Steady-State Directional Control Test Procedures for Passenger Cars and Light Trucks
This SAE Recommended Practice establishes consistent test procedures for determination of steady-state directional control properties for passenger cars and light trucks with two axles. These properties include the steering-wheel angle gradient, reference steer angle gradient, sideslip angle gradient, vehicle roll angle gradient, and steering-wheel torque gradient with respect to lateral acceleration. They also include the yaw velocity gain, lateral acceleration gain, and sideslip angle gain with respect to steering-wheel angle. Additionally, the characteristic or critical speed and the front and rear wheel steer compliances may be determined.
Rationale: The path curvature of an automobile in steady turning is determined by speed, steering-wheel angle, wheelbase, and the elastic and kinematic characteristics of the front and rear steering systems, suspensions, and tires.
When turning at very low lateral acceleration levels, tire slip angles are negligible and the vehicle follows the Ackermann turn radius, which is defined geometrically by wheelbase and by front and rear steer angles. When turning at appreciable speed, tire slip angles are no longer negligible and produce cornering forces that cause kinematic and elastic deflections in the steering, suspension, and tire systems that modify the front and rear steer angles. The difference between the total front and rear effects determine what is often referred to as the understeer / oversteer characteristic of the vehicle. The test procedures described herein are designed to measure objective responses of the vehicle that quantify this understeer / oversteer characteristic.