18-12-2012, 04:05 PM
Rollover Stability Index Including Effects of Suspension Design
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ABSTRACT
In this paper a simple yet insightful model to predict
vehicle propensity to rollover is proposed, which
includes the effects of suspension and tire compliance.
The model uses only a few parameters, usually known
at the design stage. The lateral accelerations at the
rollover threshold predicted by the model are compared
to the results of simulations, in which vehicles with the
same static stability factor, but different suspension
characteristics and payloads are subjected to rollinducing
handling maneuvers. The results of simulations
correlate well with the predictions based on the
proposed model. Design recommendations for passive
suspensions, which would increase rollover stability are
discussed.
INTRODUCTION
In recent years rollover has became an important safety
issue for a large class of vehicles. Even though rollovers
constitute a small percentage of all accidents, they have
unproportionally large contribution to severe and fatal
injuries. For example, rollover is the primary cause of
fatalities in accidents involving sport utility vehicles
(SUVs). There is an urgent need to develop both
analytical and experimental tools to predict rollover
propensity of vehicles and to improve their design from
the viewpoint of rollover resistance. Real-world rollovers
are complex events, involving a variety of factors, which
may have broad statistical distributions, and some of
these factors may be beyond control of vehicle
designers. Such factors include driver steering patterns,
type of road surface, type of shoulder, road and shoulder
inclination angles, existence of drop off in transition from
road to shoulder, coefficient of friction, presence or
absence of obstacles on the vehicle path, etc. In
addition, during rollover vehicle experiences a loss of
stability, a condition in which small changes in vehicle
parameters, inputs or environment can significantly
affect vehicle behavior. For these reasons, it is nearly
impossible to device a simple test or a method that
would reflect a majority of real-world rollover scenarios
and reliably determine rollover propensity.
ROLLOVER MODEL
Static stability factor is obtained by considering the
balance of forces acting on a rigid vehicle in steady-state
cornering. This is illustrated in Figure 1, where
deflections of tires and suspension are neglected.
During cornering the lateral tire forces on the ground
level (not shown) counterbalance the lateral inertial force
acting at vehicle center of gravity, resulting in a roll
moment. This moment is counterbalanced by the
moments of vertical forces. Taking moments about the
center of contact patches for the outside tires results:
CHANGE IN HALF-TRACK WIDTH
Due to lateral compliance of tires and suspension, as
well as changes in wheel lateral location due to
suspension kinematics and changes in camber angle,
the distance in lateral direction between the centerline of
vehicle and the tire contact patches is changed, usually
reduced, during cornering. In this paper this distance is
defined as a half-track width.
RESULTS OF SIMULATIONS
In order to verify the accuracy of the proposed model,
vehicle simulations were conducted using a full-car 16-
degree of freedom vehicle model, which was validated
against vehicle test data. The model permits simulation
of vehicle dynamics under large roll angles, significantly
exceeding the angle corresponding to two-wheel lift off
condition. The vehicle used in simulation is a midsize
sport utility vehicle with all independent suspensions and
a marginal static stability factor of only 1.09 in unladen
state. In order to make the vehicle easier to roll over
during severe handling maneuvers, the lateral
acceleration capability of the vehicle was slightly
increased by assuming more aggressive than standard
tires.