本研究主要探討自行車同時致動前、後煞車器時，其重心對煞車力與減速度的影響。騎乘自行車時，使用前、後煞車器來控制行進的速度，而重心的位置是影響煞車安全的重要因素之一，因此本文將騎乘者與自行車分成為五個部份，並由重心原理與幾何關係，推導以人體與自行車尺寸比例為變數的重心方程式，並使用Matlab 程式設計軟體進行實例運算，以探討同時致動前、後煞車器時的重心與煞車性能之關係。由結果顯示，重心影響前、後輪與地面間的制動力與減速度，調高車把高度時，可增加前、後輪最大安全煞車力相等時的減速度，亦可提高騎乘自行車之安全性；調高座墊高度時，則增加騎乘自行車之危險性，因此當必須調高座墊高度時，建議同時調高車把高度。

This study focuses on the center of gravity to the braking force and deceleration when the front and rear brakes are used simultaneously. The rider uses the front and rear brakes to control the forward velocity of abicycle as riding. The center of gravity is one of the important factors of braking performance. In order to obtain the change of the center of gravity, the rider and the bicycle are divided into five parts in this research. Both the gravity theory and geometry relations are applied to derive the equation of the center of gravity. Matlab, a programming software for instance mathematical operation, is applied to analyze the center of gravity and braking performance when the front and rear brakes are used simultaneously. From the research results, the center of the gravity is to affect braking forces of the front and rear wheels and deceleration. When the maximum safe braking force of the front and rear wheels is equal, the deceleration will be proportional to the handlebarheight, and it can also improve the safety when riding a bicycle. Increasing the seat height that will increase the dangerous when riding a bicycle, therefore, it is suggested to increase the handlebar height when it is necessary to increase the seat height.