).
In the self-levelling suspension systems
in the Audi A6 (on the rear axle) and in
the Audi allroad quattro (rear and front
axles) air springs are the only
supporting suspension elements and
these systems are therefore described
as Òfully supportingÓ.
0 1 2
3 40 102030
242_030
Spring rate
0 1 2
3 40 102030
Natural frequency of the bodywork
Supporting force
242_031
Supporting force
Steel springs (linear)
Air springsSteel springs (linear)
Air springs

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Design of the air springs:
In passenger vehicles, air springs with
U-bellows are used as suspension elements.
These allow greater spring travel in restricted
spaces.
The air springs consist of:
¥ Upper housing closure
¥ U-bellows
¥ Piston (lower housing closure)
¥ Retaining rings
The construction of the U-bellows can be
seen in Þg. 242_032.
242_032
The outer and inner surfaces are made of an
elastomer material. The material is resistant
to all weather inßuences and is largely oil-
resistant. The inner surface Þnish is designed
to be particularly air-tight.
The stability supports absorb the forces
produced by the internal pressure in the air
springs.
Upper housing closure
Retaining ring
Internal surface coating
Woven insert 1
Woven insert 2
External surface coating
Piston
Coaxial arrangement of the air springs

20
Principles of air suspension
High-quality elastomer material and
polyamide cord woven inserts (stability
supports) provide the U-bellows with good
unrolling characteristics and a sensitive
response of the spring system.
The necessary properties are ensured over a
wide temperature range between
-35 ¡C and +90 ¡C.
Metal retaining rings tension the U-bellows
between the upper housing closure and the
piston. The retaining rings are machine-
pressed by the manufacturer.
The U-bellows unrolls onto the piston.
Depending on the axle design, the air springs
are either separate from the shock absorbers
or combined as a suspension strut (coaxial
arrangement).Air springs must not be moved in an
unpressurised condition since the air
bellows cannot unroll on the piston and
would be damaged.
In a vehicle in which the air springs are
unpressurised, the relevant air springs
must be Þlled with the aid of the
diagnostic tester (see Workshop
Manual) before raising or lowering the
vehicle (e.g. vehicle lifting platform or
vehicle jack).
242_042
Separate arrangement of the air springs
PistonAir springs

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