FIELD OF THE INfVENTION This invention relates to a load carrying vehicle that has pairs of rear wheels supported by two or more longitudinally spaced axles, and in particular a vehicle having means for steering at least one pair of rear wheels.

BACKGROUND OF THE INVENTION Multiple rear axles are used in load bearing vehicles, particularly those that carry the upper range of loads transported on roads to support the weight. There is a problem associated with these multiaxle vehicles, particularly on cornering because the spaced apart axles support wheels that are non steerable.

The inherent instability of the geometry leads to a pressure on the tyres to undergo some lateral movement, this can at time be accommodated by a deformation of the tyres, but on hard cornering will usually result in a dragging or scuffing of tyres on the ground or road surface. At higher speeds and with an uneven road surface this may also result in a measure of instability of road holding capacity and lead to sideway hops on the road surface, particularly where the trailer is somewhat underweight. A further problem in unitary construction trucks relates to the tendency of the rear wheels, particularly where the load is heavy to direct the vehicle to maintain a straight trajectory despite the steering efforts by the truck, this phenomenon is known as plowing. The same general force by the load on a semi-trailer can have adverse effects on the tractor and leads to a phenomenon known as jack-knifing in which the forward momentum of the trailer is applied at an angle to the pivot point of the trailer with the tractor at the location of the so-called "fifth wheel" as the unit turns so that the weight and the momentum of the trailer tend to force the rear of the tractor around in an arc until the tractor portion of the truck is facing an almost opposite direction to the truck.

A number of suggestions have been made to alleviate this problem for semi-trailers and or other load bearing vehicles with double or more rear axles, by the having rear wheels steered. Principally, suggestions have been to link the rear steering with the manually controlled steering of the front wheels of the tractor. Some of these suggestions and further explanation of difficulties associated with multiaxle vehicles are discussed in US patent 5035439.

Many of the suggestion of linking rear steering are somewhat impractical, and none are known to the inventor to be presently implemented for that reason. Additionally, the suggestions do not allow the flexibility to take into account a range of conditions that might impact on providing for appropriate angle of steer for the rear wheels.

For the purposes of this specification the word "comprising" means "including but not limited to", and the word "comprises" has a corresponding meaning. Also a reference within this specification to a document is not to be taken as an admission that the disclosure therein constitutes common general knowledge in Australia.

OBJECT OF THE INVENTION It is an object of the present invention to provide a vehicle with rear wheel steering that obviates, minimises or reduces any one of the above problems or at least provides the public with a useful choice.

SUMMARY OF THE INVENTION This invention arises from the use of a lateral force sensor to input a signal value into a controller which calculates an angle of steer and outputs a signal to direct a steering means to move a pair of rear wheels of a multiaxle vehicle to said angle.

In a first broad form the invention could be said to reside in a load carrying vehicle comprising a driving part at a front end followed by a load carrying part, having at least one pair of steerable wheels at the front to be manually steered by a driver, and at least two or more longitudinally spaced axles each supported by a pair of rear wheels to carry the load, at least one of the axles being steerable, an angle of steer being calculated by a controller, the vehicle having at least one sensor an output of which is dependent on lateral force exerted on one or more of the rear wheels of the vehicle, the sensor output is inputted into the controller which calculates an angle of steer of the one or more pairs of rear wheels, the controller actuating a steering means to move the one or more pairs of rear wheels to the calculated angle of steer.

For a better understanding, the invention will now be described with reference to the drawings wherein,

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic side view of a semi trailer for use on the present invention,

Figure 2 is a schematic top plan view of the semi trailer showing the layout of the wheels longitudinally aligned, and

Figure 3 is a schematic top plan view of the semi trailer in a turn with the rear wheels turned at an angle determined by a controller,

Figure 4 is a schematic top plan view of a second embodiment of a semi trailer wherein the rear wheels are mounted on stub axles, with a common linkage arrangement to coordinate angling of the wheels under the influence of a hydraulic power steering unit, and

Figure 5 is a diagram showing the factors that can be used to have an influence on the degree of steering of the rear wheels.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT OF THE INVENTION This invention has particular relevance to a semi trailer where the load carrying part is trailer articulated about a fifth wheel relative to the driving part which take the form of what is termed a tractor. The tractor of the semi trailer comprises at least a further front axle and usually two further front axles beneath the fifth wheel each axle supported by at least one pair of wheels. The further front axles in known semi trailers are not steerable. Semi trailers are the principal multi-axle load carrying vehicle used on roads today. The invention may however also be applicable to vehicles of unitary construction such as trucks and buses with multiple rear axles.

Multiple axles in a load bearing vehicle refers to axles bearing weight toward the rear of the vehicle, the weight being attributable to the load and structure of the vehicle carrying the load. An axle so formed might be a single unitary axle extending across the width of the trailer of the vehicle, with suitable machined ends on which one or more typically two wheel hubs are journalled. Typically two such axles are located toward the rear of the trailer and are relatively close together to minimise the effects of tyre scuffing. Thus a typical semi-trailer will have eight or more tyres contacting the road surface toward the end of the trailer. For heavier loads a further intermediately positioned axle is also provided, but this is not as common. Other configurations of axles are also contemplated for the present invention.

The axles referred to above need not necessarily be on unitary construction but may be a pair of axle stubs extending at opposing sides of the trailer, the pair of stubs are considered for purposes of this invention to constitute a single axle. In the present invention the stubs may be steerable by reason of being separately pivotable the pivoting of a pair of stubs may be co-ordinated by suitable linkages.

Alternatively where a unitary axle is provided it may be pivotably around a longitudinal centre of the trailer. A further alternative arrangement is where two unitary axles are mounted on a bogey, the bogey being pivotably relative to the trailer, to provide an angle of steer.

It will be appreciated that not all of the multiple rear axles need be steerable, one only might be steerable, the configuration of the trailer will determine the most appropriate axle to be steerable, however to some extent that may depend on the specific distribution of the load. It may be desired to have the more than one rear axles with varying angles of steer dependant on the geometry of the position of the wheels on the trailer. Thus, for example, a first pair of wheels positioned forward of a second pair of wheels will have a different angle of steer.

A wide range of mechanism for moving wheels through a range of angles of steer are known, and a person skilled in the art will readily be able to select an appropriate form of mechanism for steering the steerable rear wheels. The motive force for such steering may be any convenient form suited to the vehicle concerned, and may be pneumatic, hydraulic or an electric motor.

There may be provided one or more types of sensors to provide input into the controller. The sensor's principal role is to provide a signal that reflects the degree of lateral force exerted on any one or more the rear wheels. Other inputs might also be relevant to a calculation of the angle of steer of the one or more rear wheels.

Thus a tyre lateral force sensor might be used to give the most straight forward measure of lateral force on the tyre, a change in that value of a time may provide a more reliable value for purposes of calculating a steering angle. Alternatively it might be desired to use a load sensor for wheels, or suspension components on opposing sides of the axle, the controller will make a comparison of the two values and a difference may be reflective of lateral forces acting on the one or more wheel, this is particularly so where there are changes or progressively increasing changes in the difference.

The tyre scuffing when turning will exert lateral forces on to the axle or bogey carrying the steerable wheels and this lateral force may conveniently be measured by a linear potentiometer or pressure sensor positioned to measure the lateral force thereon.

Other sensors that might indicate a tyre lateral force could include a roll rate sensor, or yaw sensor. These later sensors might perhaps be used a secondary input into the controller for purposes of calculating the angle of steer to be adopted. The angle of steer to be adopted might also be modified somewhat dependent on conditions of the trailer, and these include the speed of the vehicle. Thus a speed sensor such as will typically be a transmission sensor may be used. Travel over a certain speed may indicate that the vehicle is engaged in highway travel and block the activation of the rear wheel steering means, or alternatively reduce the degree of turning. The overall load of the vehicle will also impact on the degree of steering required, or may have ramifications on the accuracy of the sensors referred to above. A further input of use in the determining the angle of steer of the one or more rear wheels is the angle of steer of the front wheels, the controller may, for example, calculate a steering angle on the rear wheels when the front wheels steer angle is over a specified value, particularly at very low speed, where lateral forces may not be that high, for example, in an unloaded vehicle.

It will also be understood that there may be transient effects that cause lateral forces to be effected on the rear wheels, such as going over potholes and other unevennesses of the road surfaces, where it is undesirable for rear wheel steering to be effected. The controller might thus require the presence of the lateral force to be present at more than one time. It will be understood that the controller, when operational, will thus preferably effect the calculation at multiple times temporally spaced. The time sampling may be desired to be dependent on the speed of the vehicle but is determined to be adequately responsive to probably changes in conditions.

It may also be preferred that there be a sensor to determine the angle of the one or more rear wheels whereby the output is provided to the controller to take into account in its calculation of whether to further move the steerable rear wheels.

It may be desired that there be override means which signals to the controller not to activate the rear steering means, or alternatively, which turns off the rear steering means in another fashion, reverting the rear wheels to a longitudinal direction, that is, being in line with the trailer.

It may also be desired that the rear wheel steering is not active in other states such as when reversing the vehicle, and a further input might be provided to the controller when reverse gears are engaged, alternatively the rear steering is otherwise inactivated on engagement of reverse gear.

Shown in the drawings is a semi trailer (1) in its common configuration, with a tractor (2) and trailer (3). The tractor includes a cabin (4) in which a driver sits and drives the semitrailer. The trailer is pivotable connected by a so-called fifth wheel (5), a turntable like arrangement, to the tractor to pivot about pivot axis (6). The tractor has a single forward axle about which two steerable forward wheels (7,8) are rotatable. Two further forward axles support the tractor on eight wheels, a pair (one shown at 10) each on opposing sides of each axle, each pair of wheels is positioned approximately equidistant about pivot axis (6).

Two spaced apart rear axles support the rear of the trailer, one eight wheels a pair (one shown at 11) of each on opposite sides of each axle. The two rear axles are mounted to a bogey shown figuratively at (12), the bogey is pivotably relative to the trailer about bogey axis (13).

On turning the front wheels, as shown in figure 3, the tractor pivots relative to the trailer, as in conventional trailers, and in addition, the rear bogey rotates relative to the trailer so that both rear axles are moved to a steer angle α taken relative to a longitudinal axis of the trailer shown at (14).

Various features of the invention have been particularly shown and described in connection with the exemplified embodiments of the invention, however, it must be understood that these particular arrangements merely illustrate and that the invention is not limited thereto and can include various modifications falling within the spirit and scope of the invention.