Land vehicles – Wheeled – Running gear
Patent
1992-05-14
1994-06-21
Culbreth, Eric
Land vehicles
Wheeled
Running gear
180140, B62D 506
Patent
active
053223080
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to four wheel steering systems for vehicles and more particularly front and rear wheel steering controlled by steering input from a driver operating a steering wheel. Typically, such systems provide that the rear wheels turn in the same direction as the front wheels in modes of operation associated with high vehicle speeds and in opposite directions in modes of operation associated with low vehicle speeds and intermediate degrees of turn of the rear wheels at speeds intermediate between such high and low speeds.
2. Description of Prior Art
The prior art in this area includes U.S. Patent specifications No. 3,596,730 (Cecce); 4,313,514 (Honda); 4,467,885 (Honda); 4,483,547 (Honda); 4,660,844 (Honda); 4,572,316 (Mazda); 4,646,867 (Mazda); 4,730,839 (Mazda); 4,733,878 (Mazda); 4,781,261 (Toyota); 4,770,264 (Lotus). As exemplified by these patents, innovation in the area of four wheel steering has been extremely diverse in terms of the technical solution to the problem. Surprisingly, as yet, this has not resulted in widespread adoption of such steering systems in the market-place notwithstanding that test reports indicated that vehicles incorporating four wheel steering provide several advantages including improved safety in high speed driving Two presently marketed systems are broadly covered by Honda U.S. Pat. Nos. 4,313,514 and 4,467,885 and Mazda U.S. Pat. Nos. 4,572,316 and 4,730,839 and have been described extensively in the technical literature. Information concerning the current commercial system adopted by Mazda in their MX-6 vehicle can be found in Car Australia magazine of February 1989 at page 52 et seq and a Technical Information document published by Mazda Australia Pty Limited entitled: "The Mazda Speed Sensing Computerised 4-Wheel Steering System". The present invention will be described primarily with respect to these two examples of the prior art. In the Honda system, the rear steering is entirely mechanically driven and not directly speed dependent, whereas the Mazda system is speed dependent and therefore more relevant to the present invention.
The reluctance in the market broadly adopting four wheel steering systems may reside in that they are costly, add considerable weight to the vehicle, and have serious functional shortcomings. Also, it may be that those systems enforce compromises in the design of associated front and rear suspensions which result in less desirable suspension performance. In many cases their performance depends upon electronic signals between front and rear steering gears which, in the event of a malfunction, may cause serious vehicle control problems for the driver. Where the front and rear steering gears are mechanically linked by a rotating shaft, as is the cases of Honda and Mazda, less reliance is placed on electronic signal transmission, but there is a marked deficiency in road "feel" to the driver due to high levels of friction of the respective rotating shaft arrangements.
In the Honda Prelude arrangement, a relatively conventional rack and pinion power steering gear is provided in the front steering gear having an additional rack which drives a pinion and rotatable shaft extending to the rear of the vehicle where another full mechanical steering gear is located. The rear steering gear incorporates a function generating mechanism which provides the reversing of the direction of rear steer as a function of steering wheel angle. Since each element of the mechanical linkage imposes additional friction, the overall system is characterised by having poor steering "feel" and poor self-centering action. The front to rear steering shaft requires seals, journals and universal joints at each end which not only add to the friction but, due to the great length of such a shaft, also results in a degree of compliance. To minimize the resulting loss of steering response the components are very robust and accurately fitted, and are therefore heavy and expensive.
In the Mazda MX-6 arrangement magni
REFERENCES:
patent: 4719981 (1988-01-01), Miyoshi
patent: 4770264 (1988-09-01), Wright et al.
patent: 4874183 (1989-10-01), Chikuma et al.
patent: 4878556 (1989-11-01), Wells
Car Australia, Bum Steer, February.
The Mazda Speed Sensing Computerised 4-Wheel Steering System.
SAE-A Journal, 4 WS on track for handling, Jul./Aug. 1989.
A. E. Bishop & Associates Pty. Limited
Culbreth Eric
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