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

Service.For internal use only All rights reserved, including
the right to make technical
changes.
AUDI AG
Dept. I/VK-5
D-85045 Ingolstadt
Fax 0841/89-36367
940.2810.47.20
Technical status 11/00
Printed in Germany
Pneumatic suspension system
Par t 1
Selflevelling suspension
in the Audi A6Design and Function
Self-study programme 242
242
242

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

4
Vehicle suspension
When a vehicle travels over irregular road
surfaces, impact forces are transmitted to the
wheels. These forces pass to the bodywork
via the suspension system and the wheel
suspension.
The purpose of the vehicle suspension is to
absorb and reduce these forces.
Principles
Wheel contact with the road surface, which
is essential for braking and steering, is
maintained.
The vehicle components are protected
against excessive stresses. Unpleasant and unhealthy stresses to vehicle
passengers are minimised, and damage to
fragile loads is avoided.
242_003
Driving safety
Operating safety
Driving comfort
When we talk about the vehicle suspension
we can basically distinguish between the
suspension system
and the
vibration
damping system
.
By means of the interaction of the two
systems, the following is achieved:

5
During driving operation, the vehicle body is
subject not only to the forces which cause the
upward and downward motion of the vehicle,
but also the movements and vibrations in the
direction of the three spatial axes.
Along with the axle kinematics, the vehicle
suspension has a signiÞcant inßuence on
these movements and vibrations.
242_048
Longitudinal axis
Transverse axisVertical axis
Drift
Pitch
Swerving (yaw)
Rising and sinkingTipping (roll)
Jerking

The correct matching of the springs and
vibration damping system is therefore of
great signiÞcance.

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

7
The unsprung masses
The aim in principle is to minimise the volume
of unsprung masses and their inßuence on
the vibration characteristics (natural
frequency of the bodywork). Furthermore, a
low inertia of masses reduces the impact load
on the unsprung components and
signiÞcantly improves the response
characteristics of the suspension. These
effects result in a marked increase in driver
comfort.
Examples for the reduction of unsprung
masses:
¥ Aluminium hollow spoke wheel
¥ Running gear parts (swivel bearing, wheel
carrier, links etc.) made of aluminium
¥ Aluminium brake callipers
¥ Weight-optimised tyres
¥ Weight optimisation of running gear parts
(e.g. wheel hubs)
213_091
213_068
See also SSP 213, chapter ÒRunning
gearÓ.
213_041

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

9
The natural frequency of the bodywork is
essentially determined by the characteristics
of the springs (spring rate) and by the sprung
mass.
Greater mass or softer springs produce a
lower natural frequency of the bodywork and
a greater spring travel (amplitude).
Smaller mass or harder springs produce a
higher natural frequency of the bodywork and
a lesser spring travel.
Depending on personal sensitivity, a natural
frequency of the bodywork below 1 Hz can
cause nausea. Frequencies above 1.5 Hz
impair driving comfort and are experienced
as shudders above around 5Hz.
242_072
DeÞnitions
Vibration Upward and downward
motion of the mass
(body)
Amplitude The greatest distance of
the vibrating mass from
the rest position
(vibration extent, spring
travel)
Cycle Duration of a single
vibration
Frequency Number of vibrations
(cycles) per second
Natural
frequency of
the bodyworkNumber of vibrations of
the sprung mass (body)
per second
Resonance The mass is disturbed in
its rhythm by a force
which increases the
amplitude (build-up).
Greater mass or softer springs
Smaller mass or harder springs
Spring travel Spring travel
Low natural frequency of the
bodywork
High natural frequency of the
bodywork1 cycle
1 cycleTime
Time

10
The degree of damping of the vibration
damper has no signiÞcant inßuence on the
value of the natural frequency of the
bodywork. It inßuences only how quickly the
vibrations cease (damping coefÞcient). For
further information, see chapter ÒVibration
dampingÓ.
Matching of the natural frequency of the
bodywork
The axle loads (sprung masses) of a vehicle
vary, at times considerably, depending on the
engine and equipment installed.
To ensure that the bodywork height
(appearance) and the natural frequency of the
bodywork (which determines the driving
dynamics) remains practically identical for all
vehicle versions, different spring and shock
absorber combinations are Þtted to the front
and rear axles in accordance with the axle
load.
For instance, the natural frequency of the
bodywork of the Audi A6 is matched to 1.13Hz
on the front axle and 1.33Hz on the rear axle
(design position).
The spring rate of the springs therefore
determines the value of the natural frequency
of the bodywork.
The springs are colour-coded to differentiate
between the different spring rates (see table).
Principles
For standard running gear without self-
levelling, the rear axle is always
matched to a higher natural frequency
of the bodywork because when the
vehicle is loaded, it is principally the
load to the rear axle which increases,
thus reducing the natural frequency of
the bodywork.
242_073
Vehicle height Natural frequency of the bodyworkComponent tolerance band
Natural frequency tolerance band Usable load range
of a spring
Height tolerance
Axle load 800 kg 850 kg 900 kg 950 kg 1.13 Hz
c

F1

= 33.3 N/mmc

F2

= 35.2 N/mmc

F3

= 37.2 N/mmc

F4

= 39.3 N/mmc

F5

= 41.5 N/mmc

F6

= 43.7 N/mm

Spring rate levels of the front axle for the A6