A Study of Vehicle Response Asymmetries During Severe Driving Maneuvers

Abstract

The main objective of this research was to study vehicle response asymmetries during severe driving maneuvers. During Phase VI of the National Highway Traffic Safety Administration's Light Vehicle Rollover Research Program, such response asymmetries occurred during field tests, and consequently, the data collected in Phase VI was used in this study. Of the twenty-six light vehicles tested in Phase VI, three vehicles exhibited significant response asymmetries with respect to left versus right steer maneuvers: the 1996 Acura SLX, 1995 Mitsubishi Montero, and 1993 Ford Aerostar.An overview and an evaluation of several rollover stability indexes are provided to help explain the vehicle dynamics at the onset of rollover. No known models exist that incorporate vehicle asymmetries other than the lateral center of gravity (CG) offset. As such, this research effort investigates possible vehicle asymmetric characteristics and unintended inputs that may cause vehicle response asymmetries.The cause of vehicle asymmetric response is quite complex and cannot be narrowed down to one item alone, e.g., CG offset. Careful consideration was given to possible unintended differences in the vehicle inputs that may adversely affect the vehicle response. These unintended vehicle inputs were determined to be either negligible or to have an insignificant impact on the vehicle asymmetric response, and therefore, the cause of vehicle asymmetric response is more likely within the vehicle itself. As such, iii suspension and steering tests were conducted on the SLX and the Montero to identify any vehicle components/characteristics that may cause vehicle response asymmetries.The suspension tests showed that both vehicles exhibited less stiffness in the nonlinear region of suspension compression on the side of the suspension that was opposite to the more severe steer direction in the J-turn maneuver. For example, the SLX experiences two-wheel lift (TWL) at a lower maneuver entrance speed in a left steer versus a right steer J-turn maneuver, and its right side suspension is softer than the left side. The same phenomenon applies to the Montero: it experiences TWL at a lower maneuver entrance speed in a right steer J-turn maneuver, and its left side suspension is softer.To confirm the trend that the softer side suspension will lead to earlier TWL in the opposite steer direction, a model representative of the SLX was created and evaluated using CarSim™. Using the actual SLX suspension parameters, the model showed some differences between left and right steer response, but the effect of the lateral CG offset dominated the vehicle response.The response of the shock absorbers was not determined during the suspension and steering tests, but a simulation study was done to evaluate the effect of the shock absorbers on vehicle response. Based on the shock absorber study, it is evident that a change in the damping force response can have a significant effect on the vehicle response.iv Dedicated to my parents and Jennifer v ACKNOWLEDGMENTS I wish to thank my...