自行車後變速系統之四連桿機構的研究

Abstract

自行車變速系統的主要目的在使騎乘者能根據不同的路況和自身的體力，調整適當 之減速比，以達到省時、省力的目的。其中的後變速器的主要功能在於導引鏈條，迅速 、精確至所選擇的後齒片以完成換檔的動作，而後變速器主要機構為張力導輪、導鏈導 輪和四連桿機構。在本論文中，基於一些基本假設，導出後變速器之四連桿機構的鋼索 拉力與位移關係的理論數學模式，並經實驗成功驗得此數學模式的正確性，發現鋼索與 拉鎖調整扭間的摩擦力是關鍵因素。此數學模式並用來作為分析工具，以進行鋼索拉力 的最佳化設計，發現改變固定鋼索的支點位置，不但可降低最大鋼索力9.26％，並可大 幅縮小鋼索力的變化量65.49％。並且在此提出一種改善的設計，將鋼索與拉鎖調整扭 間，由滑動摩擦改換成滾動摩擦，可有效降低摩擦力。

The derailleur system on bicycle can makes riders time-saving or labor-saving by changing the suitable reduction ratio. The rear derailleur should guide the running chain rapidly and precisely to a selected freewheel sprocket position without disengagement from the freewheel assembly. Therefore, the design of the rear derailleurs has a great influence on the shifting process. In this study, a mathematical model of a four-bar linkage mechanism in a spatial rear derailleur is developed to simulate cable force and the displacement of the guide pulley in shifting direction. The relation between cable force and displacement is obtained and verified. And, it can be found that friction between cable and adjusting screw unit is a key factor. Then, we use the mathematical model to optimize the cable force. By changing the position of couple point, we can reduce not only the maximum cable force 9.26％ but also the difference force betwee the maximum and minimum cable force 65.49％. Finally, a roller design to replace the adjusting screw to reduce the friction force is proposed.