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| The operating mechanism of the rear steering gearbox, which connects two crank mechanisms at different phases using a planetary gear |
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The control mechanism created through the joint effort, which turned the front and rear wheels in the same direction at high speeds but in opposing directions at low speeds, was initially conceived as a "speed-linked 4WS system"-a control mechanism directly dependent on the speed of the vehicle. However, it required a gear-ratio control in order to successively link the wheel-turning actions in two different directions. Accordingly, a new mechanism had to be developed by combining an electronic control device and variable gear-ratio mechanism.
The layout of such a system would prove to be a difficult, but not impossible challenge. However, excessive complexity could result in problems during production, that would be reflected in the market. Therefore, further discussions were held in order to simplify the system. Eventually, the review process led to a theoretical shift in linking the control function to the angle of steering rather than the vehicle's speed. This became the starting point for a "steering-angle sensing 4WS," which would control the rear wheels in accordance with how much the steering wheel was turned.
This is easier to understand by imagining what one does when driving a car. When changing lanes on a highway, the steering wheel is turned only slightly. However, there are situations in which the steering wheel must be turned considerably more, such as when parking the vehicle in a garage. In view of enhanced dynamic performance, the front and rear wheels should turn in the same direction at high speeds and in opposite directions at low speeds. But when steering angles are applied to this principle, the front and rear wheels should turn in the same direction at smaller angles but opposite directions when greater angles are applied.
Honda had, in fact, already included this concept in a patent the company obtained in 1978. However, it all became a reality with the development of a new crank mechanism, which was designed to turn the wheels in the same direction initially but in opposite directions after a certain point. For example, when the steering wheel was turned to a large angle, the wheels would turn in the same direction for a brief moment after the steering wheel starts to rotate. Then, as the angle of steering increased, the rear wheels would turn toward the opposite direction. One problem had to be resolved, though, in order that the mechanism could be utilized. With a single crank mechanism, the wheels could only be turned at certain instances, regardless of whether the front and/or rear wheels were turning in the same or opposite directions. With the car traveling at high speed, the desired control could be achieved with the rear wheels turned just one or two degrees at most in crank angle. On the other hand, an offset of approximately five degrees would provide more effective control during low-speed maneuvers. In that regard it would not make sense to use only one crank. Therefore, it was decided that the combination of two cranks would overcome the drawback. This led to the development of a simple mechanical system that would not rely on an electronic control device or other such complexities.
High scores were given to the complete 4WS system at its initial evaluation. Even Tadashi Kume, then the president of Honda R&D, was impressed by its simplicity and effectiveness. The achievement was the fruit of hard work by the development staff, who at each occurrence of difficulties used a calm, analytical approach in order that the project could move forward. Simplicity was their byword, and in all respects the 4WS system satisfied that. |
