Matching of Chassis and Variable Ratio Steering Characteristics to Improve High Speed Stability

Although a vehicle with understeer is defined as a stable system, above the characteristic speed there is a reduction in yaw damping which can lead to highly oscillatory response particularly at high steering frequency inputs associated with an avoidance manoeuvre. Research was conducted to study the effect of reducing the amount of understeer to increase the characteristic speed and hence make the vehicle response less steering frequency dependent at high speed. However this also had the effect of increasing the yaw gain with regards to the steering wheel input. The yaw gain was then tuned to achieve different targets through the use of Variable Ratio (VR) steering mechanisms. This study included both a physical test of a vehicle tuned to achieve different understeer coefficients and the confirmation of the results using a vehicle model to confirm transient effects. The work indicates that by reducing the degree of understeer in conjunction with the application of a VR steering characteristic, the vehicle can be made more stable in a high speed avoidance situation.