Longitudinally-divided torsion bar optimized for weight and stability

A torsion bar has a torsion bar arm and a torsion bar back connected thereto in one piece. Successive longitudinal sections are provided with different cross-sections. Consequently, a permissible maximal tension is not exceeded at any point and a relative weight optimum is achieved.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to a torsion bar, such as a stabilizer for 
motor vehicles, and more particularly to a torsion bar having a torsion 
bar arm and a torsion bar back connected thereto in one piece as well as a 
reinforced transition area between the torsion bar arm and the torsion bar 
back configured such that, by application of force at predetermined 
points, by the arrangement is stressed with respect to torsion and/or 
bending. 
DE-OS 28 05 007 and DE-OS 28 46 445 show known torsion bars constructed of 
a massive material or in a tube shape with a small cross-section. 
Reinforcements are provided in the transition area between the torsion bar 
back and the torsion bar arm in that the cross-section of the torsion bar 
or of the tube wall is enlarged there. In this manner, a good utilization 
of material is to be achieved while the weight is comparatively low. 
If the torsion bar is constructed as a so-called stabilizer bar of a motor 
vehicle, the torsion bar back is arranged in the transverse direction of 
the vehicle and is provided on both ends with one torsion bar arm 
respectively. The torsion bar arms extend essentially in the longitudinal 
direction of the vehicle or have a component in the longitudinal direction 
of the vehicle. Close to the torsion bar arms, the torsion bar back is 
rotatably disposed on the chassis of the vehicle. The free ends of the 
torsion bar arms are connected with axle parts or suspension parts of the 
vehicle wheels of an axle. 
An object of the invention is to provide a torsion bar which is optimized 
with respect to its weight and its stability. 
According to the present invention, this object has been achieved in that 
the torsion bar can be divided into longitudinal sections at least in 
areas, which longitudinal sections, in the case of a virtual 
cross-sectional change have approximately equal values for the 
differential d(1/c.sub.i)/dV.sub.i and/or have approximately equal values 
for the mathematical product c.sub.i V.sub.i, with c.sub.i being the 
proportional spring rate relative to the force application points and 
V.sub.i being the volume of the respective longitudinal section. 
The present invention is based on the recognition that the torsion bar 
should be constructed in such a manner that the material housed within the 
outer layer of each longitudinal section is better utilized with respect 
to the spring rate of the torsion bar than the material which, in the case 
of a virtual cross-sectional enlargement of the respective longitudinal 
section, would be situated outside the outer layer of the actual 
cross-section of the longitudinal section. In the case of a torsion bar 
with a circular cross-section, the two first conditions are largely 
equivalent. 
The number as well as possibly different lengths of the longitudinal 
sections may be predetermined while taking into account additional 
requirements. For example, with a view to a simplified manufacturing, it 
could be predetermined that, within a certain longitudinal section, a 
cross-section should exist which is predetermined, for example, by torsion 
bar bearings. Outside this longitudinal section there should exist only a 
predetermined small number of longitudinal sections with different 
cross-sections. 
According to a first embodiment of the present invention, each longitudinal 
section can be constructed with a uniform cross-section so that adjacent 
longitudinal sections adjoin one another while forming a circumferential 
step. 
Instead, it is also within the scope of the present invention to construct 
the longitudinal sections between neighboring sections with different 
cross-sections conically or in a similar manner such that the 
above-mentioned circumferential steps are avoided. The optimal conical 
form is reached in this situation when the torsion bar is subdivided into 
very many and correspondingly short longitudinal sections to which the 
above-mentioned conditions apply. 
In a tube-shaped torsion bar, it is advantageous from a manufacturing 
perspective to produce the desired shape in that a tube, which is used for 
forming the torsion bar and whose longitudinal axis already largely has 
the shape of the longitudinal axis of the desired torsion bar, is placed 
into a negative mold and is then, by way of both tube ends, acted upon in 
the interior by a hydraulic pressure of a magnitude of from 2,000 to 3,000 
bar, in which case the tube is, at the same time, axially set. As a 
result, the tube, while being irreversibly deformed, will mold itself onto 
the negative mold by way of which, in the case of a corresponding 
development, basically arbitrary cross-sectional courses of the torsion 
bar may be predetermined.

DETAILED DESCRIPTION OF THE DRAWINGS 
The torsion bar 1 shown in FIG. 1 has a torsion bar back 1', which is 
rotatably disposed in pivot bearings 2 in a generally known manner on a 
chassis (not shown) or on parts of a motor vehicle connected thereto, as 
well as torsion bar arms 1", which adjoin the torsion bar back 1' in one 
piece and are connected on their free ends with wheel suspension part, 
(also not shown) for vehicle wheels arranged on the vehicle in a known 
vertically movable manner. When the wheels carry out different vertical 
lifts or move relative to the vehicle body in mutually opposite 
directions, the torsion bar arms 1" are angled relative to one another in 
an axial view of the torsion bar back 1', in which event the entire 
torsion bar 1 is stressed with respect to torsion as well as with respect 
to bending. 
In the present invention, the torsion bar 1 is divided into a plurality of 
longitudinal sections which are designated herein as L.sub.i, whereby the 
subscript i represents different numbers for different longitudinal 
sections. As illustrated in FIG. 1, these longitudinal sections L.sub.i 
have different cross-sections, in which case, for example, the 
cross-sections of the longitudinal sections L.sub.i in the area of the 
pivot bearings 2 may be predetermined by the dimensioning of the pivot 
bearings. 
In the event of different vertical lifts of the wheels, that is, different 
swivel movements of the torsion bar arms 1", all longitudinal sections 
L.sub.i are elastically deformed, i.e. twisted and/or bent. Thereby, a 
spring rate c.sub.i can be assigned to each longitudinal section L.sub.i 
for the resulting deformation which reflects the ratio between the extent 
of the deformation and the forces causing the deformation on the respect 
longitudinal section L.sub.i. The sum of the reciprocal values 1/c.sub.i 
of the spring rates of all longitudinal sections L.sub.i forms the 
reciprocal value 1/c of the overall spring rate of the torsion bar 1. 
The respective value c.sub.i of each longitudinal section L.sub.i depend 
not only on its length and cross-section but also on its position in the 
torsion bar 1, the shape of the torsion bar 1, the arrangement of its 
bearings 2, and the connections with the wheel suspension parts. The 
respective values c.sub.i therefore refer to a predetermined shape of the 
torsion bar 1 as well as to force application points predetermined by the 
bearings 2 and the arrangement of the connections with respect to the 
wheel suspension parts. 
The cross-sections of the different longitudinal sections L.sub.i are first 
predetermined such that the differentials d (1/c.sub.i)/dV.sub.i of all 
longitudinal sections L.sub.i, in the case of a virtual cross-sectional 
change, have at least approximately the same values. That is, the torsion 
bar 1 has a relative weight optimum. Specifically, if one of the 
longitudinal sections L.sub.i were constructed with an enlarged 
cross-section and, instead, another of the longitudinal sections L.sub.i 
were provided with a reduced cross-section so that the overall spring rate 
of the torsion bar 1 remains unchanged, the weight of the torsion bar is 
increased. The values dV.sub.i can be depicted as hollow cylinders whose 
length corresponds to the length of the respective section L.sub.i and 
whose wall thickness is "infinitely" small. 
In the following description, a dimensioning control will be carried out. A 
tolerable tension is predetermined first which must not be exceeded in any 
of the longitudinal sections L.sub.i when the torsion bar deforms 
maximally, as it is permitted because of its constructive arrangement and 
configuration. Then, those cross-sections are determined for the 
longitudinal sections L.sub.i in the event of which all longitudinal 
sections L.sub.i, when there is a maximal deformation of the torsion bar 
1, have approximately the same predetermined maximal value of the tension. 
Should it now be found that, in this determination of the cross-sections, 
the cross-section of one of the longitudinal sections L.sub.i is larger 
than in the case of the first described determination of the 
cross-sections, the cross-section of this latter longitudinal section is 
increased to the latter value. 
Then, after the first determination process, another calculation of the 
cross-sections of the other longitudinal sections takes place, 
specifically such that the overall spring rate of the torsion bar 1 has 
the desired value. 
Optionally, the above-mentioned calculations are repeated several times in 
order to ensure that, on one hand, the permissible maximal tension is not 
exceeded at any point of the torsion bar 1 and, on the other hand, a 
relative weight optimum exists. 
With concessions to the achievable minimal weight, a torsion bar of the 
predetermined spring rate with the same maximal tension in each of the 
selected longitudinal sections can also be obtained. Then, the tension 
value which occurs at otherwise given conditions is the lowest one 
possible. In this manner, a reasonably priced material with lower 
permissible stress values can be used. 
According to FIG. 2, the longitudinal sections L.sub.i may also each have a 
conical construction, in which case the cross-section in an axially 
central area of a conical section will then, in each case, correspond to 
the cross-section of the corresponding longitudinal section of FIG. 1. 
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.