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An overview of existing and alternative forms of vehicle understeer/oversteer expressions is presented. New forms are derived consistent with conceptual extensions to the configurations of the vehicle's steering system, the driving mode - steady-state or transient, and the responses - path curvature or yaw velocity. Derivation of all understeer expressions is presented with a consistent use of the Ackermann reference case and the related “Ackermann vehicle” construct. The vehicle is otherwise represented in a traditional manner as a bicycle model operating in the linear range consistent with small angle approximations. The vehicle's steering system is assumed to be more generally configured with four-wheel steer and active or steer-by-wire actuation at both axles. The actuation is assumed to allow the introduction of significant speed sensitivity to the effective overall steering ratios.

A set of functions for characterizing the mechanical properties of a steering “short gear” is described. They cover the kinematic, stiffness, assist, and friction performance of a power assisted (or manual) steering gear from the input shaft to the inner ends of the tie rods. Their use in describing the performance of a generalized steering gear is described. They have particular application to describing the steering feel performance of a vehicle. They can be used to specify the steering subsystem performance for desired steering feel for a given vehicle. They can also be used for experimental characterization of steering subsystems, can be used in vehicle dynamics simulations, and can be synthesized from a set of vehicle level performance targets. Along with their description, their use in simulation and methods to synthesize their values are described.

A theory of vehicle steering pull, created by asymmetrical tire cornering properties, is developed. It is validated with free control data obtained on the road. The effects of tire lateral force and aligning torque asymmetries on a car's straight line stability are analyzed for both fixed and free control. Equations for front axle lateral force, steering system moment, and sideslip angle are derived. They are based on tire properties and certain assumptions about the car's characteristics. This theory is validated using data obtained in open road testing. The test techniques, as well as alternate ones, are described. In addition, the relationships between actual front axle force and axle conicity force, ply steer force, and lateral force offset are analyzed. It is found that front axle conicity force correlates very strongly with a more accurate theoretical prediction. The axle force predicted by tire conicity force is somewhat low.

A simple, linear vehicle model is presented which incorporates the most important characteristics of contemporary passenger cars. It is a three mass model with a fixed, inclined roll axis and linear suspension geometric and compliance characteristics. Basic concepts of understeer and oversteer are presented. Static and dynamic requirements are examined, yielding expressions relating the car's design to tire lateral load transfer, total lateral force, and turn radius. Turn kinematics give expressions for the front steer angle and sideslip angle. Suspension geometric and compliance effects describe the rear steer angle, tire inclination angles, and steering wheel angle divided by the overall steering ratio.