Audi A6 242, A6 Service Manual

Service.

For internal use only

Pneumatic suspension system
Par t 1
Selflevelling suspension
in the Audi A6

Design and Function

Self-study programme 242

242

2

242_067

Pneumatic self-levelling suspension system

The 4-level air suspension of the Audi
allroad quattro is described in self­study program 243.

You will find 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_046

242_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
Note

Important:
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 significant influence on
these movements and vibrations.

242_048

Longitudinal axis

Transverse axis

Vertical axis

Drift

Pitch

Swerving (yaw)

Rising and sinking

Tipping (roll)

Jerking

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

6

Principles

The suspension system

As ”supporting” 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
finding 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
”Vibration” 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 influence on
the vibration characteristics (natural
frequency of the bodywork). Furthermore, a
low inertia of masses reduces the impact load
on the unsprung components and
significantly 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 “Running
gear”.

213_041

8

Principles

The natural frequency of the bodywork

The vibrations are defined 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 deflected 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 cycle

Amplitude

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

Definitions

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 bodywork

Number 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 travelSpring travel

Low natural frequency of the
bodywork

High natural frequency of the
bodywork

1 cycle

1 cycle

Time

Time

10

The degree of damping of the vibration
damper has no significant influence on the
value of the natural frequency of the
bodywork. It influences only how quickly the
vibrations cease (damping coefficient). For
further information, see chapter “Vibration
damping”.

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 fitted 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 self­levelling, 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 heightNatural frequency of the bodywork

Component tolerance band

Natural frequency tolerance band

Usable load range

of a spring

Height tolerance

Axle load800 kg 850 kg 900 kg 950 kg

1.13 Hz

c

F1

= 33.3 N/mm

c

F2

= 35.2 N/mm

c

F3

= 37.2 N/mm

c

F4

= 39.3 N/mm

c

F5

= 41.5 N/mm

c

F6

= 43.7 N/mm

Spring rate levels of the front axle for the A6

11

OJL

1BA

OYF

Spring allocation table (e.g. A6 front axle 1BA)

PR-No. weight
class, front axle

Axle load (kg) Suspension, left and right

(spring rate)

Colour coding

Standard
running
gear
e.g. 1 BA

OJD 739 - 766 800 411 105 AN (29.6 N/mm) 1 violet, 3 brown
OJE 767 - 794 800 411 105 AP (31.4 N/mm) 1 white, 1 brown
OJF 795 - 823 800 411 105 AQ (33.3 N/mm) 1 white, 2 brown
OJG 824 - 853 800 411 105 AR (35.2 N/mm) 1 white, 3 brown
OJH 854 - 885 800 411 105 AS (37.2 N/mm) 1 yellow, 1 brown
OJJ 886 - 918 800 411 105 AT (39.3 N/mm) 1 yellow, 2 brown
OJK 919 - 952 800 411 105 BA (41.5 N/mm) 1 yellow, 3 brown
OJL 953 - 986 800 411 105 BM (43.7 N/mm) 1 green, 1 brown
OJM 987 - 1023 800 411 105 BN (46.1 N/mm) 1 green, 2 brown

Sports
running
gear
e.g. 1BE

OJD 753 - 787 800 411 105 P (40.1 N/mm) 1 grey, 3 violet
OJE 788 - 823 800 411 105 Q (43.2 N/mm) 1 green, 1 violet
OJF 824 - 860 800 411 105 R (46.3 N/mm) 1 green, 2 violet
OJG 861 - 899 800 411 105 S (49.5 N/mm) 1 green, 3 violet
OJH 900 - 940 800 411 105 T (53.0 N/mm) 1 yellow, 1 violet
OJJ 941 - 982 800 411 105 AA (56.6 N/mm) 1 yellow, 2 violet
OJK 983 - 1027 800 411 105 AB (60.4 N/mm) 1 yellow, 3 violet

Weight class of
front axle

Running
gear

Weight class of
the rear axle

242_108

Proof of warranty

Vehicle data

Vehicle identification number

Type description

Engine capacity / gearbox / month/
year of manufacture

Engine code / gearbox
code letters

Paint no. / interior equipment no.

M-equipment number

Un-laden weight / consumption
figures / CO

2

emissions

Date of

Delivery

Stamp of the
Audi delivery

centre

12

0

0

Characteristic values of
springs

Characteristic curve/spring rate of springs

We can obtain the characteristic curve of a
spring by producing a forces/travel diagram.

The spring rate is the ratio between the
effective force and the spring travel. The unit
of measurement for the spring rate is N/mm.
It informs us whether a spring is hard or soft.

If the spring rate remains the same
throughout the entire spring travel, the spring
has a linear characteristic curve.

A soft spring has a flat characteristic curve
while a hard spring has a steep curve.

A coil spring is harder due to:

• a greater wire diameter

• a smaller spring diameter

• a lower number of coils

Principles

242_018

If the spring rate becomes greater as the
spring travel increases, the spring has a
progressive characteristic curve.

Coil springs with a progressive characteristic
curve can be recognised as follows:

a) uneven coil pitch
b) conical coil shape
c) conical wire diameter
d) combination of two spring elements
(example, see next page)

242_019

Spring travel s

Resilience F

Linear characteristic curve
Hard spring

Progressive characteristic
curve

a

b

c

Linear characteristic curve
Soft spring

13

-120 -80-400

0

3

6

9

12

15

40 80 120

(Example: Suspension strut with auxiliary
polyurethane springs).

Advantages of progressive characteristic
curve of spring:

• Better matching of the suspension system
from normal to full load.

• The natural frequency of the bodywork
remains practically constant during
loading.

• The suspension is not so prone to impacts
in the case of significant irregularities in
the road surface.

• Better use of the available spring travel.

Rebound in mm Compression in mm

Parallel springing

Lower stop

Upper stop

Rebound stop insert (in shock absorber)

Un-laden position

Design position

Auxiliary spring insert

Lower stop

242_020

Spring

Auxiliary spring

14

When the vehicle is stationary, the vehicle
body retracts by a certain spring travel
depending upon the load. In this case, we
speak of static compression: s

stat

.

The disadvantage of conventional running
gear without self-levelling is its reduced
spring travel at full load.

Conventional running gear
(steel springs) without self­levelling

Spring travel

The overall spring travel s

tot

required for
running gear without self-levelling is
comprised of the static compression s

stat

and
the dynamic spring travel caused by vehicle
vibrations s

dyn

for both laden and un-laden

vehicles.

s

tot

= s

stat

+ s

dyn(un-laden)

+ s

dyn(fully laden)

Principles

242_075

Steel suspension

fully laden
Design position

Un-laden position

Supporting force in kn.

H

V

H

H

L

dyn. rebound

s

stat

(un-laden)

dyn. compression

(un-laden)

(fully laden)

10

8

6

4

2

+80 mm

-40 mm-80 mm

H

V

= height when fully laden

H

= design position height

H

L

= height when un-laden

Characteristic curve of spring

s

stat(un-laden)

s

stat(fully laden)

+40 mm

0

15

Definitions:

The un-laden position ...
... is the compression exerted onto the wheels
when the vehicle is ready for the road (fuel
tank completely filled, spare wheel and
vehicle tools present).
The design position ...
... is defined as the un-laden position plus the
additional load of three persons, each
weighing 68 kg.

The static compression ...

... is the starting point (zero) for the dynamic
spring movements, compression travel (plus)
and rebound travel (minus).

... is dependant upon the spring rate and the
load (sprung masses).

... results from the difference between the
static compression when un-laden
s

stat(un-laden)

and the static compression when

fully laden s

stat(fully laden)

.

s

stat

= s

stat(fully laden)

- s

stat(un-laden)

In the case of a flat characteristic curve (soft
springs), the difference and thereby the static
compression between full and un-laden is
very great.

242_076

In the case of a steep characteristic spring
curve, this state of affairs is reversed and is
coupled with an excessive increase of the
natural frequency of the bodywork.

Fully laden

Un-laden position

Hard springs

Soft springs

s

stat

soft springs

s

stat

hard springs

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 benefits 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
significant 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
“Air spring characteristic values” page 21).

242_077

H = constant

fully laden
Design position H

un-laden

s

stat

0

Supporting force in kN.

10

8

6

+80 mm+40 mm-40 mm-80 mm

4

2

Air suspension

dyn. rebound dyn. compression

Spring travel

Characteristic curves
of springs

18

Principles of air suspension

Another benefit 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 find further details in SSP 243
“4-Level air suspension in the Audi allroad
quattro”.

Fully supporting means:

Self-levelling systems are often
combined with steel or gas-filled spring
devices with hydraulic or pneumatic
control. The supporting force of these
systems results from the sum of both
systems. We therefore call them
“partially supporting” (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 “fully supporting”.

0

1

2

3

4

0 102030

242_030

Spring rate

0

1

2

3

4

0 102030

Natural frequency of the bodywork

Supporting force

242_031

Supporting force
Steel springs (linear)
Air springs

Steel springs (linear)
Air springs

19

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 fig. 242_032.

242_032

The outer and inner surfaces are made of an
elastomer material. The material is resistant
to all weather influences and is largely oil­resistant. The inner surface finish 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 machine­pressed 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 filled 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

Piston

Air springs