CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This PCT Patent Application claims the benefit of U.S. Provisional Patent

Application Serial No. 61/660,151, entitled "Light Weight Tubular Twist Beam Part", filed June 15, 2012, the entire disclosure of the application being considered part of the disclosure of this application, and hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0002] The subject invention relates to twist beams for automotive vehicles, and more particularly to tubular twist beams, and methods of forming tubular twist beams.

2. Related Art

[0003] Automotive vehicles include torsion beams to connect the two rear wheels together through the use of longitudinal control arms. A particular type of torsion beam used on rear suspensions is a twist beam. Twist beams oftentimes comprise a tubular part having an O- shaped, C-shaped, U-shaped, or V-shaped cross-section, which is rigid enough to prevent bending and flexible enough to allow torsion. Accordingly, the tubular twist beam is not only a structural member, but also acts as a torsion spring. An example of a tubular twist beam is disclosed in U.S. Patent Application Publication No. 2010/0301577.

[0004] The weight of the tubular twist beam is preferably low because it contributes to the total weight of the automotive vehicle. However, tubular twist beams experience a significant amount of stress due to twisting and other factors. Therefore, maximum stress levels, especially those due to twisting, require a minimum material thickness and thus dictate the weight of the tubular twist beam. [0005] The tubular twist beam is also used to control a roll rate of the vehicle, which affects the ride and handling of the vehicle. The roll rate is analogous to a vehicle's ride rate, but for actions that include lateral accelerations, causing a vehicle's sprung mass to roll. Roll rate is expressed as torque per degree of roll of the vehicle sprung mass, and is typically measured in Nm/degree. The roll rate of a vehicle does not change the total amount of weight transfer on the vehicle, but shifts the speed at which and percentage of weight transferred on a particular axle to another axle through the vehicle chassis. Generally, the higher the roll rate on an axle of a vehicle, the faster and higher percentage the weight transfer on that axle. A slower weight transfer reduces the likelihood of vehicle rollover conditions. The dimensions and design of the tubular twist beam have a significant influence on the roll rate of the vehicle.

SUMMARY OF THE INVENTION

[0006] The invention provides a tubular twist beam comprising a tubular body extending longitudinally along a center axis between opposite end sections. Each end section presents a cylindrical opening surrounding the center axis. The tubular body includes an upper wall and side walls. The side walls are disposed on opposite sides of the upper wall. The upper wall extends from each of the end sections inwardly and toward a center point, which is disposed equally between the opposite end sections, and downwardly toward the center axis to present transition sections. Each of the side walls includes depressions along the transition sections. The upper wall of the tubular body presents a U-shaped groove between the side walls. The U- shaped groove extends longitudinally along the center axis between the transition sections. The side walls of the tubular body present a width therebetween. The width of the tubular body decreases continuously along the center axis from each of the transition sections along the U- shaped groove to the center point. [0007] The invention also provides a method of forming a tubular twist beam. The method comprises providing a tubular body extending longitudinally along a center axis between opposite end sections. The tubular body includes a center point disposed equally between the opposite end sections, and the end sections of the tubular body each present a cylindrical opening surrounding the center axis. The method next includes pressing an upper wall of the tubular body toward the center axis to form a U-shaped groove extending between side walls and longitudinally along the center axis between opposite end sections while maintaining the cylindrical opening at the end sections. The step of pressing the upper wall toward the center axis includes forming transition sections between the end sections and the U-shaped groove. In the transition sections, the upper wall extends inwardly toward the center axis and the center point. The method further includes pressing the side walls of each transition section inwardly to form depressions; and pressing the side walls of the tubular body inwardly along the U-shaped groove such that the width of the U-shaped groove decreases continuously along the center axis from each of the transition sections to the center point.

[0008] The continuously decreasing width of the tubular body along the U-shaped groove shifts stress from the transition sections to an area around the center point. Further, the depressions direct stress to a lower wall, also referred to as the underside of the U-shaped groove. The design of the tubular body balances stress levels along the length of the tubular body, and the overall stress level is reduced. Thus, the tubular body can be formed with a reduced thickness and thus a reduced weight without exceeding maximum stress levels or sacrificing performance. BRIEF DESCRIPTION OF THE DRAWING

[0009] Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing wherein:

[0010] Figure 1 is a perspective view of a tubular twist beam comprising a tubular body according to one embodiment of the invention;

[0011] Figure 2 is another perspective view of the tubular body of Figure 1 showing the cross-sectional area of an opening;

[0012] Figure 2A is an enlarged view of a portion of Figure 2 showing the depth of a depression;

[0013] Figure 3 is a side view of the tubular body of Figure 1 ;

[0014] Figure 4 is a top view of the tubular body of Figure 1 ;

[0015] Figure 4A is an enlarged cross-sectional view of the tubular body of Figure 1 along line A-A;

[0016] Figure 5 is a bottom view of the tubular body of Figure 1 ;

[0017] Figure 6 is another top view of the tubular body of Figure 1 showing an approximate stress level analysis;

[0018] Figure 7 is another bottom view of the tubular body of Figure 1 showing an approximate stress level analysis;

[0019] Figure 8 is a perspective view of a comparative tubular twist beam; and

[0020] Figure 9 is a side view of the comparative tubular twist beam of Figure 8. DETAILED DESCRIPTION

[0021] Referring to the Figures, a tubular twist beam comprising a tubular body 20 is general shown. The tubular body 20 includes a U-shaped groove 22, a pair of tubular end sections 28, and transition sections 26 between the tubular end sections 28 and the U-shaped groove 22. The width w of the U-shaped groove 22 decreases continuously from the transition sections 26 to a center point C of the tubular body 20. The width w is narrowest at the center point C to shift stress away from the transition sections 26. The width w of the U-shaped groove 22 can be adjusted to achieve the desired roll rate. Further, depressions 24, i.e. dents, are formed along side walls 38 of each transition section 26 to direct stress to a lower wall 34, also referred to as the underside of the U-shaped groove 22. The continuously changing width w of the U- shaped groove 22 and the depressions 24 balance the stress throughout the tubular body 20 and reduce the overall stress level. Thus, the tubular body 20 can be formed of less material and the total weight of the tubular body 20 may be reduced. Figures 1-7 show perspective, side, top, and bottom views of the tubular body 20 according to one exemplary embodiment.

[0022] The tubular body 20 is formed from a metal tube extending around and longitudinally along a center axis A. The tube can comprise a variety of different dimensions, but the exemplary tubular body 20 shown in Figures 1 -7 is formed form a tube having a diameter of 90 mm, a length of 930 mm, and a thickness of 2.8 mm. The tubular body 20 extends longitudinally along the center axis A between the opposite end sections 28. The center point C is disposed equally between the end sections 28, as shown in Figures 1-5. The geometry of the tubular body 20 is described by referring to the side walls 38, the lower wall 34, and an upper wall 32 facing opposite the lower wall 34. The side walls 38, upper wall 32, and lower wall 34 are integral with one another and surround the center axis A. The side walls 38 space the upper wall 32 and the lower wall 34 from one another and are disposed on opposite sides of the upper wall 32 and lower wall 34.

[0023] The ends of the metal tube used to form the tubular body 20 remain unchanged to present the tubular end sections 28. The tubular end sections 28 each present a cylindrical- shaped opening surrounding the center axis A and a diameter D extending across the center axis A, as shown in Figures 2 and 2A. The cylindrical- shaped opening of the end sections 28 presents a circular-shaped cross-section. The cylindrical-shaped opening extends longitudinally along a portion of the center axis A and then transitions to a U-shaped opening along the U- shaped groove 22. The opening has a cross-sectional area X and dimensions which are constant along the end sections 28 but vary continuously along the remaining sections of the tubular body 20, as shown in Figures 2 and 2A.

[0024] The U-shaped groove 22 and transition sections 26 are formed by pressing, pinching, or otherwise deforming the metal tube. The upper wall 32 of the tubular body 20 is typically pressed inward and downward toward the center axis A, and the side walls 38 of the tubular body 20 are pressed or pinched inwardly toward one another and the center axis A. The extent of pressing or pinching of the tubular body 20 depends on the desired roll rate. The lower wall 34, however, typically maintains a convex contour along the length of the tubular body 20.

[0025] The transition sections 26 are formed between each end section 28 and the U- shaped groove 22 and connect the end sections 28 to the U-shaped groove 22. The opposite transition sections 26 are defined by the upper wall 32 caving or collapsing inward. The upper wall 32 extending inwardly and downwardly from each of the end sections 28 toward the center axis A and toward the center point C. In the transition sections 26, the upper wall 32 has a concave contour extending longitudinally along the center axis A to the U-shaped groove 22. The upper wall 32 of the transitions sections 26 also slopes inwardly from each side wall 38 toward the center axis A. Accordingly, the configuration of each transition section 26 can be referred to as tough-shaped.

[0026] The transition sections 26 also have a height h decreasing continuously along the center axis A from the tubular end sections 28 to the U-shaped groove 22, as shown in Figure 3. The height h of the tubular body 20 extends from the upper wall 32 to the lower wall 34, and is equal to the distance between vertically aligned points along the upper wall 32 and the lower wall 34. The width w of the transition sections 26 decreases continuously along the center axis A from the tubular end sections 28 to the U-shaped groove 22, as best shown in Figures 2A and 4. The width w is measured from the outermost horizontally aligned points of the side walls 38 and is equal to the distance between those points. The cross-sectional area X of the opening also decreases continuously along the transition sections 26, as best shown in Figures 2 and 2A.

[0027] The depressions 24 are formed in each of the side walls 38 along each of the transition sections 26 to direct stress to the lower wall 34 or underside of the U-shaped groove 22. The depressions 24 of each transition section 26 are longitudinally aligned with one another on the opposing side walls 38. Each depression 24 extends from an outside edge 25 to an inside edge 27 and has a concave contour between the edges 25, 27, as shown in Figure 1. The depressions 24 are formed by pressing, pinching, or otherwise deforming the side walls 38 of the tubular body 20 such that the side walls collapse or cave inward. Each depression 24 has a depth da extending inwardly toward the center axis A, as shown in Figure 2A. In the exemplary embodiment, the depressions 24 each have a depth d _d of 17.18 mm. The location of each transition section 26 along the center axis A can be partially defined by the inner edge 27 of each depression 24. The transition section 26 begins where the upper wall 32 begins to cave inwardly, and the transition section 26 ends at the inner edge 27 of the depression 24. The upper wall 32 continues to cave or collapse inward and extend downwardly toward the lower wall 34 along the transition sections 26. Eventually, the upper wall 32 extends below the center axis A and presents the U-shaped groove 22.

[0028] The U-shaped groove 22 presented by the upper wall 32 of the tubular body 20 is disposed the between the side walls 38. The U-shaped groove 22 is also referred to as a torsion section, and the dimensions of the U-shaped groove 22 are designed to achieve the desired roll rate. The U-shaped groove 22 extends continuously between the opposite transition sections 26, which typically end at the inner edge 27 of each depression 24. Each U-shaped groove 22 extends longitudinally along the center axis A between the depressions 24 of the transition sections 26 and the center point C. The side walls 38 of the U-shaped groove 22 also present the width w extending therebetween, and the width w decreases continuously along the center axis A from the inner edge 27 of each depression 24 along the length of the U-shaped groove 22 to the center point C, as shown in Figures 2A and 4. The side walls 38 of the tubular body 20 are not parallel to one another at any point along the U-shaped groove 22 between each transition section 26 and the center point C.

[0029] The U-shaped groove 22 also presents a depth d _g extending from a point aligned with the top of the upper wall 32, between the side walls 38, to the lower wall 34, as shown in Figures 2A and 4A. The depth d _g of the U-shaped groove 22 increases continuously from each of the transition sections 26 to the center point C. The cross-sectional area X of the opening also continues to decrease along the U-shaped groove 22 toward the center point C, and the cross- sectional area X of the opening is smallest at the center point C. In one embodiment, the walls 32, 34, and 38 come together at the center point C so that the opening is completely closed at the center point C. As shown in Figure 3, the height h of the tubular body 20 increases slightly along a portion of the U-shaped groove 22, around the center point C. The upper wall 32 of the tubular body 20 also presents a bulge 36 between the side wall 38 and the U-shaped groove 22, as best shown in Figure 4A. The transition sections 26 may also include the bulge 36.

[0030] Each wall 32, 34, 38 of the tubular body 20 presents a thickness t, and the thickness t of each wall 32, 34, 38 is typically equal to the thickness t of the other walls 32, 34, 38, as shown in Figure 4A. For example, the tubular body 20 can have a thickness t of 2.8 mm, 3.2 mm, or 3.6 mm. However, the thickness t of the tubular body 20 can be increased or decreased, depending on the forming process, desired roll rate, or other factors. The thickness t along the length of the tubular body 20 is typically constant, but may vary slightly. The thickness t of the tubular body 20 around the center axis A is also typically constant, but may vary around the center axis A.

[0031] The tubular twist beam comprising the tubular body 20 of the present invention provides several advantages over comparative twist beams. An example of a comparative twist beam is shown at 120 in Figures 8 and 9. The comparative twist beam 120 also has tubular end sections 128, a U-shaped groove 122, and transition sections 126. However, in the comparative twist beam 120, the U-shaped groove 122 has a constant width w and side walls 134 are parallel to one another along a significant portion of the U-shaped groove 122.

[0032] The twist beam comprising the tubular body 20 of the present invention experiences less stress in the transition sections 26 than the comparative part 120 because the width w of the U-shaped groove 22 continuously decreases from the transition section 26 to the center point C and is narrowest at the center point C. Therefore, the twist strain is directed away from the transition sections 26 and the total peak stress level in the transition sections 26 is reduced. The depressions 24 also reduce the total peak stress level in the transition sections 26 by directing the stress to the underside of the tubular body 20.

[0033] Figure 6 is a top view and Figure 7 is a bottom view showing the approximate stress levels along the exemplary tubular body 20, in MPa/Mises, when the tubular body 20 is used in a vehicle having a roll rate of 404 Nm/degree. A legend for the approximate stress levels is also provided in the Figures. The stress levels along the length of the tubular body 20 are more balanced than those of the comparative twist beam 122 and other tubular twist beams of the prior art. Further, test results indicate that the thickness of the tubular body 20 can be lower than the thickness of the comparative twist beam 122, without increasing the total stress levels.

Accordingly, the twist beam comprising the tubular body 20 of the present invention can have a reduced weight and thus provide significant costs savings without exceeding maximum stress levels or sacrificing performance. Furthermore, the roll rate provided by the tubular body 20 can be adjusted by adjusting the width w of the U-shaped groove or thickness t of the tubular body along the U-shaped groove 22. For example, the roll rate can be decreased by decreasing the thickness t or decreasing the width w of the U-shaped groove 22.

[0034] The invention also provides a method of forming the tubular twist beam comprising the tubular body 20. The method first includes providing the tubular body 20 extending longitudinally along the center axis A between the opposite end sections 28. Before pressing the upper wall 32 and side walls 38 inward, the tubular body 20 has a thickness of 2.8 mm, a length of 930 mm, and a diameter of 90 mm.

[0035] The method next includes pressing the upper wall 32 of the tubular body 20 toward the center axis A to form the U-shaped groove 22, while maintaining the cylindrical opening at the opposite end sections 28. The step of pressing the upper wall 32 toward the center axis A also includes forming transition sections 26 between the end sections 28 and the U-shaped groove 22.

[0036] The method also includes pressing the side walls 38 of the tubular body 20 inwardly along the U-shaped groove 22 such that the width w of the U-shaped groove 22 decreases continuously along the center axis A. The step of pressing the upper wall 32 typically includes increasing the depth d _g of the U-shaped groove 22 continuously from each of the transition sections 26 to the center point C. The method further includes pressing a portion of the side walls 38 of each transition section 26 inwardly to form the depressions 24.

[0037] Obviously, many modifications and variations of the present invention are possible in light of the above teachings and may be practiced otherwise than as specifically described while being within the scope of the claims.