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
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 self-
levelling
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-ladenCharacteristic curve of spring
s
stat(un-laden)
s
stat(fully laden)
+40 mm
0
