Steering systems for highway vehicles and the like are designed primarily for driver control. In these systems, the steering force required on the steering member and the ratio between steering member movement and movement of the steerable ground wheel takes into consideration the characteristics of the particular vehicle and the conditions under which it will usually be operated. A wide variety of forces can act on vehicle steering systems and these must be dealt with satisfactorily in order to provide a stable and controllable vehicle. As vehicle speed increases, the effects of any adverse steering inputs are multiplied, making it necessary for the driver to exercise more precise and careful driving control.
Vehicles with stable steering systems track relatively straight ahead and generally resist all steering inputs away from center, including those of the driver. Intentional turning maneuvers by the driver therefore require sufficient turning force to overcome this positive resistance to movement away from center. When the driver relaxes the turning force applied to the steering wheel, a stable system has a strong tendency to return to its straight ahead position and normally does not overshoot the neutral or center position. By contrast, an unstable steering system provides relatively weak or no positive return to center and may overshoot and sometimes oscillate about the center position. Therefore, with an unstable system, a straight, unswerving vehicle track can be maintained only by constant driver control through precision steering inputs to the steering system. The amount of driver attention required to keep the less stable vehicle tracking straight may vary considerably because forces of many different magnitudes and types can produce spurious steering inputs into the vehicle steering system.
Some examples of less stable steering systems include standard highway-type vehicles with little or no positive caster or overpowered steering systems and various types of motor vehicles using offset wheels and/or oversized tires. Overpowered steering systems override any return or centering forces so strongly that driver feel for the center position is virtually eliminated. Zero or negative caster deprives the steering system of the centering forces discussed below with regard to positive caster. The destabilizing effects of offset wheels and/or oversized tires involve a more complex interaction of forces. Offset wheels are offset from the pivotal wheel mounting at the kingpin and can produce turning moments about the kingpin. Road forces acting along the outer portions of the extra-wide tread of oversized tires can also generate turning moments about the kingpin. These turning moments can lead to instability by dramatically multiplying any force tending to turn the steerable wheels away from center. Furthermore, offset wheels sometimes have a tendency to turn away from center without driver input, a tendency which can be aggravated by either positive or negative wheel caster. Since these destabilizing forces are magnified by speed, vehicle steering with offset wheels can be extremely difficult and actually unsafe at highway speeds.
The ideal situation is one where the steering system inherently causes the vehicle to travel in an unswerving straight line unless the driver intentionally turns the vehicle in another direction. Thus, stable steering systems require relatively little attention from the driver as the vehicle progresses along a straight path down the roadway. In other words, from a steering standpoint, the stable vehicle should not respond to anything but the driver's steering commands and those commands must be of sufficient magnitude to overcome the resistance to turning away from center. In the absence of a steering input by the driver, the vehicle should literally do nothing but progress straight ahead.
In the past, vehicles that were inherently stable usually employed a generous positive caster, among other things, to achieve straight or true track characteristics. While positive caster is desirable in some respects, it is not without compromise over the full steering spectrum. For example, the adverse effects of strong, gusty crosswinds are usually more pronounced on vehicles with positive caster. As its name would imply, the vehicle tends to caster toward the side it is being pushed by the wind. Similarly, a high crown at the center of the roadway will cause vehicles to caster toward the edge of the roadway, that is, in the downhill direction from the crown. These two adverse effects are some of the negative features of achieving steering stability through generous amounts of positive caster. On the plus side, except under the foregoing conditions or where offset wheels are used, motor vehicles with positive caster are less fatiguing to drive over long distances and are safer and more controllable at highway speeds. One reason such vehicles are more controllable is that by tracking straight, virtually no driver effort is required to keep the vehicle from swerving unless the foregoing extraneous forces are present.
In marked contrast, weak or soft centering systems, such as those employing little or no positive caster, excessive power steering, and/or offset wheels, change direction almost continuously so that constant driver manipulation is required for straight tracking of the vehicle. This kind of steering uses up more of the driver's energy than is generally realized and makes a long drive much more fatiguing. Such weak systems are also more susceptible to unintentional driver steering inputs, such as unstable driver-induced oscillations. Single car accidents have been caused by such adverse driver inputs simply because the overly soft directional stability of the vehicle was not sufficient to resist an upset steering input initiated unintentionally by the driver.
Centering devices of the prior art have been used primarily on specialty vehicles as illustrated by the Quayle U.S. Pat. No. 3,056,461 of Oct. 2, 1962, and the Schwenk U.S. Pat. No. 3,583,515 of June 8, 1971. Quayle shows a power steering mechanism for an industrial truck in which a double-acting spring assembly returns a traction wheel to its straightforward position upon removal of fluid pressure from a hydraulic steering ram. Schwenk shows a double-acting air pressure assembly for returning a steering axle to its center position upon removal of forces serving to rotate the axle away from center. The two devices are also quite similar in that both bias two piston components away from each other against respective stops near each end of a cylinder component. Thus, the spring of Quayle and the pressure chamber of Schwenk are both located between the piston components. The center position of these prior art devices is fixed at the time of installation.
Bishop U.S. Pat. No. 3,333,863 of Aug. 1, 1967, is of interest as showing a wheel stabilizer with a spring arrangement for counteracting torque around the kingpin due to the vehicle weight. The spring of FIG. 3 includes hydraulic dampening. However, the spring arrangement is single-acting and the device is not employed as a centering mechanism.
In an attempt to overcome some of the problems discussed above, Henry-Biabaud U.S. Pat. No. 3,171,298 of Mar. 2, 1965, No. 3,373,631 of Mar. 19, 1968, and No. 3,426,612 of Feb. 11, 1969, suggest that a cam mechanism may be mounted directly on the steering column to help center the steering wheel. Because such a system is located on the steering wheel side of the steering gear assembly, spurious steering inputs from the steering column and/or from a power steering unit are magnified before reaching the steerable wheels by a factor corresponding to the vehicle steering ratio, which may be as large as 18:1 or greater. As a result, relatively weak spurious inputs can be transformed into strong wheel positioning forces which may adversely affect the stability of the vehicle. One such spurious input is that generated when the steering wheel overshoots its center position as discussed in the latter two Henry-Biabaud patents referenced above. Another drawback of the Biabaud type centering device resides in the absence of a means for the driver to quickly and easily trim the center position of the steering system in response to varying road conditions and steering system characteristics.
A still further drawback is that spurious forces transmitted from the roadway through the steerable wheels affect substantially the entire steering assembly before encountering any stabilizing resistance from either the steering wheel or the Biabaud type of centering device. The transmission of these spurious forces between the wheels and the steering column cause the inter-connected components of the steering assembly to repeatedly oscillate between states of tension and compression. Such oscillations cause wear and slack in ball joints and other connections and have long been considered a primary source of stress fatigue which may lead to premature failure of various steering assembly components.