A bicycle frame comprises a main frame having a head tube portion and a seat tube support portion integrally formed of a single blank tube, a seat tube inserted in the seat tube support portion of the main frame and having a cross-shaped lower end formed from a single blank tube by bulge-forming, a metal fitting having an aperture adapted to receive a lower end of the seat tube and chain stay supports on both sides of the aperture, two chain stays having front ends fixed to a mid portion of the main frame and mid portions abutting against the chain stay supports of the metal fitting, and rear forks connecting rear ends of the chain stays and an upper end of the seat tube. Two lateral expanded portions of the cross-shaped lower end are pierced to form apertures into which commercially available bearings are press-fitted to form a hanger portion or bottom bracket of the seat tube. Production processes are simplified to improve the productivity of bicycles. Increased strength of the frame makes it possible to use thinner tubes for the frame to reduce the entire frame weight and hence cost.

A bicycle frame includes a top tube provided with bulge-processed regions at its front and rear end portions, formed with bores having axes which intersect a longitudinal axis of the top tube, for accommodating therein the upper end portions of the head tube and the seat tube, respectively. The frame further includes a down tube provided with a bulge-processed region at its front end portion, formed with a bore having an axis which intersects a longitudinal axis of the down tube, for accommodating therein the lower end portion of the head tube, and optionally provided with another bulge-processed region at its rear end portion, formed with a bore having an axis which intersects the longitudinal axis of the down tube, for accommodating therein the lower end portion of the seat tube. The frame has an optimized distribution of effective section modulus in the longitudinal direction of the top and down tubes, to realize a markedly improved rigidity and mechanical strength.

This invention relates to a method for molding a bicycle tube and the method comprises the following steps: first, providing a tube and then controlling thickness of the tube by a drawing and extruding process for forming a molding portion. Second, plasticize other portions of the tube into particular diameters and shapes by plasticizing processes. Third, hydroform the molding portion of the tube by a mold and a hydroforming device for forming particular three-dimensional shapes. By applying to the above-mentioned method, the tube could be molded into a three-dimensional, unique and delicate shape.

A method for making a bicycle frame part includes the steps of preparing a malleable tubular blank that has an outer surface and that is made from an alloy selected from the group consisting of Al--Mg--Sc alloy, Al--Mg--Zr alloy, and Al--Mg--Li--Zr alloy; placing the tubular blank in a mold such that the tubular blank is surrounded by an inner surface of the mold; heating the tubular blank to a working temperature ranging from 200 to 500.degree. C; and injecting a high-pressure fluid into the tubular blank so as to permit expansion and permanent deformation of the tubular blank in the mold to an extent that the outer surface of the expanded tube abuts against and conforms to the inner surface of the mold.

A bicycle frame part made via a process that includes the steps of placing a one-piece metallic tube blank in a die; hydraulically forming the tube blank by introducing a hydraulic pressure into the tube blank to deform and expand the tube blank so as to form at least one expanded wall part projecting from the tube blank along a direction different from the direction of extension of the tube blank; and machining the expanded wall part to form a connection tube. A bicycle frame includes a plurality of frame parts connected to each other. At least one of the frame parts is formed via the aforementioned process, and includes a tube body and at least one connection tube projecting from the tube body in a direction different from the direction of extension of the tube body.