Manufacturing method and related structure of bicycle frame lower pipe five-way assembly

Abstract

The invention relates to a bicycle frame lower pipe five-way assembly manufacturing method and a related structure, and belongs to the technical field of bicycle frame manufacturing. The method comprises the steps that a metal blank is subjected to casting and forging treatment, and an integral piece comprising a lower pipe part and a solid five-way part which are integrally formed is formed; in the subsequent process, the solid five-way part provides machining support for the integral part, and finally numerical control machining is conducted on the solid five-way part to form the center cavity. Through the casting and forging integrated forming process, the lower pipe part and the five-way part are made into a seamless connection integral structure, and the stress concentration problem caused by traditional welding can be relieved. Moreover, in the machining process, the five-way part is firstly kept to be of a solid structure, the machining rigidity and stability of the whole part are improved through the solid five-way part, and the manufacturing precision of a final product can be improved. The anti-fatigue performance and the structural strength of the frame can be improved.

Description

Technical Field

[0001] This application relates to the field of bicycle frame manufacturing technology, and in particular to a method for manufacturing a bicycle frame bottom tube assembly and related structures. Background Technology

[0002] In the research and development of high-performance bicycles, the frame, as the core load-bearing component, is designed to achieve extreme lightweighting while maintaining high structural strength. Currently, the mainstream frame manufacturing process in the industry involves welding pre-prepared components such as the downtube, bottom bracket, and seat tube together to form a complete frame structure. This modular manufacturing and subsequent welding approach is widely used.

[0003] However, this traditional welding connection method has inherent technical bottlenecks. The welding process creates weld seams and heat-affected zones at the pipe joints. The metallographic structure of these areas differs from that of the base pipe material, making them prone to stress concentration points under stress. During cycling, the frame repeatedly bears alternating loads from road impacts and pedaling forces. Fatigue cracks will first develop in these stress concentration areas and gradually propagate, potentially leading to structural failure.

[0004] Regarding the aforementioned technologies, there is a problem of stress concentration caused by welding the various pipes, which in turn affects the overall fatigue resistance of the frame. Summary of the Invention

[0005] To address the issues of stress concentration and poor fatigue resistance in existing bicycle frames caused by welded connections between tubes, this application provides a method for manufacturing a bicycle frame bottom tube assembly, a bicycle frame bottom tube assembly, and a bicycle frame.

[0006] This application provides a method for manufacturing a bicycle frame bottom bracket assembly, which adopts the following technical solution: A method for manufacturing a bicycle frame downtube bottom bracket assembly includes: S1, melting a metal blank into a molten metal casting; S2, injecting the molten metal casting into a casting and forging mold to cast an integral part blank; S3, forging the integral part blank in the casting and forging mold to form an integral part; wherein the integral part includes an integrally formed downtube section and a bottom bracket section, the downtube section having an integrally formed head tube assembly port, and the bottom bracket section having an integrally formed center tube assembly port and a fork assembly port; the downtube section is hollow, and the bottom bracket section is solid; S9, CNC machining the solid bottom bracket section to form a central cavity penetrating the bottom bracket section.

[0007] By adopting the above technical solution and using a combination of casting and forging processes, the lower tube section and bottom bracket section, which are subject to complex stresses in the frame, are made into a seamless integral component. The casting process facilitates the formation of complex initial shapes, while the forging process densifies the metal grains, thus replacing the traditional weld seam between the two with a one-piece structure. This allows stress to be transferred in continuous metal flow lines, helping to alleviate stress concentration. Designing the bottom bracket section as a solid component before machining helps maintain rigidity during subsequent processes such as tube pulling and bending. Finally, the central cavity is formed through CNC machining.

[0008] Optionally, in step S2, the casting and forging die includes a steel core for forming the hollow structure of the lower tube.

[0009] By adopting the above technical solution and using a steel core in the casting process, the hollow structure of the lower tube can be formed directly in the casting stage, which helps to simplify subsequent processes and improve material utilization.

[0010] Optionally, in step S3, the middle tube assembly port and the lower fork assembly port are configured as insert-type welding interfaces.

[0011] By adopting the above technical solution and setting up standardized insert welding interfaces, a positioning and matching benchmark is provided for the connection of other tubes such as the center tube and bottom fork, which helps to simplify the subsequent welding and assembly process of the frame and improve assembly accuracy.

[0012] Optionally, after step S3, the method further includes: S4, applying a film treatment to the lower tube portion; S5, performing a tube-pulling process on the lower tube portion to obtain a tube-pulling integral component.

[0013] By adopting the above technical solutions, the film treatment can provide lubrication and protection for subsequent tube drawing, and the tube drawing treatment can thin the lower tube section while maintaining structural integrity, so as to achieve weight reduction.

[0014] Optionally, step S5 includes: performing a pipe drawing and diameter reduction process on the lower pipe section to give the lower pipe section a pipe wall thickness that varies along the axial direction.

[0015] By adopting the above technical solution, differentiated wall thickness designs can be carried out based on the stress analysis of different locations in the lower tube section. This approach allows for the retention of thicker tube walls in critical stress areas to maintain strength, while reducing material thickness in non-critical areas to achieve weight reduction, thus helping to optimize the mechanical properties and weight ratio of the assembly.

[0016] Optionally, after step S5, the method further includes: S6, performing a bending pretreatment on the integral tube part, including: S61, performing an annealing treatment on the integral tube part; and S62, performing a film treatment on the integral tube part; and S7, performing a bending treatment on the lower tube part to form a bent integral part.

[0017] By employing the above technical solutions, the bending process can create a specific curvature in the lower tube to accommodate different frame geometries or to provide interference space for functions such as front wheel steering. The annealing treatment before bending reduces material hardness, which facilitates plastic forming and reduces the risk of cracking.

[0018] Optionally, after step S7, the method further includes: S8, performing a water swelling treatment on the bent integral part, wherein the bent integral part is placed in a forming mold with a preset inner cavity contour, and liquid is injected into the bent integral part to obtain a formed integral part.

[0019] By adopting the above technical solutions, the hydroforming process can form a complex irregular pipe surface in the lower pipe section, which can be used to optimize aerodynamic performance or structural strength, while making the internal stress distribution of the pipe after bending more uniform.

[0020] Optionally, in step S61, the integral tube assembly is hardened to a Weber hardness of 3 or below.

[0021] By adopting the above technical solution, the hardness is controlled within a specific range, providing a suitable material processing window for subsequent bending and hydroforming processes, which helps to improve forming accuracy and yield.

[0022] To achieve the above objectives, the second aspect of this application provides a bicycle frame bottom tube assembly, which adopts the following technical solution: a bicycle frame bottom tube assembly, wherein the bicycle frame bottom tube assembly is manufactured using any of the aforementioned manufacturing methods.

[0023] By adopting the above technical solution, a product obtained through a specific manufacturing method is provided. The bicycle frame bottom tube assembly, due to its integrated casting and forging process, features a seamless connection between the bottom tube and the bottom tube section. Compared to traditional welded structures, this facilitates the smooth transfer of stress within the structure, thereby improving the assembly's fatigue resistance and structural strength.

[0024] To achieve the above objectives, the third aspect of this application provides a bicycle frame with the following technical solution: A bicycle frame includes: a bottom bracket assembly as described above, a head tube connected to the bottom tube, a center tube connected to the bottom bracket, and a chain fork connected to the bottom bracket; the head tube is connected to the bottom tube via a head tube mounting port; the center tube is connected to the bottom bracket via a center tube mounting port and is connected to the head tube via an upper tube; the chain fork is connected to the bottom bracket via a chain fork mounting port and is connected to the center tube via an upper fork.

[0025] By adopting the above technical solution, the bicycle frame downtube bottom bracket assembly is applied to the construction of the bicycle frame. Since the core stress-bearing area of ​​the frame uses this one-piece molded assembly, compared to frames using traditional separate welded parts, the connection area between the downtube and bottom bracket of this bicycle frame has better stress distribution, which helps to improve the structural strength and fatigue resistance of the entire frame.

[0026] In summary, this application includes at least one of the following beneficial technical effects: 1. By forging the lower tube section and the bottom bracket section into a single piece, the weld between the two is replaced with a one-piece structure, which helps to alleviate stress concentration and thus helps to improve the fatigue resistance and structural strength of key parts of the frame. 2. By performing tube pulling, bending, and water expansion on the lower tube section, lightweight design can be achieved while meeting the requirements of strength and complex shape, which helps to optimize the weight and performance ratio of the assembly; 3. By directly molding the head tube assembly port, middle tube assembly port, and lower fork assembly port on the integral component, it helps to simplify the subsequent frame assembly and welding process and improve the efficiency and precision of the whole vehicle manufacturing. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the bicycle frame bottom tube assembly in an embodiment of this application.

[0028] Figure 2 This is a schematic diagram illustrating the application of the bicycle frame bottom tube assembly according to an embodiment of this application in a bicycle frame.

[0029] Figure 3 This is a cross-sectional view of the bicycle frame bottom tube assembly according to an embodiment of this application.

[0030] Figure 4 This is a flowchart illustrating the manufacturing method of the bicycle frame bottom tube assembly according to an embodiment of this application.

[0031] Figure 5 This is a schematic diagram of the structure of a casting and forging mold for forging in an embodiment of this application.

[0032] Figure 6 This is a schematic diagram of the integral part obtained by casting and forging in the embodiments of this application.

[0033] Figure 7 This is a schematic diagram of the integral tube-drawing component obtained after tube-drawing processing in the embodiments of this application.

[0034] Figure 8 This is a schematic diagram illustrating the principle of bending and hydroforming of the integral tube in the embodiments of this application.

[0035] Figure 9 This is a schematic diagram of the integral molded part obtained after water expansion molding in the embodiments of this application.

[0036] Explanation of icon numbers: 10. Lower pipe section; 11. First pipe assembly port; 12. Non-uniform pipe wall; 13a. First-order first pipe wall; 13b. First-order second pipe wall; 13c. First transition pipe wall; 14a. Second-order first pipe wall; 14b. Second-order second pipe wall; 14c. Second-order third pipe wall; 14d. Second transition pipe wall; 14e. Third transition pipe wall; 15. Third-order non-uniform pipe wall; 15a. Curved pipe wall; 16. Water injection component of water expansion mold; 16a. Water injection port; 20. Bottom joint; 21. Middle tube assembly port; 22. Lower fork assembly port; 30. First pipe; 40. Top tube; 50. Medium tube; 60. Upper fork; 70. Rear hook claw; 80. Downward fork; 91. Outer mold; 92. Steel core; 93. Injection port; 94. Integral blank. Detailed Implementation

[0037] The following is in conjunction with the appendix Figure 1-9 This application will be described in further detail.

[0038] The technical solutions of this application embodiment relate to the field of metal forming and processing technology, and to a bicycle frame downtube bottom bracket assembly and related structures. Specifically, it includes a manufacturing method for manufacturing the bicycle frame downtube bottom bracket assembly and a bicycle frame containing the bicycle frame downtube bottom bracket assembly.

[0039] First refer to Figures 1 to 3 , Figure 1 This is a schematic diagram of the bicycle frame bottom tube assembly in an embodiment of this application. Figure 2 The application of the bicycle frame bottom tube assembly in a bicycle frame is demonstrated. Figure 3 This is a cross-sectional view of the bicycle frame downtube bottom bracket assembly. As can be seen from the figure, the bicycle frame downtube bottom bracket assembly of this embodiment is an integral structural component, mainly including a downtube portion 10 and a bottom bracket portion 20 integrally connected to the downtube portion 10. To achieve lightweighting while ensuring structural strength, such as... Figure 3 As shown, the lower tube portion 10 has a non-uniform tube wall 12 with varying wall thickness along its length.

[0040] This application provides a method for manufacturing a bicycle frame bottom tube assembly, the overall process of which can be referred to... Figure 4 As shown, the specific steps include the following: First, step S1 is performed to melt the metal billet into molten metal; then step S2 is performed to pour the molten metal into a casting and forging mold to cast and form an integral part blank 94; then step S3 is performed to forge the integral part blank 94 in the casting and forging mold to form an integral part. These three steps constitute a casting and forging combined forming process, which aims to manufacture the lower tube section and the bottom bracket section into an initial integral part in one go. (Refer to...) Figure 5 The casting and forging mold used includes an outer mold 91 for forming the outer contour and a sprue 93 for injecting molten metal. The casting step facilitates the formation of complex initial shapes, while the subsequent forging step applies high pressure while the integral blank 94 is still in a high-temperature plastic state, causing the internal metal grains to be compressed and refined, resulting in a denser structure. In this embodiment, the casting and forging mold used in step S2 also includes a steel core 92, which is pre-placed in the mold cavity during casting to directly form the hollow structure of the lower tube portion 10. (Refer to...) Figure 6 The integral component obtained after step S3 includes an integrally formed lower tube section 10 and a five-way connector section 20. The lower tube section 10 has an integrally formed head tube assembly port 11, and the five-way connector section 20 has an integrally formed middle tube assembly port 21 and a lower fork assembly port 22. According to the design, the lower tube section 10 is hollow at this time, while the five-way connector section 20 is a solid structure. The solid structure helps to provide sufficient rigid support for subsequent plastic processing steps.

[0041] In some preferred embodiments, to achieve lightweighting and optimize mechanical properties, the manufacturing method may further include subsequent plastic processing of the integral part. This plastic processing may include: step S4, applying a film treatment to the lower tube portion; and step S5, performing a tube-drawing process on the lower tube portion to obtain a tube-drawn integral part. The film treatment in step S4 forms a lubricating protective film on the surface of the lower tube portion 10. This helps reduce the friction between the tube and the mold during the tube-drawing process in step S5, improving surface quality and extending mold life. Specifically, in step S5, the lower tube portion 10 may undergo a tube-drawing and diameter-reducing process. Using specialized tube-drawing equipment, a tensile force is applied to the lower tube portion 10, causing it to pass through one or more molds with gradually decreasing outer diameters, thereby achieving tube wall thinning and length increase. By controlling the parameters of the tube-drawing process, the lower tube portion 10 can form a non-uniform tube wall 12 that varies axially. (Refer to...) Figure 7 By drawing the tube, regions with different tube wall thicknesses are formed, such as first-order first tube wall 13a, first-order second tube wall 13b, or second-order first tube wall 14a, second-order second tube wall 14b, etc., which realizes the optimized design of retaining thicker tube wall in critical stress areas and thinning tube wall in non-critical areas.

[0042] To obtain a specific frame geometry or appearance, the manufacturing method may further include: step S6, performing a bending pretreatment on the tube-drawing integral component; step S7, performing a bending treatment on the lower tube portion to obtain a bent integral component. In this embodiment, the pretreatment in step S6 may include annealing and a second coating treatment. For example, annealing can heat the tube-drawing integral component to a specific temperature and then slowly cool it to eliminate the internal stress generated during the tube-drawing process and reduce the material hardness. For example, the Weber hardness can be annealed to below 3 degrees, thereby improving the plasticity of the material. The purpose of step S7 is to give the tube-drawing integral component a preliminary geometric shape, which should be able to be smoothly placed into the cavity of the forming mold used in the subsequent hydroforming process, thereby forming a bent integral component suitable for subsequent hydroforming. This helps to prevent wrinkling or cracking caused by excessive differences between the tube and the mold cavity during the hydroforming process.

[0043] Furthermore, to create more complex tube shapes such as aerodynamic profiles, the manufacturing method may further include: step S8, performing a water-expanding treatment on the bent integral part, specifically, placing the bent integral part in a forming mold with a preset inner cavity contour, and injecting high-pressure liquid into the bent integral part to obtain the formed integral part. (Refer to...) Figure 8During the hydroforming process, the bent integral part is placed in the sealed cavity of the forming mold. The inner wall contour of the forming mold constitutes the preset inner cavity contour, which corresponds to the complex curved surface shape that the lower tube 10 is ultimately expected to achieve. The hydroforming mold water injection component 16 is installed at the end of the lower tube 10 away from the five-way connector 20. High-pressure liquid is injected into the interior of the lower tube 10 through the water injection port 16a on the hydroforming mold water injection component 16. The huge internal pressure causes the tube wall of the lower tube 10 to expand outward until it completely conforms to the preset inner cavity contour defined by the inner wall of the forming mold. This process can form complex curved surfaces (such as the bent tube wall 15a) that are difficult to achieve with traditional machining. Figure 9 As shown, this also helps to make the stress distribution inside the pipe more uniform.

[0044] After all plastic forming processes are completed, step S9 is performed to CNC machine the formed integral part. This step is crucial in determining the final product's precision. Using a high-precision CNC machine tool (e.g., a 5-axis milling machine), the previously solid bottom bracket 20 is drilled and bored to form a central cavity that runs through it for mounting the bottom bracket assembly. Simultaneously, in this embodiment, CNC machining is also used to finish the end faces and mating surfaces of the head tube assembly port 11, the middle tube assembly port 21, and the lower fork assembly port 22. Through precise milling, the middle tube assembly port 21 and the lower fork assembly port 22 are constructed as precisely sized insert-type welding interfaces, providing a reliable positioning reference for the subsequent welding and assembly of the remaining frame components.

[0045] The implementation principle of the bicycle frame downtube bottom bracket assembly manufacturing method in this application embodiment is as follows: the core idea of ​​the method lies in "integral molding and local optimization". First, through a composite process combining casting and forging, the downtube and bottom bracket, which are the most stress-prone parts of the frame, are manufactured into a single integral part without any welds. This replaces the heat-affected zone and abrupt structural change zone caused by welding in the traditional process with a continuous metal crystal streamline structure, thus alleviating the stress concentration problem from the structural root. Compared with the traditional technical route of welding parts separately and then machining them as a whole, the manufacturing sequence of this application also helps to avoid the adverse effects of welding thermal deformation on the subsequent machining accuracy. The release of thermal stress generated by welding may cause slight deformation of the component, and precision machining of a deformed thin-walled hollow structure presents challenges in positioning and clamping. This application, by completing the main plastic deformation first and then performing the final cutting, uses the solid bottom bracket as a stable and rigid machining reference, which helps to ensure the dimensional and positional accuracy of the final functional cavity and assembly interface. Secondly, in a series of subsequent plastic processing steps (tube drawing, bending, and hydroforming), precise morphological and performance optimization can be performed on specific areas without compromising the overall structural integrity of the component. For example, tube drawing enables lightweight design with non-uniform wall thickness, while hydroforming creates complex aerodynamic tubing. Finally, high-precision CNC machining completes the manufacturing of functional cavities and high-precision interfaces. This entire method organically combines the advantages of multiple processes, contributing to a comprehensive improvement in performance, weight, and precision.

[0046] This application also provides a bicycle frame bottom tube assembly. (See attached document.) Figure 1 , Figure 2 and Figure 3 The bicycle frame downtube bottom bracket assembly is manufactured using the aforementioned method. The bicycle frame downtube bottom bracket assembly is a single piece made from a metal blank through a casting and forging process, possessing a denser metallic crystalline structure formed by forging compared to pure castings. The bicycle frame downtube bottom bracket assembly includes: a downtube section 10, which is integrally connected to the bottom bracket section 20, and the downtube section 10 has a non-uniform tube wall 12 formed by tube drawing and having a non-uniform distribution along its length; and a bottom bracket section 20, which has a head tube mounting port 11, a middle tube mounting port 21, and a fork mounting port 22 formed on it, and the bottom bracket section 20 has a central cavity formed by CNC machining.

[0047] The implementation principle of the bicycle frame downtube bottom bracket assembly in this embodiment is as follows: As a seamless integral component, the internal metal crystal structure of the bicycle frame downtube bottom bracket assembly is continuous. When subjected to external loads, stress can be smoothly transmitted in the continuous material, without stress concentration caused by material discontinuity at traditional welded joints. This is the structural basis for the assembly's improved fatigue resistance. Simultaneously, the non-uniform tube wall 12 structure of the downtube 10 is a direct physical manifestation of optimized engineering mechanics design. Its principle is based on stress distribution diagrams obtained through finite element analysis and other methods. Thicker tube walls are retained in high-stress areas of the frame (such as near the head tube and both ends of the bottom bracket) to maintain strength and rigidity, while thinner tube walls are used in lower-stress areas (such as the middle of the tube) to significantly reduce weight. This helps maximize structural efficiency without sacrificing key performance characteristics.

[0048] This application also provides a bicycle frame. (See attached image.) Figure 2 The bicycle frame structure includes the aforementioned bottom bracket assembly as its core load-bearing component. Specifically, the bicycle frame includes: the bottom bracket assembly; a head tube 30 connected to the bottom tube 10 via a head tube mounting port 11; a bottom tube 50 connected to the bottom bracket 20 via a bottom tube mounting port 21; a top tube 40 connecting the head tube 30 and the bottom tube 50; a chainstay 80, one end of which is connected to the bottom bracket 20 via a chainstay mounting port 22; and a top fork 60 connecting the bottom tube 50 and the other end of the chainstay 80 (e.g., via a dropout 70).

[0049] The implementation principle of the bicycle frame in this embodiment is as follows: by integrating the high-performance downtube bottom bracket assembly into the bicycle frame design, it contributes to the structural stability of the entire frame. During bicycle riding, the majority of the combined torques from pedaling, steering, and road impacts converge at the connection area between the downtube and the bottom bracket. Using this one-piece molded assembly is equivalent to replacing the weakest welded section of a traditional frame with a high-strength, high-fatigue-resistant "structural hub." Therefore, the excellent mechanical properties of this assembly can be transmitted to the entire frame system, helping to improve the overall rigidity, power transmission efficiency, and long-term durability of the frame. Specifically, higher bottom bracket area rigidity means that the elastic deformation of the frame due to force is smaller when the rider pedals forcefully, thus allowing more pedaling energy to be directly transferred to the drive wheel, reducing energy loss during frame deformation. Furthermore, higher bottom bracket area rigidity means that the elastic deformation of the frame due to force is smaller when the rider pedals forcefully, thus allowing more pedaling energy to be directly transferred to the drive wheel, improving power transmission efficiency. This provides the bicycle with a more direct power response.

[0050] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A method for manufacturing a bicycle frame bottom bracket assembly, characterized in that, include: S1. Melt the metal billet into a molten metal casting liquid; S2. The molten metal is injected into a casting mold to form an integral blank: S3. Forging the integral part blank in the forging mold to form an integral part; wherein, the integral part includes an integrally formed lower tube part (10) and a five-way part (20), the lower tube part (10) has an integrally formed head tube assembly port (11), and the five-way part (20) has an integrally formed middle tube assembly port (21) and a lower fork assembly port (22); the lower tube part (10) is hollow, and the five-way part (20) is solid; S9. The solid five-way section (20) is CNC machined to form a central cavity that runs through the five-way section (20).

2. The manufacturing method of the bicycle frame bottom bracket assembly according to claim 1, characterized in that, In step S2, the casting and forging mold includes a steel core (92) for forming the hollow structure of the lower tube (10).

3. The manufacturing method of the bicycle frame bottom tube assembly according to claim 1, characterized in that, In step S3, the middle tube assembly port (21) and the lower fork assembly port (22) are configured as insert-type welding interfaces.

4. The manufacturing method of the bicycle frame bottom tube assembly according to claim 1, characterized in that, The process after step S3 also includes: S4. Apply a coating to the lower tube section (10); S5. Perform tube extraction on the lower tube section (10) to obtain the tube extraction integral part.

5. The manufacturing method of the bicycle frame bottom bracket assembly according to claim 4, characterized in that, Step S5 includes: performing a pipe drawing and diameter reduction process on the lower pipe section (10) so that the lower pipe section (10) has a pipe wall thickness that varies along the axial direction.

6. The manufacturing method of the bicycle frame bottom bracket assembly according to claim 1, characterized in that, The process after step S5 also includes: S6. Perform a bending pretreatment on the integral tube drawing component, including: S61. Annealing the integral tube drawing component; and S62. Applying a film treatment to the integral tube drawing component. S7. The lower tube (10) is bent to form a bent integral part.

7. The manufacturing method of the bicycle frame bottom tube assembly according to claim 1, characterized in that, The process after step S7 also includes: S8. Perform water swelling treatment on the bent integral part, wherein the bent integral part is placed in a forming mold with a preset inner cavity contour, and liquid is injected into the bent integral part to form a molded integral part.

8. The method for manufacturing the bicycle frame bottom bracket assembly according to claim 5, characterized in that, In step S61, the integral tube assembly is hardened to a Weber hardness of 3 or less.

9. A bicycle frame bottom tube five-way assembly, characterized in that, The bicycle frame bottom tube assembly is manufactured using the manufacturing method described in any one of claims 1-8.

10. A bicycle frame, characterized in that, include: The bicycle frame bottom tube bottom bracket assembly as described in claim 9 includes an integrally formed bottom tube portion (10) and a bottom bracket portion (20). The first tube (30) is connected to the lower tube (10) through the first tube assembly port (11). The middle tube (50) is connected to the five-way connector (20) through the middle tube assembly port (21) and to the head tube (30) through the upper tube (40). The lower fork (80) is connected to the five-way connector (20) through the lower fork assembly port (22) and to the middle tube (50) through the upper fork (60).

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