Motor vehicle

For avoiding torsional vibrations of the vehicle body, the motor vehicle of the present invention uses a drive assembly as an absorber which is held via three bearings in the vehicle body. The bearings having a characteristic which determines a high natural frequency of the assembly roll mode and is modulated precisely to the vehicle body torsion and has a low vertical bearing stiffness.

BACKGROUND AND SUMMARY OF THE INVENTION 
The invention relates motor vehicle comprising a motor vehicle body subject 
to torsional and bending vibrations, particularly a convertible body on 
which, via elastic bearings, a drive assembly is suspended which acts as 
an absorber mass, the drive assembly being held by three elastic bearings 
between side and cross members of the vehicle body. 
A typical vehicle comprise of a plurality of vibrating systems. One of the 
systems is the motor vehicle body itself. This motor vehicle body 
interacts with other vehicle components, such as the chassis or the 
elastically disposed assembly, and these components influence one another. 
For example, the low frequency translational and rotational motor vehicle 
body vibrations are influenced by the modulation of the chassis. The shock 
absorbers dampen disturbing lifting and rolling motions. 
However, the bending and torsional vibrations of the vehicle body are 
almost undamped. In the driving operation, they are excited by uneven road 
conditions and are transmitted in the form of vibrations by way of the 
seat, the back rest, and the steering system to the driver. In the case of 
convertibles the visually discernable shaking is also annoying. 
Depending on the vehicle design, the vehicle modes have different 
amplitudes and resonances. As a function of the interaction with other 
vibration systems and the road excitations, these amplitudes and 
resonances are of importance particularly in the case of convertibles. 
They contribute to the comfort of the vehicle and to the impression of 
quality of the vehicle and are therefore becoming more and more important. 
Because of the open structural shell without the reinforcing effect of a 
firm roof, convertibles usually have a lower body stiffness than coupe 
vehicles. 
The shaking of the vehicle body which occurs particularly on bad roads is 
significantly determined by the amplitudes and resonance frequency of the 
bending and torsional vibrations of the first degree of the vehicle body. 
Not only the body vibrations of the first degree, which are so important 
with respect to convertibles, are in the frequency range of approximately 
11 to 24 Hz. Also, the natural resonances of the unsprung vehicle masses 
as well as the rigid body modes of the drive assembly which is elastically 
disposed in its rubber elements are in this frequency range. 
Tests on road surfaces and of the resulting vehicle excitation have shown 
that, as the wavelength of the unevenness becomes smaller, the spectral 
density of the unevenness will decrease. This means that, while the 
driving speed is the same, lower frequency vehicle body modes are excited 
to a greater degree in the case of convertibles than the higher frequency 
body modes in the case of coupes. 
In convertibles, an interaction of different vibration systems will 
necessarily result in increased vibration amplitudes in the vehicle body 
modes. While in the case of a coupe the torsion of the vehicle body is not 
excited or is excited only insignificantly during road travel, it is 
usually of great importance in the case of convertibles. 
In many cases, a convertible is derived from a vehicle body of a coupe, so 
that basic structural changes are rarely possible. Reinforcing measures on 
the shell can often be carried out only to a limited degree because of 
many different marginal conditions. 
Apart from the resulting high tool costs, a considerable increase in weight 
and packaging problems, the expected demands are met only with difficulty 
or only partially. 
For this reason, secondary measures are increasingly used, such as vehicle 
body vibration absorbers. These absorbers reduce the maximum amplitude of 
the vehicle body torsional vibration and, depending on the number and 
weight of the used masses, have had considerable success while the 
modulation was optimal. Their effect, however, is usually of a narrow-band 
type so that an exact frequency modulation is required. 
Variations in series, material fatigue and temperature influences may 
result in a considerable reduction of efficiency. In addition, the weight 
may also be increased considerably depending on the number and design of 
the absorber masses. 
In order to achieve an absorption of torsional vibrations of the vehicle 
body, according to German Patent Document DE-41 11 613 A1, the drive 
subassembly is used as an absorber mass. The drive subassembly is disposed 
via elastic bearings in the vehicle body in such a manner that torsional 
vibrations of the vehicle body are converted to longitudinal vibrations of 
the drive unit, for the purpose of which mutually sloped elastic bearings 
are provided. 
It is an object of the invention to largely reduce the motor vehicle body 
vibrations by means of a drive assembly which acts as an absorber. 
This and other objects are achieved by the present invention which provides 
a motor vehicle comprising a vehicle body subject to torsional and bending 
vibrations and having side and cross members, a drive assembly suspended 
via elastic bearings to act as an absorber mass, the drive assembly being 
held by three elastic bearings between the side and cross members of the 
vehicle body. The three elastic bearings include one engine bearing which, 
with respect to the driving direction, is situated in front, and is 
arranged in a low vibration range in the drive assembly roll axis. Two 
rearward transmission bearings with a large base are held by rigid vehicle 
body elements which hold the rearward transmission bearings in a 
transverse plane and outside the roll axis in an area with maximal 
torsional vibration amplitudes. The two transmission bearings have a 
characteristic which determines a high natural frequency of a drive 
assembly roll mode and is precisely modulated to vehicle body torsion and 
has a low vertical bearing stiffness such that the drive assembly as an 
absorber swings freely about the roll axis. The three elastic bearings are 
arranged in a diagonal plane that rises towards the rear of the vehicle, 
with respect to the driving direction, and is set at an angle. The 
bearings are positioned such that a main axis of inertia of the drive 
assembly is situated in the driving direction and an overall center of 
gravity is situated in the diagonal plane. 
Some of the principal advantages achieved by means of the invention are 
that the mass of the drive assembly is held in a swivelling manner by the 
elastic bearings about a roll axis in the vehicle body, so that the 
vehicle body vibrations, such as the vehicle body torsional vibrations, 
are reduced to a significant extent. For this purpose, measures are 
required with respect to the bearing sites, the stiffness modulation, and 
the position of the assembly roll axis with respect to the overall center 
of gravity of the drive assembly. 
Thus, particularly the drive assembly is aligned as a compact assembly and 
is suspended by two elastic bearing elements close to the two rearward 
vehicle body corners as well as an engine bearing arranged in the 
longitudinal center axis of the vehicle. 
By means of the variation of the vertical stiffnesses of the two rear 
transmission bearings--with respect to the driving direction--the roll 
mode of the assembly about the longitudinal axis of the vehicle can be 
modulated to the vehicle body torsion. In particular, the engine bearing 
situated in the roll axis will be inoperative, while the two transmission 
bearings are arranged in high vibration ranges, in which case a base 
between these bearings which is as large as possible is extremely 
important. It is only in this manner that the bearing stiffnesses at the 
two transmission bearings which are required for the absorber modulation 
can be kept low. 
The bearings are arranged in a diagonal plane in which the main axis of 
inertia of the assembly is also situated and which extends through the 
center of gravity of the assembly. 
For achieving a reduction of the torsional vibrations in the vehicle body, 
the three bearings are constructed with different stiffnesses in order to 
improve the vertical comfort during road travel, to reduce the 
structure-borne noise transmitted via the bearings as well as for 
achieving a broad-band absorber effect. The bearings are therefore 
constructed to be stiffer in the vertical direction than in the 
longitudinal direction and have an approximately identical stiffness in 
the transverse and vertical directions. 
The transmission bearings are held in transversely disposed consoles which 
each include of a spring leaf standing in a vertical plane. These leaves 
are stiff about the roll axis in the vertical load direction and are 
constructed to be conditionally deflectable in the longitudinal direction 
of the vehicle for the absorption of longitudinal movements. A fastening 
to the transmission housing takes place by way of screws which are 
supported in the console by means of partially enclosed sleeves. 
The absorber affect of the drive assembly may be used for a rear engine as 
well as for a front engine, in which case the engine may be constructed as 
a transversely installed engine or as a longitudinally installed engine. 
The bearings may be constructed as rubber bearings or as hydrobearings. 
For using the drive assembly as an absorber mass for vehicle body 
vibrations, the dynamics of vertical stiffnesses of the two exterior 
transmission bearing elements are of decisive importance. When 
hydrobearings are used, the acoustic coupling system must be adapted to 
the special conditions. By the selection of a suitable uncoupling play, 
the dynamic bearing stiffness must be modulated in the relevant vibration 
path range. 
A bearing damping which is modulated in a targeted manner results in a low 
vertical engine stuttering, improves the broad-band capacity and therefore 
the insensitivity to series variations. When the damping is excessive, 
however, the absorber affect is reduced because no assembly roll mode can 
develop. The maximal effect of an assembly modulated as an absorber will 
be achieved in the case of an optimal assembly bearing concept, of a 
suitable dynamic vehicle body stiffness and an assembly bearing which is 
modulated in detail. In the case of a disadvantageous bearing concept, the 
necessity of a sufficient structure worn noise insulation represents a 
limit. 
In principle, the drive assembly will advantageously act as an absorber 
when the two transmission bearing sites have a maximally large base. They 
are situated on the outside on the side members of the motor vehicle body 
or on corresponding lateral body parts. This permits a high natural 
frequency of the assembly roll mode while the bearing stiffness is low so 
that an effective acoustic insulation of the bearings can also be 
achieved. 
The position of the transmission bearings on the high-vibration vehicle 
body corners permits an effective absorption of the torsion mode. 
The third bearing, the engine, is situated in the assembly roll axis. The 
absorbing forces are transmitted only by way of the transmission bearings, 
that is, at points with a high vehicle body sensitivity. 
The sporty chassis modulation, extreme low cross-section tires, and the 
wide track width result in an increased excitation of the vehicle body 
torsion during road travel which is considerably reduced by the drive 
assembly acting as the absorber. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 schematically illustrates a motor vehicle comprising a rear drive 
assembly 1, consisting of an engine 2 and a transmission 3. This assembly 
1 is held by way of a, with respect to the driving direction F, forward 
engine bearing 4 and two rearward transmission bearings 5 and 6 in the 
vehicle body K. 
The engine bearing 4 is arranged in a low vibration area A in the rear 
drive assembly roll axis 7 and is supported on a cross member 8 of the 
vehicle body. By means of a traverse 10 (FIG. 2), the bearing 4 is 
connected with the engine casing 11. 
The transmission 3 is supported on the vehicle body K (FIG. 3) via the 
bearings 5 and 6 which are held on their ends by transversely extending 
consoles 12, 13 which are connected with the transmission housing 14. 
These bearings 5 and 6 are arranged in a high vibration range B which is 
arranged outside the roll axis 7 preferably in vehicle body corner areas 
in which there are maximal torsional vibration aptitudes. 
The bearings 4, 5 and 6 are provided in a diagonal plane X--X set at an 
angle .alpha., which diagonal plane X--X extends approximately through the 
overall center of gravity S of the drive assembly 1 and is situated in the 
assembly roll axis 7. In particular, the transmission bearings 5, 6 with a 
large base b (FIG. 2), (i.e. the bearings 5, 6 are situated as far apart 
as possible), are fastened to the vehicle body K because the torsional 
deflections are highest at this point and the absorption of the torsional 
vibrations of the vehicle body are therefore most effective at this point. 
The transmission bearings 5, 6 are modulated with respect to the drive 
assembly or to the vehicle body in such a manner that they have a 
characteristic which determines a high natural frequency of the assembly 
roll mode and which is modulated precisely to the vehicle body torsion and 
has a low vertical bearing stiffness. 
As a result, the drive assembly 1 can freely swing about its roll axis 7 in 
such a manner that it acts as an absorber, but at the same time a vertical 
comfort is achieved with respect to disturbance quantities coming from the 
road. 
The vertical stiffnesses in the direction of arrow 15 of the transmission 
bearings 5, 6 are coordinated such that they are higher with respect to 
the longitudinal stiffnesses in the direction of arrow 16. The transverse 
stiffness in the direction of arrow 17 (FIG. 3) corresponds approximately 
to the vertical stiffness in the direction of arrow 15. The drive assembly 
1, which is adapted in such a manner as an absorber, results in a 
considerable reduction of the vehicle body torsion by means of counter 
vibrations of the drive assembly 1, in which case the engine bearing 4 
remains at rest. 
By means of a special adaptation of the vertical stiffnesses of the two 
transmission bearings 5 and 6, the assembly roll mode about the vehicle 
longitudinal axis can be modulated more easily to the vehicle body 
torsion. 
The bearings 4, 5 and 6 are supported on members of the vehicle body or on 
stiff body elements or on interposed structural elements. These supports 
have a stable and rigid construction. 
The transmission bearings 5, 6 are connected with the transmission housing 
14 via the transversely extending consoles 12, 13. In the illustrated 
exemplary embodiment, these consoles 12, 13 are constructed as leaf 
springs and are set in a perpendicular plane Y--Y so that they are stiff 
in the vertical direction, that is when the assembly is moved, about the 
roll axis 7 and can be conditionally bent in the longitudinal direction of 
the vehicle. 
As illustrated in detail in FIGS. 4 to 6, the bearing 6 is housed in a 
receiving device 18 of the console 13. The fastening of the console 13 on 
the transmission housing 14 takes place via screws 19, for example. The 
screws 19 are supported on sleeves 21 which are partially surrounded by 
shaped-out portions 20 of the console and rest directly against the 
housing wall by means of a face. The sleeves 21 are fixably connected with 
the console 13. 
A deformation of the assembly in the driving direction F as well as against 
the driving direction F is almost excluded as a result of the leaf spring 
12, 13 which is soft in these directions. An additional tolerance 
compensation in the driving direction and against the driving direction in 
bearings 5 and 6 will not be necessary because the longitudinal forces are 
absorbed mainly by the engine bearing. 
Although the invention has been described and illustrated in detail, it is 
to be clearly understood that the same is by way of illustration and 
example, and is not to be taken by way of limitation. The spirit and scope 
of the present invention are to be limited only by the terms of the 
appended claims.