Influence of aerodynamic lift and centre of pressure position in motorcycle stability

Motorcycles are systems with complex dynamic behaviour that can become unstable under certain driving conditions. Avoiding such instabilities from the design stage is not trivial since they depend on various interrelated parameters, one of which is aerodynamics. Aerodynamic forces in a vehicle can be essentially described by its longitudinal (drag) and vertical (lift) components acting in a point known as the centre of pressure (CoP). Additionally, several authors explain that drag influences stability through four mechanisms: dampening lateral motion and changing weight distribution, tire cornering stiffness, and rake geometry. On the other hand, the lift force, which has been used importantly in sports motorcycles in recent years, can also influence stability, however, its effect has not been described in the literature. Therefore the aim of this research is to analyse the influence of lift magnitude and CoP position on motorcycle stability in straight-running conditions. To this end, we develop a motorcycle stability model and perform an analysis on a motorcycle with several CoP and downforce values. We consider the CoP ahead, aligned, and behind the motorcycle centre of mass, together with multiple lift coefficients. Results showed that CoP towards the front end stabilises wobble mode, while rear CoP may cause instability on weave mode. The result contributes to the understanding of motorcycle aerodynamics providing new insights into how to use aerodynamics to enhance stability.