Description
TUBULAR TORSION BEAM FOR REAR SUSPENSIONS OF
VEHICLES AND MANUFACTURING METHOD THEREOF
Technical Field
[ 1 ] The present invention relates, in general, to a tubular torsion beam for rear sus-
pensions of vehicles and a manufacturing method thereof and, more particularly, to the
provision of tubular torsion beams having excellent roll stiffness and excellent roll
strength, produced through hydroforming.
Background Art
[2] Suspensions for vehicles are structural devices used for suspending a vehicle body
and absorbing shocks from the road during the operation of a vehicle, thereby
preventing the shocks from being applied to the vehicle body and to passengers. Thus,
the suspensions must be designed such that they can attenuate shocks from a road and
make passengers feel comfortable despite the shocks, and improve steering stability,
determined by the ground contact force of tires during running of vehicles. Another
important factor to be considered while designing suspensions is that the suspensions
must maintain desired stiffness and desired durability despite the repeated application
of shocks from roads thereto. Deformations or cracks formed in the suspensions may
impose fatal effects on vehicle stability, and thus the durability design of the sus-
pensions plays an important role in the functional design of the suspensions.
[3] Particularly, a torsion beam suspension, typically used as a rear suspension of a
small-sized vehicle, must be designed to have high durability because a torsional load
is repeatedly applied to a torsion beam of the suspension. In the torsion beam
suspension, the cross-sectional shape of the torsion beam plays an important role in the
durability performance of the beam. The cross-sectional shapes of torsion beams may
be variously designed according to the different characteristics of vehicles. However,
in the initial stage of designing a torsion beam, the cross-sectional shape of the torsion
beam must be determined in relation both to the roll stiffness and to the roll strength of
a vehicle using the torsion beam, and thus it is required to carefully study the roll
stiffness and the roll strength.
[4] In other words, the torsion beam of a rear suspension, which couples a left wheel and
a right wheel together, is an important element in maintaining the stiffness of the
suspension and in determining the dynamic characteristics of the suspension during the
operation of a vehicle. Thus, the torsion beam must be designed such that it has ap-
propriate roll stiffness, determined by the weight of the vehicle, so as to resist torsional
deformation and bending deformation, which take place when the left wheel and the

right wheel execute respective motions in opposite directions. Further, because normal
stress and shear stress are concentrated on the torsion beam, it is required to design the
torsion beam such that the beam has appropriate roll strength and has fatigue resistance
determined in consideration of running-induced fatigue.
[5] Hereinbelow, the construction and problem of a prior art torsion beam suspension
will be described with reference to FIG. 1. which shows a suspension equipped with a
conventional plate-type torsion beam. The prior art torsion beam suspension, typically
used as a rear suspension in a small-sized vehicle, comprises two trailing arms, which
are left and right trailing arms 2 coupled together by a plate-type torsion beam 3, and a
bush sleeve 1. which is provided at the front end of each of the two trailing arms 2 and
pivots on a vehicle body using a rubber bush. Further, both a spring seat 4 for
supporting a suspension spring thereon and a damper bracket 5 for supporting a shock
absorber are mounted to the inner side of the rear end of each of the two trailing arms
2. Both a wheel carrier 6 and a spindle plate 7 for holding the rear wheels of a vehicle
are mounted to the outer side of the rear end of each of the two trailing arms 2. The
above-mentioned bush sleeves 1, trailing arms 2, spring seats 4, damper brackets 5.
wheel carriers 6 and spindle plates 7 form basic elements constituting the torsion beam
suspension.
[6] The conventional plate-type torsion beam 3 is typically produced using a thick iron
plate having a thickness of about 4-6 mm through pressing such that the beam 3 has an
open cross-section in a shape of D, c. A, <. >. etc. The plate-type torsion beam 3.
having the above-mentioned open cross-section, has low stiffness and low strength,
resisting torsional deformation or bending deformation, so that, to increase the stiffness
and strength of the torsion beam 3. a reinforcement, such as a torsion bar 8, must be
mounted to the torsion beam 3. However, due to the reinforcement, the plate-type
torsion beam 3 is problematic in that the increased number of elements constitutes the
beam 3, complicates the production process of the beam 3, and increases the weight of
a final product.
[7] To solve the problem of the plate-type torsion beam 3, a suspension having a tubular
torsion beam has been used in recent years. An example of suspensions having con-
ventional tubular torsion beams is illustrated in FIG. 2. As shown in FIG. 2. a bush
sleeve 1. a trailing arm 2. a spring seat 4. a damper bracket 5, a wheel carrier 6 and a
spindle plate 7 are used as basic elements constituting a conventional tubular torsion
beam suspension.
[8] The tubular torsion beam 10 of the suspension is produced through pressure-forming
using a tubular steel member having a circular cross-section. During the pressure-
forming, the tubular steel member is shaped into a torsion beam having a cross-section
varying along the entire length thereof. The tubular torsion beam 10 comprises

opposite ends 11, which have a closed cross-section, such as a triangular, rectangular
or circular cross-section, and are mounted to respective trailing arms 2 of the
suspension, a middle portion 13, in which a first semicircular surface part 13a is
deformed so as to be in close contact with a second semicircular surface part 13b such
that they form a V-shaped cross-section, which is open to one side, and a transitional
portion 12, the size of the cross-section of which continuously varies and executes a
natural transition from the middle portion 13 to each of the opposite ends 11.
Described in detail, the middle portion 13 has a small-sized closed cross-section at
each end of the V-shaped cross-section. However, because most of the first semi-
circular surface 13a is in close contact with most of the second semicircular surface
13b, the middle portion 13 is regarded as a part having an open cross-section.
[9] In FIG. 2, each of the opposite ends 11 is illustrated as having a closed rectangular
cross-section with rounded corners. However, it should be understood that the cross-
section of the opposite ends 11 is not limited to the rounded rectangular cross-section,
but may be configured to have some other closed cross-section, such as a triangular,
angled rectangular or circular cross-section, according to the type of vehicle. When the
tubular torsion beam 10 having the above-mentioned construction is compared to the
plate-type torsion beam 3 having only an open cross-section, the tubular torsion beam
10 has higher stiffness and higher strength, resisting torsion and bending. Thus, the
tubular torsion beam 10 may be used without additional reinforcement.
[10] As described above, the tubular torsion beam 10 is produced through shaping such
that the torsion beam 10 has a cross-section continuously varying along the entire
length thereof. To produce such a tubular torsion beam in the prior art. conventional
pressing or hydroforming has been used. An example of conventional pressing
techniques is disclosed in Korean Patent No. 554310. The pressing technique disclosed
in Korean Patent No. 554310 will be described hereinbelow with reference to FIG. 3.
[11] To produce such a tubular torsion beam through conventional pressing, first, a
tubular steel member 20 is placed between upper and lower molds 21 and 22, which
have specified shaping surfaces configured to shape opposite ends having a closed
cross-section, a transitional portion having a varying cross-section, and a middle
portion having a V-shaped open cross-section. After placing the steel member between
the two molds, upper and lower pad molds 23 and 24 are actuated so as to shape
opposite ends having closed cross-sections through pressing [FIG. 3(a)]. Thereafter,
cylinder actuators 26 are operated so as to insert left and right cores 27 into respective
opposite ends of the tubular steel member. After the insertion of the cores, the upper
and lower molds 21 and 22 are actuated so as to shape a transitional portion and a
middle portion through pressing, thus producing a desired tubular torsion beam [FIG.
3(b)]. Thereafter, the upper mold 21 is lifted upwards prior to removing the tubular

torsion beam from the lower mold 22 using a push rod 25.
[12] However, the conventional pressing requires a complex molding technique but nev-
ertheless, fails to realize high processing precision, so that the pressing cannot provide
a product having a precise cross-sectional shape or a uniform thickness, thus increasing
the defective proportion of products.
[ 13] In an effort to solve the problems of the conventional pressing, hydroforming has
preferably been used in recent years. Korean Patent Laid-open Publication No.
2004-110247 discloses an example of a conventional hydroforming technique. The hy-
droforming technique disclosed in Korean Patent Laid-open Publication No.
2004-110247 will be described with reference to FIG. 4. As shown in FIG. 4, to
produce a tubular torsion beam through hydroforming, first, a tubular- steel member is
placed on a lower mold 32. Thereafter, upper and lower molds 31 and 32 are actuated
in cooperation with two guide molds 33, thus shaping opposite ends having a
rectangular closed cross-section through pressure forming [FIG. 4(a).(b)l. After
shaping the opposite ends, elliptical axial punches 36, which are attached to respective
mandrel units, operated in a lengthwise direction relative to the tubular steel member,
are advanced so as to seal the opposite ends of the tubular steel member. After sealing
the opposite ends, actuation oil is fed into the tubular steel member through inlet holes
formed through central axes of the axial punches 36, thus applying hydraulic pressure
to the inner surface of the tubular steel member. Thereafter, upper and lower punches
34 and 35 are actuated so as to shape both a middle portion and transitional portions,
thus producing a desired tubular torsion beam 30 through pressure forming [FIG. 4(c)].
[14] In the hydroforming technique, pressure of the actuation oil is evenly and con-
tinuously applied to the entire inner surface of the tubular steel member, so that it is
possible to precisely control the shape and thickness of a tubular torsion beam, thus re-
markably reducing the defective proportion of products in comparison with the con-
ventional pressing techniques. Thus, the technique of producing tubular- torsion beams
through hydroforming has been actively and variously studied recently.
[15] To realize desired vehicle stability, a highly durable design of tubular torsion beams
for suspensions has been required. In the prior art, the design of highly durable tubular-
torsion beams has concentrated on the use of high strength materials or thick materials
as materials for the beams. However, the use of hish strength materials reduces work
efficiency during hydroforming and the use of thick materials increases the weights of
car bodies, thus limiting the design of durable tubular- torsion beams.
Disclosure of Invention
Technical Problem
[16] Accordingly, the present invention has been made keeping in mind the above

problems occurring in the related art, and is intended to provide a tubular torsion beam
for rear suspensions of vehicles, which is produced through hydroforming and has an
optimal shape, capable of reinforcing a stress-concentrated portion of the beam, with a
cross-section varying along the entire length thereof, thus having improved durability.
The present invention is also intended to provide a method of manufacturing the
tubular torsion beam.
Technical Solution
[ 17] In order to achieve the above object, the present invention provides a tubular torsion
beam for rear suspensions of vehicles and a method of manufacturing the tubular
torsion beam, in which the tubular torsion beam is produced by pressure-forming a
tubular steel member through hydroforming such that the tubular torsion beam has a
cross-section varying along the entire length thereof, with opposite ends having a
closed cross-section and being mounted to respective trailing arms, a middle portion
having a V-shaped open cross-section, and a transitional portion having a varying
cross-section and connecting the middle portion to each of the opposite ends, wherein
opposite ends of the tubular steel member are fed using axial punches of a hy-
droforming machine, so that the opposite ends of the tubular torsion beam are thicker
than the middle portion.
[18] In an aspect, an inclined offset may be formed between the middle portion and each
of the transitional portions, so that both the transitional portions and the opposite ends
are enlarged outwards compared to the middle portion.
[19] In another aspect, a bead may be formed on the surface of each of the transitional
portions.
Advantageous Effects
[20] The tubular torsion beam for rear suspensions of vehicles and the method of manu-
facturing the tubular torsion beam according to the present invention are advantageous
in that the roll stiffness and roll strength of the tubular torsion beam are improved by
increasing the thickness of a stress-concentrated portion of the torsion beam or by
forming an offset or a bead in the stress-concentrated portion, thus realizing high
durability.
Brief Description of the Drawings
[21] The above and other objects, features and other advantages of the present invention
will be more clearly understood from the following detailed description when taken in
conjunction with the accompanying drawings, in which:
[22] FIG. 1 is a perspective view illustrating a rear suspension for vehicles having a plate-
type torsion beam:
[23] FIG. 2 is a perspective view illustrating a rear suspension for vehicles having a con-

ventional tubular torsion beam:
[24] FIG. 3 is a view illustrating a process of manufacturing a conventional tubular
torsion beam according to an embodiment of the prior art;
[25] FIG. 4 is a view illustrating a process of manufacturing a conventional tubular
torsion beam according to another embodiment of the prior art;
[26] FIG. 5 is a perspective view illustrating finite element modeling of torsion beams;
[27] FIG. 6 is a view illustrating stress concentration according to finite element modeling
of a torsion beam:
[28] FIG. 7 is a perspective view illustrating the construction of tubular torsion beams
according to embodiments of the present invention;
[29] FIG. 8 is a perspective views illustrating the construction of a tubular torsion beam
according to another embodiment of the present invention:
[30] FIG. 9 is a view illustrating a preforming machine according to the present invention:
[31] FIG. 10 is a view illustrating a hydroforming machine according to the present
invention;
[32] FIG. 11 is a view illustrating an axial punch used in the hydroforming machine
according to the present invention: and
[33] FIG. 12 is a view illustrating a tubular torsion beam manufacturing method using the
axial punch of FIG. S.
[34] Description of reference characters of important parts
[35] 1: bush sleeve 2: trailing arm
[36] 3: plate-type torsion beam 4: spring seat
[37] 5: damper bracket 6: wheel carrier
[38] 7: spindle plate 10. 20, 30: tubular torsion beam
[39] 11: opposite ends 12: transitional portion
[40] 13: middle portion 14: bead
[41] 15: offset 40: preforming machine
[42] 50: hydroforming machine 60: axial punch
[43] 61: cylinder rod 62: support bar
[44] 63: punch unit 64: close contact surface
[45] 65: inlet hole
Mode for the Invention
[46] Hereinbelow, a tubular torsion beam for rear suspensions of vehicles and a manu-
facturing method thereof according to preferred embodiments of the present invention
will be described in detail with reference to the accompanying drawings.
[47] The inventor of the present invention used FEM (Finite Element Modeling) to design
a durable tubular torsion beam. FEM is a method that analyzes actual arising physical

variation by inputting the physical phenomena of problems to be solved, by modeling
the physical phenomena with finite elements having mathematical concepts, and by
simulating the physical phenomena based on the finite elements.
[48] Finite element modeling for durable suspension design is executed as follows. A link
system is modeled with beam elements, and a tire, a wheel and a brake are modeled
with concentrated mass elements in consideration of the characteristics of mass and
inertia. Further, a suspension spring and a shock absorber are modeled with linear stiff
spring elements in consideration of equivalent damping stiffness relative to a
maximum damping coefficient. Thus, the finite element modeling is executed so as to
be almost equal to the actual behavior of the suspension.
[49] A bush is modeled with linear stiff spring elements between two nodes of a
connected pan in consideration of linear stiffness values obtained through tests.
Further, a position at which a bush sleeve is pivoted to a vehicle body using the bush,
is connected over all of the nodes of an actually connected pait using the beam
elements, such that a load can be uniformly distributed over the portion. Structural
parts, such as a knuckle, a trailing arm. and a torsion beam, which may be deformed by
the direct application of a load thereto and impose effects on the behavior charac-
teristics of the suspension, are modeled with solid elements or shell elements.
[50] FIG. 5 illustrates respective methods of determination of roll stiffness of a plate-type
torsion beam 3 and determination of roll strength of a tubular torsion beam 10 in sus-
pensions through FEM so as to design a durable torsion beam suspension according to
the present invention. A bush sleeve 1. a trailing ami 2, a torsion beam 3 or 10, a
spring seat 4. a damper bracket 5, a wheel carrier 6 and a spindle plate 7, which
constitute a torsion beam suspension, are modeled with respective finite elements
according to the above-mentioned suspension modeling method.
[51] Thereafter, roll stiffness is determined by calculating reaction forces at respective
nodes in a restricted state in which the bush sleeves 1 are set to fixing points (I) and 1°
rolling (II) (Z - 12.96 mm) is executed in a direction perpendicular to the central axis
between two spindle plates 7 [FIG. 5(a)]. Further, roll strength is determined by
measuring the stress distribution on the torsion beam in a restricted state, in which the
bush sleeves 1 are set to fixing points (I) and 4° rolling (III) (Z = 51.92 mm) is
executed in a direction perpendicular to the central axis between the two spindle plates
7 [FIG. 5(b)].
[52] FIG. 5(a) illustrates a method of determining roll stiffness of a suspension having the
plate-type torsion beam 3. while FIG. 5(b) illustrates a method of determining roll
strength of a suspension having the tubular torsion beam 10. However, it should be un-
derstood that both the roll stiffness and the roll strength of each of the plate-type
torsion beam 3 and the tubular torsion beam 10 may be determined through FEM. In

the process of determination of roll stiffness and roll strength of the tubular- torsion
beam 10, the roll stiffness and the roll strength are measured while changing the
thickness of the tubular torsion beam 10 within a range from 2.6 mm to 4.0 mm, and
the results are given in Table 1.
[53] Table 1
[Table 1]
[Table ]

[54] As shown in Table 1, when a tubular torsion beam is used, it can realize higher roll
stiffness than that of a plate-type torsion beam even though it is thinner (lower weight).
For example, when a plate-type torsion beam having a 6.0 mm thickness is used, it
weighs 19.72kg and roll stiffness of 227 Nm/deg can be realized. However, when a
tubular torsion beam having a 2.6 mm thickness is used, it weighs 17.57kg and higher
roll stiffness of 496 Nm/deg can be realized. Thus, when a tubular torsion beam is
used, it is possible to design a suspension having excellent durability while reducing
the weight of the suspension, so that torsion beam suspensions have been preferably
used in recent years.
[55] Further, when compared to a plate-type torsion beam, the tubular torsion beam has
higher roll strength. Roll strength means maximum stress that acts in the torsion beam,
so that the durability of the torsion beam is increased in inverse proportion to the
maximum stress. This is because when the maximum stress acting on the torsion beam
exceeds the yield stress of the material of the torsion beam, the torsion beam may be
plastically deformed during the operation of a vehicle, and fatally affect vehicle

stability. Thus, it is necessary to design the roll strength of a torsion beam such that it
is less than the yield stress of a material of the torsion beam as less as possible.
[56] In the prior art. to reduce the roll strength of a tubular tore ion beam, a material
having a large thickness or a high strength material having high yield stress is used as
the material of the tubular torsion beam. However, the use of a thick material or a high
strength material cannot solve the problem. In other words, when a material having a
large thickness is used, the weight of the torsion beam is increased, thus reducing the
running performance of a vehicle. Further, when a high strength material is used,
workability during a forming process is reduced.
[57] Thus, while keeping in mind the above problems, the inventor of the present
invention has studied a method of efficiently reducing the roll strength of a tubular
torsion beam while using the same material as in the prior art. The high roll strength of
a tubular torsion beam results from the fact that the shape of the tubular torsion beam
repeatedly varies along the lengthwise direction thereof, so that stress-concentrated
portions are formed in the torsion beam. The above-mentioned fact can be clearly un-
derstood from FIG. 6, which shows a stress distribution in a tubular torsion beam
obtained through a definite element modeling. As shown in FIG. 6, the stress in the
tubular torsion beam 10 is increased in the direction from the middle portion to the
opposite ends and. particularly, the maximum stress (roll strength) acts in a lower part
of the transitional portion, at which the V-shaped open cross-section is changed into
the closed cross-section.
[58] According to the first embodiment of the present invention, a design for a durable
tubular torsion beam, which can increase the roll stiffness of the torsion beam and can
reduce the roll strength thereof, based on the above-mentioned stress distribution, thus
realizing excellent durability of the torsion beam, can be provided.
[59] Described in detail, as shown in FIG. 7(a). the thickness T of each of the opposite
ends 11, which has higher roll strength in the tubular torsion beam 10, is increased to
be higher than the thickness t of the middle portion. Further, as shown in FIG. 7(b). a
bead 14 is formed on the surface of the transitional portion 12, at which the maximum
stress acts. The bead 14 may be exclusively formed on the transitional portion 12, at
which the maximum stress acts, or may be formed so as to extend from the transitional
portion 12 to each of the opposite ends 11.
[60] To measure the effects of the above-mentioned durable design, roll stiffness and roll
strength are measured using tubular torsion beam samples made of a material having
2.6 mm thickness while varying the thickness of the opposite ends 11 from 2.60 mm to
3.90 mm and dividing the samples into two groups having respective beads or no beads
on the surface of the transitional portions, and the results are given in Table 2.
[61] Table 2

[Table 2]
[Table ]

[62] As shown in Table 2, when a tubular torsion beam is designed such that the opposite
ends and the middle portion thereof have a thickness of 2.6 mm and no bead is formed
on the surface of the transitional portion, in other words, when the durable design of
the present invention is not adapted to the tubular torsion beam, the roll stiffness
thereof is measured to be 342 Nm/deg and the roll strength thereof is measured to be
390 MPa [for reference, the difference in roll stiffness and roll strength between the
Tabular torsion beam 2.6 mm thick in Table 2 and the tubular torsion beam 2.6 mm
thick in Table 1 is induced by the shape optimization.
[63] When the thickness of the opposite ends of the above-mentioned tubular torsion
beam is increased to 3.90 mm, the roll stiffness thereof is increased to 430 Nm/deg and
the roll strength is reduced to 350 Mpa. That is, when the thickness of the opposite
ends of the tubular- torsion beam is increased relative to the thickness of the middle
portion according to the durable design of the present invention, the roll stiffness is
increased and the roll strength is reduced, so that the durability of the tubular torsion
beam can be improved.
[64] Further, when the thickness of the opposite ends is increased to 3.90 mm in a state in
which a bead is formed on the surface of the transitional portion, roll stiffness is
increased in comparison with a tubular torsion beam having the same thickness and no
bead. In the above case, until the thickness of the opposite ends has been increased to
3.12 mm, the roll strength is reduced in comparison with a tubular torsion beam having
the same thickness and no bead. However, in the case where the thickness of the
opposite ends exceeds 3.38 mm. the roll strength is increased in comparison with a
tubular torsion beam having the same thickness and no bead. This may result from the

fact that when a thickness difference between the middle portions and the opposite
ends of the tubular torsion beam exceeds a predetermined reference level, stress is con-
centrated on the bead.
[65 ] Described in detail, it is preferred that the thickness of the opposite ends be increased
to be 1.2 ~ 1.5 times the thickness of the middle portion. As shown in Table 2. when
tests are executed while the thickness of the opposite ends of the tubular torsion beam
is increased relative to the thickness 2.6 mm of the middle portion so as to become
2.86 mm, which is 1.1 times the thickness 2.6 mm of the middle portion, 3.12 mm,
which is 1.2 times thereof. 3.38 mm, whch is 1.3 times thereof. 3.64 mm, which is 1.4
times thereof, and 3.90 mm, which is 1.5 times thereof, it is noted that the roll stiffness
and the roll strength are improved.
[66] However, when the thickness of the opposite ends is increased to 2.86 mm. which is
1.1 times the thickness 2.6 mm of the middle portion, the improvement in the roll
stiffness and the roll strength is not recognized as significant. Further, in the case
where the thickness of the opposite ends is increased to become 1.6 times (no data), the
feeding distance of an axial punch of a hydroforming machine is excessively long, thus
causing a problem in that folds may be formed in opposite ends of the tubular steel
member. The hydroforming method, which includes the control of the feeding distance
of the axial punch, will be described in detail later herein, with reference to FIG. 9
through FIG. 12.
[67] In the tubular torsion beam, when the bead 14, formed on each of the transitional
portions 12. is configured to have a ridge shape not exceeding a height of 35 mm, a
width of 125 mm and a length of 550 mm, and having a radius of curvature equal to or
higher than 2.2 times the thickness of the tubular steel member, the durability of the
tubular torsion beam can be optimally improved. In other words, when the height of
the bead 14 exceeds 35 mm, the width thereof exceeds 125 mm or the length thereof
exceeds 550 mm, the bead 14 acts as a stress-concentrated portion, thus reducing the
durability of the tubular torsion beam. Further, when the radius of curvature of the
bead 14 is less than a value that results from 2.2 times the thickness of the tubular steel
member, the ridge of the bead 14 becomes too sharp, so that it is almost impossible to
form a precise bead shape through hydroforming.
[68] According to the above-mentioned results, it is noted that, when the thickness of the
opposite ends of the tubular torsion beam is increased relative to the thickness of the
middle portion according to the durable design of the present invention, the roll
stiffness of the tubular torsion beam can be increased and the roll strength thereof can
be reduced, thus significantly improving the durability of the tubular torsion beam.
Further, when a bead is formed on the surface of each of the transitional portions, the
roll stiffness can be increased and roll strength may be increased or reduced according

to the thickness difference between the middle portion and the opposite ends. Thus,
during a design of a durable tubular torsion beam, when the thickness of the opposite
ends of the tubular torsion beam is increased relative to the thickness of the middle
portion, and a bead is formed on the surface of each of the transitional portions in con-
sideration of the thickness difference between the middle portion and the opposite
ends, excellent roll stiffness and excellent roll strength of the tubular torsion beam can
be realized, resulting in optimal durability of the torsion beam.
[69] Meanwhile, according to a second embodiment of the present invention, a durable
design capable of improving the durability of a tubular torsion beam 10 by reinforcing
the lower part of each transitional portion, in which maximum stress (roll strength)
acts, as shown in FIG. 6, can be provided.
[70] Described in detail, as shown in FIG. 8. in a tubular torsion beam 10 produced by
pressure-forming a tubular steel member through hydrofoming such that the torsion
beam 10 has a cross-section varying along the entire length thereof, with opposite ends
11 having a closed cross-section and mounted to respective trailing arms 2. a middle
portion 13 having a V-shaped open cross-section, and a transitional portion 12 having a
varying cross-section and connecting the middle portion 13 to each of the opposite
ends 11 while executing a natural transition from the middle portion to the opposite
end, an inclined offset 15 is formed between the middle portion 13 and each of the
transitional portions 12. so that both the transitional portions 12 and the opposite ends
11 are enlarged outwards compared to the middle portion 13.
[71 ] When the inclined offset 15 is formed in each of the transitional portions 12 of the
tubular torsion beam 10, in which the maximum stress acts, the closed cross-sectional
areas of both the transitional portions 12 and the opposite ends 11 are increased, and
the bending stiffness thereof can be increased in proportion to the increase in the
closed cross-sectional areas, so that the durability of the torsion beam can be improved.
When the offset 15 is configured to have right-angled comers, the comers may act as
stress-concentrated portions, so that it is preferred that the offset 15 be configured to
have an inclined shape.
[72] Further, it is preferred that the increase in the circumference of the tubular- torsion
beam due to the offset 15 be within 35% of the initial circumference of the tubular
steel member and that the height difference between the middle portion 13 and each
transitional portion 12 due to the offset 15 not exceed 50 mm.
[73] According to the hydroforming method of the present invention, high pressure
actuation oil is fed into a tubular steel member seated in a cavity between molds, thus
pressurizing the inner surface of the tubular steel member and expanding the wall of
the steel member, and thus forming a desired tubular torsion beam, the shape of which
varies along the entire length thereof. Therefore, a limitation exists in the process of

expanding only the transitional portions 12 and the opposite ends 13 by forming the
offset 15 in a tubular steel member having a constant thickness, so that it is necessary
to control both the increase in the circumference and the height difference such that
they do not exceed the above-mentioned values. In other words, when the increase in
the circumference of the tubular torsion beam due to the offset 15 exceeds 35% of the
initial circumference of the tubular steel member, or the height difference between the
middle portion 13 and each transitional portion 12 due to the offset 15 exceeds 50 mm,
breakage may occur in the portion having the offset 15.
[74] Hereinbelow, the manufacturing method of a tubular torsion beam for rear sus-
pensions of vehicles according to the present invention will be described with
reference to FIG. 9 through FIG. 12.
[75] The manufacturing method according to the first embodiment of the present
invention comprises the steps of: preforming a tubular steel member having a circular
cross-section prior to seating the tubular steel member in a mold of a hydroforming
machine; preparing hydroforming by seating the preformed tubular steel member in a
lower mold of the hydroforming machine and by lowering an upper mold so as to close
the molds: and hydroforming a tubular torsion beam by sealing opposite ends of the
preformed tubular steel member, seated in the cavity between the upper and lower
molds, using axial punches placed at opposite ends of the molds, by feeding actuation
oil into the preformed tubular steel member seated in the molds so as to pressurize the
inner surface of the tubular steel member, thus forming the tubular torsion beam
having opposite ends having a closed cross-section and mounted to trailing arms, a
middle portion having a V-shaped open cross-section, and a transitional portion having
a varying cross-section and connecting the middle portion to each of the opposite ends
while executing a natural transition from the middle portion to the opposite end, and. at
the same time, by feeding the opposite ends of the tubular steel member using the axial
punches so as to increase the thickness of the opposite ends of the tubular torsion beam
compared to the middle portion.
[76] Hereinbelow, the preforming step will be described in detail, with reference to FIG.
9, showing a preforming machine.
[77] First, the preforming machine 40 is a conventional press machine, which comprises a
lower mold 42 securely mounted on a fixed base 41. a lower punch 43 provided on the
upper surface of the lower mold 42 so as to form a V-shaped concave part of a tubular
torsion beam, and a holder 44 provided on each end of the lower mold 42 so as to hold
a tubular steel member without allowing the steel member to move after an upper mold
45 is lowered to close the lower mold 42. In the above state, the upper mold 45 is
placed on the lower mold 42 such that it can be moved upwards or downwards within a
predetermined stroke by a plurality of cylinder actuators 46 placed around respective

comers of the upper mold 45. A depression for forming a V-shaped convex part of the
tubular torsion beam is formed on the lower surface of the upper mold 45.
[78] A tubular steel member, having a circular cross-section, is preformed using the
preforming machine having the above-mentioned construction prior to seating the
tubular steel member in a cavity between molds of a hydroforming machine. The hy-
droforming machine is a machine that feeds high pressure actuation oil into a
preformed tubular steel member seated in a cavity between molds so as to pressurize
the inner surface of the preformed tubular steel member and expand the wall of the,
steel member, as will be described in detail later herein, so that the hydroforming
machine has a limitation in its forming capacity and, therefore, it cannot shape a
tubular material having a circular cross-section into a desired final shape at one time.
Further, the upper and lower molds of the hydroforming machine are provided with a
plurality of curved surfaces corresponding to the final shape of a tubular torsion beam,
so that a tubular steel member having a circular cross-section cannot be stably seated in
the cavity between the upper and lower molds. Thus, in the present invention, the
tubular steel member is preformed to have a shape similar to that of a desired tubular
torsion beam prior to executing a hydroforming step.
[79] Hereinbelow, both the hydroforming preparation step and the hydroforming step will
be described in detail with reference to FIG. 10. which shows a hydroforming machine.
[80] The hydroforming machine 50 comprises a lower mold 52. which is securely
mounted on a fixed base 51 so as to seat a preformed tubular steel member 10a on the
upper surface thereof. On the upper surface of the lower mold 52, a protrusion for
finally forming the V-shaped concave part of the tubular torsion beam is formed. An
upper mold 53 is placed on the lower mold 32 such that the upper mold 53 can move
upwards or downwards within a predetermined stroke. On the lower surface of the
upper mold 53, a depression for finally forming the V-shaped convex part of the
tubular- torsion beam is formed. Further, at opposite ends of the two molds 52 and 53,
axial punches 60 for closing the cavity between the two molds 52 and 53, so as to
prevent the leakage of actuation oil from the molds, and hydraulic axial cylinder
actuators 54. for actuating respective axial punches 60 so as to feed the tubular steel
member, are provided. The construction and operation of the axial punches 60 will be
described in detail later herein with reference to FIG. 11 and FIG. 12.
[81] To manufacture a tubular torsion beam using the preforming machine having the
above-mentioned construction, a preformed tubular steel member 10a is seated on the
lower mold 52 of the hydroforming machine and the upper mold 53 is lowered so as to
close the molds. In the above state, to prevent the upper mold 53 from being lifted
upwards by the high pressure applied to the preformed tubular steel member 10a. a
high press load is applied to the upper mold (hydroforming preparation step).

[82] Thereafter, the opposite ends of the preformed tubular steel member seated in the
cavity between the upper and lower molds are sealed by the axial punches provided at
the opposite ends of the molds, and actuation oil is fed into the preformed tubular steel
member seated in the molds, thus pressurizing the inner surface of the preformed
tubular steel member 10a. Thus, a tubular torsion beam, which has opposite ends 11
that have closed cross-sections and are mounted to trailing arms, a middle portion 13
having a V-shaped open cross-section, and a transitional portion 12 having a varying
cross-section and connecting the middle portion to each of the opposite ends while
executing a natural transition from the middle portion to the opposite end, is formed,
and, at the same time, the opposite ends of the preformed tubular steel member 10a are
fed using the axial punches 60. thus increasing the thickness of the opposite ends 11
compared to the middle portion 13 (hydroforming step).
[83] Hereinbelow, a method of increasing the thickness of the opposite ends of the tubular
torsion beam will be described in detail. A conventional pressing method cannot
realize precise shape control, so that it is difficult to increase the thickness of only the
opposite ends of a tubular torsion beam through the conventional pressing method. In
an effort to solve the problem, the inventor of the present invention has developed a
method of increasing the thickness of only the opposite ends of a tubular torsion beam
using the axial punches of a hydroforming machine.
[84] Each of the axial punches 60 used in the present invention comprises a cylinder rod
61, provided in the front of a body, and a support bar 62 provided on each side of the
cylinder rod 61. as shown in FIG. 1 1. Further, a punching tip 63 is mounted to the end
of the cylinder rod 61 such that the tip 63 can be moved forwards and backwards by a
cylinder actuator installed in the body. An inlet hole 65 for feeding actuation oil is
formed through a central axis of the punching tip 63, with a close contact surface 64
formed around the inlet hole 65 so as to come into contact with an associated end of a
tubular steel member and seal the interior of the tubular steel member.
[85] FIG. 12 illustrates a method of increasing the thickness of only the opposite ends of a
preformed tubular torsion beam using the axial punches 60 having the above-
mentioned construction. First, a preformed tubular steel member 10a is seated in the
hydroforming machine and the upper mold is lowered. Second, each axial punch 60 is
moved forwards so as to bring the close contact surface 64 of the punching tip 63 into
close contact with an associated end of the preformed tubular steel member 10a. Third,
actuation oil is fed into the preformed tubular steel member 10a through the inlet hole
65, which is formed through the central axis of the punching tip 63, thus pressurizing
the inner surface of the tubular steel member 10a. When the pressure that is applied to
the inner surface of the tubular steel member 10a by the actuation oil exceeds a prede-
termined reference level, the tubular steel member 10a comes into close contact with

the inner surfaces of the upper and lower molds while being plastically deformed, thus
being shaped into a desired tubular- torsion beam [FIG. 12 (a)].
[86] During the above-mentioned process, when the cylinder rods 62 of respective axial
punches 60 are moved forwards and feed the punching tips 63 forwards, only the
opposite ends 11 of the preformed tubular steel member 10a are plastically deformed,
resulting in an increase in the thickness of the opposite ends of the tubular steel
member. In the above state, if the force and time to be consumed to feed the punching
tips 63 of the axial punches 60 are controlled, it is possible to control the length and
thickness of portions subjected to thickness increase, in the tubular torsion beam [FIG.
12(b)]. The process of increasing the thickness of the opposite ends of the tubular steel
member by feeding the axial punches may be executed simultaneously with the process
of forming the tubular torsion beam using the pressure of actuation oil. as shown in
FIG. 12(a), or may be separately executed as a post process.
[87] In the above state, it is preferred that the feeding distance of each axial punch 60 be
set to 2 ~ 150 mm and that the thickness of each end of the tubular steel member be
increased to be 1.2 - 1.5 times the thickness of the middle portion. The increase in the
thickness of the opposite ends is in proportion to the feeding distance using the axial
punches 60. When the feeding distance is less than 2 mm, the increase in the thickness
of the opposite ends is not significant. On the contrary, when the feeding distance
exceeds 150 mm, folds may be formed in the surfaces of the opposite ends due to ex-
cessively high plastic deformation, as described above.
[88] In the method according to the first embodiment of the present invention, to form a
bead 14 on the surface of each transitional portion 11 of the tubular torsion beam 10.
an optimized bead shaping surface may be formed on the lower surface of the upper
mold 53 of the hydroforming machine.
[89] In the above state, it is preferred that the bead be configured such that it has a ridge
shape not exceeding a height of 35 mm, a width of 125 mm or a length of 550 mm, and
such that it has a radius of curvature equal to or greater than 2.2 times the thickness of
the tubular- steel member. When the height of the bead 14 exceeds 35 mm. the width
thereof exceeds 125 mm or the length thereof exceeds 550 mm, stress is concentrated
on the bead 14, thus reducing the durability of the tubular tore ion beam. Further, when
the radius of curvature of the bead 14 is less than a value equal to 2.2 times the
thickness of the tubular steel member, the ridge of the bead 14 becomes too sharp, so
that it is almost impossible to form a precise bead shape through hydroforming. as
described above.
[90] The manufacturing method according to the second embodiment of the present
invention comprises the steps of: preforming a tubular steel member having a circular
cross-section prior to seating the tubular steel member in a mold of a hydroforming

machine: preparing for hydroforming by searing the preformed tubular steel member in
a lower mold of the hydroforming machine and by lowering an upper mold so as to
close the molds; and hydroforming a tubular torsion beam by sealing opposite ends of
the preformed tubular steel member, seated in the cavity between the upper and lower
molds, using axial punches placed at opposite ends of the molds, by feeding actuation
oil into the preformed tubular steel member seated in the molds so as to pressurize the
inner surface of the tubular steel member, thus forming the tubular torsion beam
having opposite ends having a closed cross-section and mounted to trailing arms, a
middle portion having a V-shaped open cross-section, and a transitional portion having
a varying cross-section and connecting the middle portion to each of the opposite ends,
and, at the same time, forming an inclined offset at a location between the middle
portion and each of the transitional portions, so that both the transitional portions and
the opposite ends are enlarged outwards compared to the middle portion.
[91] The general shapes of both the preforming machine 40 and the hydroforming
machine 50. used in the second embodiment of the present invention, remain the same
as those described above with reference to FIG. 9 and FIG. 10. However, an offset
shaping portion for forming the inclined offset 15 is formed on the surface of each of
the lower and upper molds 52 and 53. as shown in FIG. 8.
[92] In the above state, it is preferred that the increase in the circumference of the tubular
torsion beam due to the offset 15 be within 35% of the initial circumference of the
tubular steel member, and that the height difference between the middle portion 13 and
each transitional portion 12 due to the offset 15 not exceed 50 mm. The height
difference between the middle portion 13 and each transitional portion 12 due to the
offset 15 is in proportion to the increase in the circumference of the tubular torsion
beam due to the offset 15. When the increase in the circumference of the tubular
torsion beam due to the offset 15 exceeds 35% of the initial circumference of the
tubular steel member, or the height difference between the middle portion 13 and each
transitional portion 12 due to the offset 15 exceeds 50 mm, breakage may occur in the
portion having the offset 15, as described above.
Industrial Applicability
[93] Although the embodiments of the present invention have been disclosed for il-
lustrative purposes, those skilled in the art will appreciate that various modifications,
additions and substitutions are possible, without departing from the scope and spirit of
the invention as disclosed in the accompanying claims. Therefore, it is obvious that,
although the thickness or the shape of a material of a tubular torsion beam produced
through hydroforming may be variously changed according to the class or
aerodynamic volume displacement of a vehicle using the tubular torsion beam, as long

as the tubular torsion beam adopts the construction disclosed in the accompanying
claims, the tubular torsion beam is included in the scope of the protection of the
invention.

Claims
[ 1 ] A tubular torsion beam for rear- suspensions of vehicles, which is produced by
pressure-forming a tubular steel member through hydroforming such that the
tubular torsion beam has a cross-section varying along an entire length thereof,
with opposite ends having a closed cross-section and mounted to respective
trailing arms, a middle portion having a V-shaped open cross-section, and a
transitional portion having a varying cross-section and connecting the middle
portion to each of the opposite ends,
wherein each end of the tubular steel member is fed using an axial punch of a hy-
droforming machine, so that the opposite ends of the tubular torsion beam have a
larger thickness than a thickness of the middle portion.
[2] The tubular torsion beam for rear suspensions of vehicles according to claim 1.
wherein a feeding distance of the axial punch is set to 2 ~ 150 mm.
[3] The tubular torsion beam for rear suspensions of vehicles according to claim 1 or
claim 2, wherein the thickness of the opposite ends of the tubular torsion beam is
increased to be 1.2 ~ 1.5 times the thickness of the middle portion.
[4] The tubular torsion beam for rear suspensions of vehicles according to claim 1.
further comprising:
a bead formed on a surface of the transitional portion.
[5] The tubular torsion beam for rear suspensions of vehicles according to claim 4.
wherein the bead is configured to have a ridge shape not exceeding a height of 35
mm. a width of 125 mm and a length of 550 mm.
[6] The tubular torsion beam for rear suspensions of vehicles according to claim 4 or
claim 5, wherein the bead is configured to have a radius of curvature equal to or
greater than 2.2 times the thickness of the tubular steel member.
[7] A tubular torsion beam for rear suspensions of vehicles, which is produced by
pressure-forming a tubular steel member through hydroforming such that the
tubular torsion beam has a cross-section varying along an entire length thereof,
with opposite ends having a closed cross-section and mounted to respective
trailing arms, a middle portion having a V-shaped open cross-section, and a
transitional portion having a varying cross-section and connecting the middle
portion to each of the opposite ends,
wherein an inclined offset is formed between the middle portion and the
transitional portion, so that both the transitional portions and the opposite ends
are enlarged outwards compared to the middle portion.
[8] The tubular torsion beam for rear suspensions of vehicles according to claim 7,
wherein an increase in a circumference of the tubular torsion beam due to the

offset is within 35 % of an initial circumference of the tubular steel member, and
a height difference between the middle portion and the transitional portion due to
the offset is within 50 mm.
[9] A method of manufacturing a tubular torsion beam for rear suspensions of
vehicles, comprising:
preforming a tubular steel member having a circular cross-section prior to seating
the tubular steel member in a mold of a hydroforming machine;
preparing for hydroforming by seating the preformed tubular steel member in a
lower mold of the hydroforming machine and by lowering an upper mold so as to
close the molds; and
hydroforming a tubular torsion beam by sealing an interior of the preformed
tubular steel member, seated in a cavity between the upper and lower molds,
using axial punches placed at opposite ends of the molds, and by feeding
actuation oil into the preformed tubular steel member so as to pressurize an inner
surface of the tubular steel member, thus forming the tubular torsion beam
having opposite ends having a closed cross-section and mounted to trailing arms,
a middle portion having a V-shaped open cross-section, and a transitional portion
having a varying cross-section and connecting the middle portion to each of the
opposite ends, and. at the same time, feeding opposite ends of the tubular steel
member using the axial punches so as to increase thickness of the opposite ends
of the tubular torsion beam relative to the middle portion.
[10] The method of manufacturing the tubular torsion beam for rear suspensions of
vehicles according to claim 9, wherein, during the hydroforming, a feeding
distance of each of the axial punches is set to 2 ~ 150 mm.
[11] The method of manufacturing the tubular torsion beam for rear suspensions of
vehicles according to claim 9 or claim 10. wherein, during the hydroforming, the
thickness of the opposite ends of the tubular torsion beam is increased to become
1.2-1.5 times a thickness of the middle portion.
[12] The method of manufacturing the tubular torsion beam for rear suspensions of
vehicles according to claim 9. wherein the hydroforming further comprises:
forming a bead on a surface of the transitional portion.
[ 13] The method of manufacturing the tubular torsion beam for rear suspensions of
vehicles according to claim 12, wherein, during the hydroforming. the bead is
formed to have a ridge shape not exceeding a height of 35 mm, a width of 125
mm and a length of 550 mm.
[14] The method of manufacturing the tubular torsion beam for rear suspensions of
vehicles according to claim 12 or claim 13, wherein, during the hydroforming,
the bead is formed to have a radius of curvature equal to or greater than 2.2 times

a thickness of the tubular steel member.
[15] A method of manufacturing a tubular torsion beam for rear suspensions of
vehicles, comprising:
preforming a tubular steel member having a circular cross-section prior to seating
the tubular steel member in a mold of a hydroforming machine;
preparing for hydroforming by seating the preformed tubular steel member in a
lower mold of the hydroforming machine and by lowering an upper mold so as to
close the molds: and
hydroforming a tubular torsion beam by sealing an interior of the preformed
tubular steel member, seated in a cavity between the upper and lower molds,
using axial punches placed at opposite ends of the molds, by feeding actuation
oil into the preformed tubular steel member so as to pressurize an inner surface
of the tubular steel member, thus forming the tubular torsion beam having
opposite ends having a closed cross-section and mounted to trailing arms, a
middle portion having a V-shaped open cross-section, and a transitional portion
having a varying cross-section, and connecting the middle portion to each of the
opposite ends, and, at the same time, forming an inclined offset at a location
between the middle portion and the transitional portion, so that both the
transitional portions and the opposite ends are enlarged outwards compared to
the middle portion.
[ 16] The method of manufacturing the tubular torsion beam for rear suspensions of
vehicles according to claim 15, wherein, during the hydroforming, an increase in
a circumference of the tubular torsion beam due to the offset is within 35% of an
initial circumference of the tubular steel member, and a height difference
between the middle portion and the transitional portion due to the offset is within
50 mm.

The present invention provides a tubular torsion beam for rear suspensions of vehicles, which is produced according
to a more durable design capable of improving roll stiffness and roll strength of the tubular torsion beam. The tubular torsion beam
is produced by pressure-forming a tubular steel member through hydroforming such that the tubular torsion beam has a cross-section
varying along an entire length thereof, with opposite ends having a closed cross-section and mounted to respective trailing arms, a
middle portion having a V-shaped open cross-section, and a transitional portion having a varying cross-section and connecting the
middle portion to each of the opposite ends. During the process of manufacturing the tubular torsion beam, the opposite ends of the
tubular steel member are fed using respective axial punches of a hydroforming machine, so that the opposite ends are thicker than
the middle portion.