posted on 2011-02-01, 16:59authored byGeorge Mavros
The work presented in this thesis is devoted to the study of mechanism of tyre force
generation and its influence on handling dynamics of ground vehicles. The main part
of the work involves the development of tyre models for use under steady-state and
transient operating conditions. The general capability of these models is assessedin a
full vehicle simulation environment. The interaction between tyre and vehicle
dynamics is critically evaluated and the observed vehicle behaviour is related to the
inherent characteristics of different tyre models.
In the field of steady-state tyre modelling, two versions of a numerical tyre model are
developed. The modelling procedure is carried out in accordance with the viscoelastic
properties of rubber, which influence the mechanical properties of the tyre structure
and play a significant role in the determination of friction in the tyre contact patch.
Whilst the initial simple version of the tyre model assumes a parabolic pressure
distribution along the contact, a later more elaborate model employs a numerical
method for the calculation of the actual normal pressure distribution. The changes in
the pressure distribution as a result of variations in the rolling velocity and normal
load influence mainly the levels of self-aligning moment, whilst the force
characteristics remain practically unaffected. The adoption of a velocity dependent
friction law explains the force generating behaviour of tyres at high sliding velocities.
The analysis is extended to the area of transient tyre behaviour with the development
of a tyre model appropriate for the study of transient friction force generation within
the contact patch. The model incorporates viscoelasticity and inertial contributions,
and incorporates a numerical stick-slip law. These characteristics are combined
together for the successful simulation of transient friction force generation. The
methodologies developed for the modelling of transient friction and steady-state tyre
force generation are combined and further extended in order to create a generic
transient tyre model. This final model incorporates a discretised flexible viscoelastic
belt with inertia and a separate fully-dynamic discretised tread, also with inertia and
damping, for the simulation of actual prevailing conditions in the contact patch. The
generic tyre model appears to be capable of performing under a variety of operating
conditions, including periodic excitations and transient inputs which extend to the
non-linear range of tyre behaviour.
For the evaluation of the influence of the aforementioned tyre models on the handling
responses of a vehicle, a comprehensive vehicle model is developed, appropriate for
use in handling simulations. The two versions of the steady-state models and the
generic transient model are interfaced with the vehicle model, and the response of the
vehicle to a step-steer manoeuvre is compared with that obtained using the Magic
Formula tyre model. The comparison between the responses is facilitated by the
definition of a new measure, defined as the non-dimensional yaw impulse. It is found
that the transience involved in tyre behaviour may largely affect the response of a
vehicle to a prescribed input.
History
School
Mechanical, Electrical and Manufacturing Engineering