Land vehicles – Wheeled – Running gear
Patent
1993-03-16
1994-11-08
Culbreth, Eric
Land vehicles
Wheeled
Running gear
180140, 280 99, B62D 715
Patent
active
053620801
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to front and rear wheel steering systems for vehicles, commonly called Four Wheel Steering (4WS) systems, specifically "speed dependent" systems in which the magnitude and direction of the rear-steer is made to be a function of not only front steering angle but also vehicle speed and/or other related parameters.
BACKGROUND ART
Typically such systems provide that the rear wheels steer in the same direction as the front wheels in modes of operation associated with high vehicle speed, providing a sideshifting characteristic to the vehicle so that the same level of lateral acceleration response is achieved with a lesser degree of yawing. The resulting enhanced lateral response of the vehicle to steering wheel inputs gives the driver a sensation of stiffness and security, particularly in lane changing at high speeds. On the other hand some such systems provide that the rear wheels steer in opposite direction to the front wheels in modes of operation associated with low vehicle speed, providing an enhanced yaw response with consequential good maneuverability and minimum width of the "swept path" of the vehicle through corners. In this specification "swept path" is defined as the envelope of the vehicle's trajectory projected with respect to the road during a maneuver. The numerical ratio of rear steer angle to front steer angle, measured instantaneously over a small incremental range of these variables at a given vehicle speed, will be termed "rear/front steering ratio". This ratio is therefore the effective instantaneous slope of the rear steer angle function when plotted against front steer angle at a given vehicle speed. According to this definition, rear/front steering ratio is usually designed to be positive at high vehicle speeds and negative at low vehicle speeds. Also the term "on-center" will be used to describe the driving condition when the steering wheel is positioned in or around its central operating region, as associated with straight-ahead driving.
The optimum rear/front steering ratio for high-speed on-center driving has been found-to be approximately 0.4. Ratios higher than 0.4, although further improving lane holding and lane changing during highway driving, lead to inadequate vehicle yaw response during negotiation of highway curves. Similarly the optimum rear/front steering ratio for low speed parking maneuvers has been found to be approximately -0.3. Values lower than -0.3 (for example -0.5 or -1.0), although providing excellent vehicle maneuverability, may cause the rear of the vehicle to "sweep-out" and rub against the curb in parking maneuvers. Non-experienced drivers find excessively low values of rear/front steering ratio difficult to master.
The additional major constraint on virtually all 4WS systems is that rear steer angles must be limited to about 5 degrees in order to avoid the rear wheels intruding into potential rear trunk compartment space. Although 5 degrees rear steer angle represents a practical limit in low speed maneuvering consistent with the aforementioned sweep-out problem during parking, 5 degrees rear steer angle will be extremely dangerous in high speed driving. In some prior art speed dependent 4WS systems, 5 degrees rear steer will inevitably occur for large magnitudes of steering wheel input because of the particular speed dependent reversing mechanisms employed. The design philosophy in this case is simply based on the fact that such large steer wheel angles will never be used in normal high speed driving. This is of course correct, however, in emergency evasive maneuvers at high speed, or recovery from skidding, large steering angles will often be necessary. In such critical driving situations it is essential that large rear steer angles do not occur in order that the response of the 4WS vehicle is not disconcerting to the average driver.
Mechanical sliding mechanisms such as disclosed in U.S. Pat. No. 4,313,514 (Honda), U.S. Pat. No. 4,552,239 (Mazda), and west German Patent 3,837,141 (ZF) maintain a constant r
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Baxter John
Bishop Arthur E.
A. E. Bishop & Associates Pty Limited
Culbreth Eric
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