Steering gear ratios in existing automobiles are made almost constant. Therefore, at low speed, the larger steering wheel angle needed increases drivers' physical workload especially in the case of a maneuver with large directional angle change such as parking, and may impair quickness in response necessary for a sudden lane change maneuver such as obstacle avoidance at low speed.
On the contrary, at high speed, a relatively small steering angle input can generate comparatively large vehicle lateral acceleration, and this larger overall steering gain may necessitate drivers' more meticulous steering operation which can produce increased mental workload.
In addition, for understeer vehicle, steering gain tends to decrease as lateral acceleration increases because of saturation in lateral force of the tire, and this may also reduce maneuverability in directional change, especially during cornering.
In order to address these possible factors, a steering system in which the equivalent steering gear ratio increases (becomes lower geared) with the increase of vehicle speed, and decreases ( becomes higher geared ) with the increase of steering wheel angle was created, and its effects on driver-vehicle response and performance were studied.
This paper presents the selection of variable characteristics of steering gear ratio derived from observations of drivers' steering behavior and from the results of proving ground tests, and describes the mechanism of a steering gearbox which could realize the proposed steering characteristics, and the construction of such a steering system. The results of simulations and proving ground tests, which indicate improvement in performance as expected, are shown as well.