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

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

11
OJL1BA
OYF

Spring allocation table (e.g. A6 front axle 1BA)
PR-No. weight
class, front axleAxle load (kg) Suspension, left and right
(spring rate)Colour coding
Standard
running
gear
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
Sports
running
gear
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
front axleRunning
gear
Weight class of
the rear axle
242_108
Proof of warranty
Vehicle data
Vehicle identiÞcation 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
Þgures / CO
2
emissionsDate of
Delivery
Stamp of the
Audi delivery
centre

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

27
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

32
Principles of air suspension
Measures are taken during the design stage
to adapt the characteristic curves to the
requirements of suspension matching.
Shock absorbers with decreasing
characteristic curves are normally used.
Normal shock absorbers have predetermined
characteristic curves. They are adapted to
normal bodywork weights and can cope with
a wide range of driving situations in a well-
matched running gear.
Running gear matching is always a
compromise between driving safety (driving
dynamics) and driving comfort.
The degree of damping (damping effect of
sprung masses) is lessened as the load
increases, which affects the driving dynamics.
In contrast, the degree of damping is greater
when the vehicle is un-laden, which lessens
driving comfort.Note:
A distinctive feature of damper
matching is described in SSP 213,
page 28, ÒShock absorbers with load
and travel-dependent damping
characteristicsÓ.

33
The PDC damper
In order to maintain the degree of damping
and thereby the handling characteristics at a
constant level between partially and fully
laden, the Audi A6 self-levelling air suspen-
sion and the Audi allroad quattro 4-level air
suspension both have a continuously variable
load recognition system Þtted to the rear axle.
Along with the constant natural frequency of
the bodywork, the vehicle bodywork
maintains virtually constant vibration
characteristics irrespective of the load thanks
to the air springs.
When the vehicle is partially-laden, good
driving comfort is achieved and body
movements are damped sufÞciently Þrmly at
full load.
The PDC damper (Pneumatic Damping
Control) is responsible for this. The damping
force can be varied according to the air spring
pressure.
242_043
PDC valve
Hoses
Air springs
1 1,2 1,4 1,6 1,82,0Body weight ratio
Degree of damping D
PDC damper
Conventional dampers242_057
Coaxial arrangement of air springs/PDC damper

47
The air dryer
The air must be dehumidiÞed in order to
prevent condensate and the associated
problems of corrosion and freezing. The
system used here is a so-called regenerative
air dryer system. A synthetically
manufactured silicate granulate is used as the
drying agent. This granulate can, depending
on the temperature, store up to 20% of its
own weight in water.
As the air dryer operates regeneratively, and
is only operated with oil-free, Þltered air, it is
not subject to replacement intervals and is
therefore maintenance-free.Because the air dryer is regenerated
only with waste air, the compressor
cannot be used to Þll any other
components. As this compressed air is
not fed back via the air dryer, no
regeneration can take place. For this
reason, the manufacturers do not Þt a
pressure connection for external
components.
Water/moisture in the system indicates
a fault in the air dryer or the system.
242_056 Granulate Þlling