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.
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