BICYCLE FRAME

摘要

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.