自行车已成为人们代步和短途运送货物的主要工具。近年来,随着城市经济的发展,如何针对国情开发适应山区、丘陵、平原、河川等各种地形、多种用途的农用变速自行车已成为一个重要课题。目前,国外自行车变速器主要分两大类,一类是外变速器,俗称拔链器,一般用于赛车或运动车上。另一类是我们常见的内变速器,如内三飞,由于结构强度上的限 Bicycles have become a major tool for people to transport and transport goods short distances. In recent years, with the development of urban economy, how to develop agricultural variable speed bicycle adapted to various terrains, hills, plains, rivers and other topographical and multi-purposes according to the national conditions has become an important issue. At present, the foreign bicycle transmission is mainly divided into two categories, one is the external transmission, commonly known as pull chain, generally used for racing or sports car. The other is our common internal transmission, such as Nei Sanfei, due to the structural strength of the limit