This study investigates the dynamic characteristics of the steering handlebar, termed "lean-over characteristics," by combining unmanned bicycle experiments with frequency response analysis. The focus is on the frequency response function from external lateral force to roll angle and steering angle, with particular attention to the relationship between these outputs. Subjective evaluations conducted by test riders revealed noticeable differences in steering feel between the two bicycle configurations. These differences were quantitatively explained by the gain and phase characteristics of the FRF between roll angular angle and steering angle, especially at approximately 7 Hz.
The origin of this dynamic behavior was identified as zeros in the transfer function of roll angle. At this frequency, the external moment input and the inertial response of the vehicle body cancel each other out, resulting in suppressed roll motion and an enhanced steering response. Numerical simulations confirmed that changes in the product of inertia directly affect the location of these zeros. Furthermore, the same mechanism was observed in motorcycle models based on Sharp’s equations, indicating that the zeros are a fundamental control parameter in handling design not only for bicycles but also for motorcycles.
These findings suggest that the design of lean-over characteristics requires control of physical parameters such as the location of zeros and inertia coupling, in order to achieve a steering response that aligns with rider perception.