AUDI A6 ALLROAD 1999 C5 / 2.G Pneumatic Suspension System
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Spring allocation table (e.g. A6 front axle 1BA)
class, front axleAxle load (kg) Suspension, left and right
(spring rate)Colour coding
e.g. 1 BAOJD 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
e.g. 1BEOJD 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
Weight class of
the rear axle
Proof of warranty
Vehicle identiÞcation number
Engine capacity / gearbox / month/
year of manufacture
Engine code / gearbox
Paint no. / interior equipment no.
Un-laden weight / consumption
Þgures / CO
Stamp of the
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Characteristic values of
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 ßat 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
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)
Spring travel s
Linear characteristic curve
c Linear characteristic curve
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-120 -80-400 0 3 6
9 12 15
(Example: Suspension strut with auxiliary
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
¥ The suspension is not so prone to impacts
in the case of signiÞcant irregularities in
the road surface.
¥ Better use of the available spring travel.
Rebound in mm Compression in mm
Upper stop Rebound stop insert (in shock absorber)
Auxiliary spring insert Lower stop
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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
The disadvantage of conventional running
gear without self-levelling is its reduced
spring travel at full load.
Conventional running gear
(steel springs) without self-
The overall spring travel s
running gear without self-levelling is
comprised of the static compression s
the dynamic spring travel caused by vehicle
for both laden and un-laden
Supporting force in kn.
+80 mm-40 mm -80 mm
= height when fully laden
= design position height
= height when un-ladenCharacteristic curve of spring
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... is the compression exerted onto the wheels
when the vehicle is ready for the road (fuel
tank completely Þlled, spare wheel and
vehicle tools present).
... is deÞned 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
and the static compression when
fully laden s
In the case of a ßat characteristic curve (soft
springs), the difference and thereby the static
compression between full and un-laden is
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.
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Principles of air suspension
Air suspension is a controllable form of
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.
¥ Ground clearance is maintained, whatever
¥ There are no track or camber changes
when vehicle is laden.
¥ The c
w value is maintained, as is the visual
¥ Less wear to ball joints due to reduced
¥ Greater loads are possible if required.
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In addition to the main advantages offered by
self-levelling, its realisation by means of air
suspension (Audi A6) offers another
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
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.
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
Design position H
Supporting force in kN.
+80 mm+40 mm-40 mm -80 mm4
dyn. rebound dyn. compression
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Principles of air suspension
Another beneÞt is the principle-related
progressive characteristic curve of an air
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
¥ 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/
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
0 1 2
3 40 102030
Natural frequency of the bodywork
Steel springs (linear)
Air springsSteel springs (linear)
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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
The air springs consist of:
¥ Upper housing closure
¥ Piston (lower housing closure)
¥ Retaining rings
The construction of the U-bellows can be
seen in Þg. 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
Upper housing closure
Internal surface coating
Woven insert 1
Woven insert 2
External surface coating
Coaxial arrangement of the air springs
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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
Separate arrangement of the air springs