In motorcycles, when they are traveling at medium to high speed, the roll stability is usually maintained by the restoration force generated by self-steering effect. However, when the vehicle is stationary or traveling in low speed, sufficient restoring force does not occur because some of the forces, such as centrifugal force, become small. In our study, we aimed at prototyping a motorcycle having a roll stability realized by a steering control when the vehicle is stationary or traveling in low speed.
When we considered a mathematical control model to be applied, general models of four-degree-of-freedom had a critical inconvenience that the formulae include nonlinear second derivatives making them excessively complicated for deriving a practically applicable control method. Accordingly, we originally constructed a new control model which has equivalent two point masses (upper and lower from the vehicle’s center of gravity). With this model, the formula can be easily converted into mathematical expressions applicable for the actual control. From this model, we found that if the trail length has a negative value, only by steering control, the total roll moment, which is large enough to restore the imposed inclination of the vehicle, is producible. However, with negative trail length, when the vehicle is traveling in medium to high speed, the self-steering effect does not work and the roll stability cannot be maintained.
Based on these investigations, a prototype vehicle was produced equipped with a mechanically arranged switching function from negative to positive trail lengths activated when traveling speed exceeds above a predetermined speed. In addition, a handlebar angle compensator was adopted to prevent interferences between the steer control forces and rider’s maneuvers. With this prototype and control method, the motorcycle was realized which has the normal maneuverability at medium to high speed and has self-standing function in stationary state.
