Only when the height of the saddle is adjustable can we meet our needs for a safe and relaxing biking tour. The saddle height (Y’) is simply related to the leg length (Y), while the arm length and the length of the upper body (X) are related to length of the top tube (T/T) between handlebar and the saddle (X’). A proper combination of Y’ and X’ will ensure an optimum frame size that best suits bike riders’ needs.