本文以力學原理分析機車碰撞或擦撞後的倒地方向，詳細介紹陀螺的特性與陀螺力矩形成的原理。研究表明若碰撞為機車前部（前輪、前輪蓋、前叉、把手、照後鏡等），機車倒地方向主要由陀螺力矩決定；若碰撞為機車後部（車架、車殼、引擎、傳動系統、後懸吊系統、排氣管與後輪等），機車倒地方向主要由碰撞高度與機車重心高度決定。將機車與機車碰撞，機車與汽車碰撞常見的形式，分別加以說明，輔以肇事實例，驗證機車的倒地方向，並製成表格方便參考。由機車倒地方向與車損部位，可以推測交通事故時，機車碰撞時的行向與碰撞形式。

The study used principles of mechanics to analyze the direction of a motorcycle falls to ground due to collision and introduced the characteristics of gyroscope and derived how the gyroscopic moment would occur in detail. The research results showed that when impact was taken place in front assembly (front wheel, front wheel cover, fork, hand bar and mirror etc.) of a motorcycle, direction of the motorcycle falls to ground would depend on the gyroscopic moment. If the collision was occurred in rear assembly (frame, body shell, engine, powertrain, rear suspension system, muffler and real wheel etc.) of a motorcycle, the direction of the motorcycle falls to ground would be determined by the heights of the impact and the center of gravity of the motorcycle. This study analyzed the direction of a motorcycle falls to ground on common types of collision between motorcycle and motorcycle, as well as motorcycle and car with real examples to validate the developed theory. Based on the analyzed results, this study also established tables for easy reference. From the falling direction and damage area of a motorcycle, one can apply the studied results to estimate the impact direction and collision type of the motorcycle.