Motor vehicle forward structure having a knee restraint with defined yielding capability

A forward structure of a motor vehicle has a cross member which connects the two A-columns with one another and which is used for fixing a steering column and a knee restraint. A strut is fixed on the forward structure and extends from the cross member to a transmission tunnel extending in the longitudinal direction of the vehicle. At least two mutually overlapping strut sections of the strut have a connection area which, under a load during a crash and after overcoming a force threshold, can be elongated in a longitudinal direction while absorbing energy. A force limiting device is provided in the connection area of the strut sections in the force transmission path. Consequently, after the force threshold is exceeded, an energy reduction is achieved during a predetermined lengthening path of the connecting strut and at a constant force level so that a yielding of the impact-loaded knee restraint together with the cross member takes place with a force-limited resistance along a predetermined path distance.

BACKGROUND OF THE INVENTION 
The present invention relates to a forward structure of a motor vehicle and 
more particularly, to a structure having a cross member which connects the 
two A-columns with one another, for fixing a steering column and a knee 
restraint, and having a strut which extends from the cross member to a 
transmission tunnel extending in the longitudinal direction of the vehicle 
and which has at least two mutually overlapping strut sections whose 
connection area in the case of a load during a crash can be elongated in 
the longitudinal direction after overcoming a force threshold while 
absorbing energy. 
In Mercedes-Benz vehicles, the steering column and the knee restraint for 
the driver and the front passenger are fixed on the vehicle cross member 
extending between the A-columns in a freely supporting manner in the 
vehicle transverse direction. A connecting strut projects from the vehicle 
cross member and is fastened on the transmission tunnel which extends in 
the longitudinal direction of the vehicle and is shaped out of the floor 
panel. The strut dampens vibrations introduced into the vehicle cross 
member and from there into the steering column. 
In the event of a vehicle crash, the transmission tunnel is lowered in the 
known construction and, in the process, by way of the connecting strut, 
advantageously pulls down the cross member and the steering column. As a 
result, where, in a lower position, the steering wheel is directed against 
the occupant and, when stressed by the occupant, does not tilt 
upwards/toward the front. Thereby, a triggered steering wheel air bag also 
receives better alignment toward the occupant, and a pushing-away of the 
air bag over the steering wheel rim can be avoided. 
When being stressed by the occupant, the knee restraint yields on its 
possible path in an energy reducing manner to the extent permitted by the 
knee restraint's construction in the assigned space. The connecting strut 
consists of two strut parts which overlap one another in the connection 
area and which are screwed to one another. One of the connecting struts 
has an oblong hole to compensate for the spacing tolerances between the 
vehicle cross member and the transmission tunnel during the fixing of the 
connecting strut. Also this frictional screwed connection can yield in the 
event of a crash as a result of the high forces whereby an elongation of 
the strut is caused. 
In the known construction, however, the path of force and the energy 
reduction resulting from this elongation cannot be predetermined because, 
for example, the material structure of the struts and the tightening 
moment of the screw can change the coefficient of friction. For this 
reason, how far and with which force the knee restraint will yield when 
stressed by the occupant can be precalculated only imprecisely. 
SUMMARY OF THE INVENTION 
It is an object of the invention to achieve, in a forward structure of a 
motor vehicle, a defined yielding of the knee restraint. 
This object has been achieved in accordance with the present invention by 
providing that in the connection area, a force limiting device is provided 
in the force transmission path, by way of which, after the exceeding of 
the force threshold, during a predetermined elongation path of the strut, 
an energy reduction is achieved at a constant force level so that a 
yielding of the impact-loaded knee restraint together with the cross 
member takes place with a force-limited resistance along a predetermined 
path. 
Corresponding to a targeted construction, the connection area of the strut 
parts of the present invention in the event of a tensile load during a 
crash will yield until a first force threshold is reached while, after 
overcoming this force threshold, a further elongation with a constant 
force level is permitted. As the result of the deformation of the force 
limiting device, more energy is absorbed in the force transmission path. 
Therefore, when an impact onto the knee restraint occurs, the stressing of 
the occupant's knees rises to this force threshold which is below a 
serious injury. Subsequently, a largest possible elongation path of the 
connecting strut assures that no higher stressing, which exceeds this 
threshold value of the occupant's knees, takes place. 
The stressing threshold value, like the one for the course of the energy 
reduction with the further elongation of the connecting strut, can be 
adjusted by the force limiting device in the force transmission path of 
the connection area. Therefore, the value can be coordinated in a defined 
manner to the stressing relationships in the vehicle crash. 
To easily implement the force limiting device in the force transmission 
path in the connection area of two strut sections, to carry out the 
present invention, the strut sections are constructed as flat profiles 
which overlap in the connection area and fixed to one another by at least 
one fastening bolt. The oblong hole, which is provided on at least one 
strut section, will then have an area into which a fastening bolt is 
pulled when the strut is stressed by tension. Thereby, this area has a 
smaller width than the bolt diameter. Along this elongation path, the 
fastening bolt causes a uniform expansion or deformation of the oblong 
hole area with a constant force level. This force level characterizes also 
the stress value for the occupant's knees which impact on the knee 
restraint, pushes it away from the occupant and thereby causes the strut 
to elongate. 
For a constant course of the movement during the strut elongation, 
upward-projecting guide edges can be provided on a flat profile for 
displaceably receiving the other strut section. Likewise, plates with 
predetermined coefficients of friction can be inserted between mutually 
displaceable surfaces of the flat profiles. 
Another way to provide a force limiting device in the force transmitting 
path of the connection area of the strut sections is with a connecting 
tube whose force necessary for a pulling apart while absorbing energy can 
be predetermined. Also, in the connection area of the strut sections, a 
rod with a piston on the end could be implemented on a strut section. The 
rod projects into a tube which is constructed on another strut section. 
The end section of the tube, during piston displacement, absorbs energy 
and is expanded by the introduction of force at the same force level.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a view of a cutout of a forward structure 1 of a motor vehicle in 
which the arrangement of a tube-shaped cross member 2 can be viewed from 
the vehicle interior. Each end of the cross member 2 is fastened on the 
two vehicle A-columns 3 which are shown here only in a section. A steering 
column 4, which on its upper end carries the steering wheel (not shown), 
is fastened on the cross member 2. Furthermore, a knee restraint 5 for the 
front passenger is fastened on the cross member 2 and has two holders 6 
bent in a U-shape and which are covered by a deflector plate 7 toward the 
interior. The bent holders 6 make possible that the deflector plate 7, 
when loaded during a crash, can yield corresponding to the construction of 
the holders. As a result, impact energy and load to the knees is reduced. 
The possible path of the deflector plate 7 in the upward direction and in 
the direction of the vehicle front, with a simultaneous reduction of 
energy and with a limited maximal effect of force onto the occupant's 
knees, is further improved with the present invention as hereinafter 
described. 
A strut 9 is a connection part between the cross member 2 and a raised 
transmission tunnel 8 extending in the longitudinal direction of the 
vehicle along the vehicle floor. During a crash which causes a lowering of 
the transmission tunnel 8 during the deformation of the forward structure 
1 of the motor vehicle, the strut 9 pulls the cross member 2 a distance 
farther in the downward direction. Consequently, the steering column 4 
close to the driver is also lowered which is advantageous with respect to 
his or her impact on the steering wheel or on an inflating steering wheel 
air bag. This lowering prevents the steering column 4, when loaded, from 
being pushed away in the upward direction, and the air bag can also not be 
pushed away along the steering wheel rim. 
With an increased tension exercised onto the strut 9 by virtue of the 
loading of the steering column 4, the knee restraint 5, the cross member 2 
or other parts operatively connected therewith, the strut 9 will yield in 
its connection area 10 between the two strut sections 11, 12. In order to 
achieve an introduction of force at a constant force level 13 (FIG. 2) 
after a short time and also a constant energy reduction, a force limiting 
device 14 is provided in the connection area 10. 
FIG. 2 shows one embodiment of a path or course of force F over the 
elongation path s of the strut 9. The rising tension force exercised on 
the strut 9 results in a slight elongation. After a constructively 
determinable force threshold 15 is reached, characterizing the maximal 
value of the force to be introduced, the force limiting device 14 is 
activated. This activation limits the force introduction as well as the 
energy reduction to a force level 13 in a uniform amount when the strut 9 
is elongated further. This means that, when loaded by the occupant's 
knees, the knee restraint 5, together with the cross member 2, can be 
displaced in the direction of the vehicle front and in the upward 
direction. Thereby, impact energy is reduced and the counterforce onto the 
occupant is limited to the predetermined force level 13. The force level 
13 is set, of course, at a value at which the occupant is exposed to a 
load which is physically still bearable. 
FIGS. 3a and 3b show an embodiment of a force limiting device 14 in which 
the strut sections 11, 12 are formed by flat profiles 16, 17, 
respectively, which are disposed on one another in the connection area 10 
and which are firmly connected with one another by two bolts 18. The flat 
profile 17 has an oblong hole 19 through which the bolt shanks 20 project 
which also extend through bores 21 of the other flat profile 16. When a 
tensile force is exercised on the strut 9 in excess of the holding force 
of the bolts 18, the flat profile 17 on the flat profile 16 with a certain 
frictional resistance of the bolt heads and the nuts is pulled downward. 
Thereby, the upper one of the bolt shanks 20 is pulled into an oblong hole 
area 22 which has a smaller width than the bolt shank 20. Now, in its 
further movement, the bolt shank 20 deforms the oblong hole area 22, with 
the force required for this purpose being constructionly set to the height 
of the intended force level 13. 
In order to achieve a uniform displacing movement, the flat profile 16 has 
upward-projecting guide edges 23 for displaceably receiving the flat 
profile 17. Plates 24 with a predetermined friction coefficient are placed 
between mutually displaceable surfaces of the flat profiles 16, 17. 
FIG. 4 shows another embodiment of a force limiting device 14' which is 
formed by a connecting tube 25 but which is provided here as a connection 
area 10' between the two strut sections 11', 12' of the strut 9'. The 
characteristic of the connecting tube 25 is that, with a tensile load, the 
tube 25 can be pulled apart in the direction of its longitudinal axis 
while absorbing energy with constant resistance. 
FIG. 5 shows yet another embodiment of a usable force limiting device 14". 
In the connection area 10", a strut section 12" ends in a rod 26 with an 
end having a piston 27 projecting into the hollow space of a tube 28 on 
the other strut section 11". A tensile load exercised on the strut 9" in 
the axial direction causes the piston 27 to be pulled into a plastic 
insert 29 in the end section of the tube 28 with constant resistance, and 
the plastic insert 29 deforms while absorbing energy. 
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.