Motorcycle Steering Oscillations Due to Road Profiling
Prof. David Limebeer
Imperial College
A study of the effects of regular road undulations on the dynamics of a cornering motorcycle will be presented. This work makes use of a motorcycle-tyre model that was built using the automated multibody modelling package AUTOSIM. We make use of root-locus and frequency response plots that were derived from a linearized version of this model. The root-locus plots provide information about the damping and resonant frequencies of the key motorcycle modes at different machine speeds, while the frequency response plots are used to study the propagation of road forcing signals to the motorcycle steering system. Our results are based on the assumption that there is road forcing associated with both wheels and that there is a time delay between the front and rear wheel forcing signals---this is sometimes referred to as wheelbase filtering. Control systems are used in the simulation model to maintain both the machine speed and the machine camber angle at preset values for flat road running.
The results show that at various critical cornering conditions, regular road undulations of a particular wavelength can cause severe steering oscillations. At low speeds the machine is susceptible to road forcing signals that excite the lightly damped wobble and front suspension pitch modes. At higher speeds it is the weave and front wheel hop modes that become vulnerable to a similarly dangerous phenomenon. We believe that the results and theory presented here explain most of the stability related accidents that have been reported in the popular literature and police accident reports, and are therefore of practical import.
