<p class="MsoNormal">The vast majority of research into ground vehicle lateral-directional stability has primarily focused on vehicles operating on paved roads. In comparison there has been very little published research in the lateral directional stability and manual control of off-road vehicles operating in their designed-for off-road conditions. Therefore the open-loop and closed-loop stability of four-wheeled ground vehicles with understeer and oversteer characteristics in off-road conditions were investigated using a locally linearized lateral-directional vehicle dynamics model with two degrees-of-freedom. This model, which was parameterized in terms of the effective front and rear axle cornering compliances (including tire and suspension geometry and compliance effects) by Bundorf&nbsp; and Leffert, is well suited for investigating the effects of vehicle understeer and oversteer and the choice of steer angle reference on lateral-directional stability and control. The choice of steer angle reference affects the quasi-steady open-loop stability analysis and conclusions. Traditionally, the steer angle reference is the vehicle centerline, but an alternative can be the local path tangent (i.e., a path-centered instead of a vehicle centered reference). Analytical results confirm the widely accepted understanding that the open-loop response for an understeering vehicle with the steered wheels fixed parallel to the vehicle centerline is stable for all vehicle speeds, but an oversteering vehicle is unstable above a certain “critical speed” for the same fixed steering condition. However, results for both understeering and oversteering vehicles using a path-centered steer angle input, defined relative to the path tangent (offset from the vehicle centerline by the sideslip angle), were also found to be stable for all forward speed conditions. These results also indicated that the path-centered steering can be used for lateral-directional control with suitable gain. Therefore these results indicate that a simple manual control strategy based on path-centered steer angle inputs can be used to stably control a vehicle for all forward speeds in both understeering and oversteering conditions provided the tire cornering force continues to increase with slip angle, which is generally the case for vehicle off-road or snow conditions. This strategy simply requires the driver to keep the front wheels pointed in the direction of travel for stability (which may involve “counter steering”) with appropriate steering adjustments to maintain lateral-directional control. This strategy is especially relevant to off-road vehicle conditions where the cornering stiffness is always positive up to 90 degrees of tire slip angle. These results are also consistent with ground vehicle operator acceptance and preference of vehicles with oversteering characteristics in off-road conditions. The results also explain why some vehicles, such as commonly used off-road vehicles, can be easily controlled by typical drivers even when operating above their “critical speeds”.</p>

Lateral-Directional Stability and Manual Control of Understeering and Oversteering Vehicles in Off-Road Conditions Based on a 2-DOF Cornering Compliance Vehicle Dynamics Model

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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