Lateral dynamics of a bicycle with a passive rider model: stability and controllability

This paper addresses the influence of a passive rider on the lateral dynamics of a bicycle model and the controllability of the bicycle by steer or upper body sideway lean control. In the uncontrolled model proposed by Whipple in 1899, the rider is assumed to be rigidly connected to the rear frame of the bicycle and there are no hands on the handlebar. Contrarily, in normal bicycling the arms of a rider are connected to the handlebar and both steering and upper body rotations can be used for control. From observations, two distinct rider postures can be identified. In the first posture, the upper body leans forward with the arms stretched to the handlebar and the upper body twists while steering. In the second rider posture, the upper body is upright and stays fixed with respect to the rear frame and the arms, hinged at the shoulders and the elbows, exert the control force on the handlebar. Models can be made where neither posture adds any degrees of freedom to the original bicycle model. For both postures, the open loop, or uncontrolled, dynamics of the bicycle-rider system is investigated and compared with the dynamics of the rigid-rider model by examining the eigenvalues and eigenmotions in the forward speed range 0-10 m/s. The addition of the passive rider can dramatically change the eigenvalues and their structure. The controllability of the bicycles with passive rider models is investigated with either steer torque or upper body lean torque as a control input. Although some forward speeds exist for which the bicycle is uncontrollable, these are either considered stable modes or are at very low speeds. From a practical point of view, the bicycle is fully controllable either by steer torque or by upper body lean, where steer torque control seems much easier than upper body lean.