GB2420325A

GB2420325A - Self-steering axles for vehicles

Info

Publication number: GB2420325A
Application number: GB0425333A
Authority: GB
Inventors: Chi-Hang Ryan Shek; Mark Glazier
Original assignee: Glide Rite Products Ltd
Current assignee: Glide Rite Products Ltd
Priority date: 2004-11-17
Filing date: 2004-11-17
Publication date: 2006-05-24
Also published as: WO2006054069A1; CN1651294A; GB0425333D0

Abstract

There is described a self-steering axle for a vehicle, and a control apparatus for the self-steering axle which provides for locking the wheels of the self-steering axle in a central position while the vehicle is being reversed, and freeing the wheels 6 for castoring self-steering action when the vehicle is travelling forward. In modifications of the control apparatus, active steering of the self-steering axle during forward and reverse motion of the vehicle is provided. The invention further provides a positionable linear actuator, used to control the steering action of the self-steering axle and comprising opposed airbags 10a, 10b operating on an actuating element 9 connectable to a track rod 6b of the steering linkage.

Description

SELF-STEERING AXLE

The present invention relates to self-steering axles for vehicles, and is particularly concerned with axles for multi-axle commercial vehicles such as trucks and buses.

In a conventional multi-axle truck, at least one steerable front axle is provided, together with one or more fixed (i.e. non-steerable) rear axles, some or all of which may be driven. A particular difficulty with such a layout of the vehicle wheels is that when the vehicle is performing a turn, lateral movement of the fixed wheels across the ground occurs, in a phenomenon known as "scrubbing". This leads to excessive wear on the rear tyres, decreasing their useful life and increasing the running costs for the vehicle.

One approach adopted in the prior art to reduce tyre wear due to scrubbing in multi-axle vehicles is the provision of a "lift axle". In this approach, one of the fixed axles is mounted to the vehicle chassis such that, when the vehicle is only lightly loaded, the axle may be lifted to bring its wheels clear of the ground. This lifting of the axle not only eliminates tyre wear from the tyres of the lift axle while they are not in contact with the ground, but by leaving only one fixed axle in ground contact also eliminates "scrubbing" during turning of the vehicle and thus reduces wear on the tyres of the fixed axle which remains in contact with the ground. However, when the vehicle is heavily loaded and the lift axle is lowered to distribute the load over all axles, the problem of scrubbing still arises when the vehicle is turned.

A further approach to the reduction of tyre wear has

been the introduction of self-steering axles in

vehicles with multiple rear axles. Such axles are typically provided as one or more of a number of rear axles for a vehicle, in conjunction with one fixed axle. The self-steering axle comprises castoring wheels which follow the direction of travel of the vehicle such that, when the front steering axle is used to make a turn, the castoring effect causes the wheels of the self-steering axles to follow the direction of the turn and thus eliminate scrubbing of the wheels of the two (or more) rear axles. A disadvantage of the "self-steering" rear axle is that, when the vehicle is reversed, the castoring angles of the wheels of the self-steering axle are such that the wheels of the self-steering axle tend to turn to the fullest extent of their travel in one or other direction, due to the instability of the reversed castoring effect. This disadvantage may be partially overcome by mounting the self-steering axle as a lift axle, and lifting the self-steering axle out of contact with the ground while the vehicle is reversing. This is however only available if the 1.0 vehicle is lightly loaded since otherwise lifting the axle may increase the loadings on the axles remaining on the ground beyond permitted limits.

The present invention seeks to provide in one aspect a control system for a self-steering axle which allows the self-steering effect to be utilised during forward motion of the vehicle, but urges the self-steering axle to align its wheels in the fore-and-aft direction of the vehicle when the vehicle is reversing.

It is a further objective of the invention to provide a control system for a self-steering axle which operates actively on the self-steering axle to cause steering movements which correspond to the steering movements of a steerable axle of the vehicle, when running in the forward direction and/or in reverse.

According to one aspect of the present invention, there is provided a control apparatus for a self- steering axle of a vehicle, the apparatus comprising a fluid actuator having first and second opposed fluid elements each operable to urge a self-steering axle toward a central position, valve means to supply fluid pressure to the fluid elements at a first, higher, pressure or a second, lower pressure, sensor means to detect reversing movement or intended reversing movement of the vehicle, and control means responsive to the sensor means and operable to control the valve means to provide the first higher fluid pressure to the fluid elements when reversing or intended reversing movement is detected and to provide the second lower fluid pressure to the fluid elements when reversing is not detected. Preferably the sensor means detects selection of reverse gear by the driver of the vehicle. Most preferably the sensor detects the position of the gear selector lever in the position for selecting reverse gear. The valve means may comprise a variable reducing valve, or a regulator valve operable to provide one of a plurality of reference pressures to the fluid elements. The fluid elements are preferably air bags, and most preferably act in opposition on a component in the steering linkage of the self-steering axle.

A further aspect of the invention provides an active control apparatus for a self-steering axle of a vehicle, the apparatus comprising a steering angle sensor for detecting the position of the steered wheels of the vehicle, a first fluid actuator operable to urge the self-steering axle toward one end of its steering movement, a second fluid actuator operable to urge the self-steering axle toward the other end of its steering movement, valve means for supplying pressurised fluid independently to the first and second fluid actuators, and control means operable to control the valve means on the basis of the output of the steering angle sensor to provide different fluid pressures to the first and second fluid actuators in dependence on the sensed steering angle.

The fluid actuators are preferably air bags, and may be independently provided with continuously variably fluid pressures from respective regulator valves.

Alternatively a regulator valve arrangement may provide a number of different fluid pressures, and the control means may be operable to select a first fluid pressure for the first actuator and a second fluid pressure for the second fluid actuator in dependence on the sensed steering angle.

In a modification of this aspect, the vehicle's speed may be sensed and fluid pressures applied to the first and second actuators in dependence on the sensed speed and the sensed steering angle. The magnitude of one of the fluid pressures may be selected on the basis of the vehicle speed, and the difference between the fluid pressures may be selected to correspond with the sensed steering angle. The control means may be operable to provide different pressures only at speeds below a threshold speed, with the actuators being provided with equal pressures when the vehicle speed exceeds the threshold. In a further development, the control means may be arranged to provide different pressures only at forward speeds below a threshold speed, with the actuators being provided with equal pressures when the vehicle's forward speed exceeds the threshold and when the vehicle is reversing A further aspect of the invention comprises a selfsteering axle assembly for a vehicle, comprising a pair of stub axles pivotally mounted for steering movement from a first end position to a second end position, a track rod joining the stub axles, a first fluid actuator operable to urge the stub axles away from the first end of the steering movement, a second fluid actuator operable to urge the stub axles away from the second end of the steering movement, valve means for providing fluid pressure to the fluid actuators. The fluid actuators are preferably air bags, most preferably mounted in Opposition in a housing and acting on an operating element fixed in relation to the track rod. The axle assembly may further comprise a sensor for detecting the direction of rotation of a wheel mounted to the axle, and a control means responsive to the sensor output to operate the valve means to provide high or low pressure to the fluid actuators depending on the rotation direction.

Yet another aspect of the invention provides a vehicle having such a selfsteering axle and a control apparatus therefor, the control apparatus including a sensor to detect selection of reverse gear, a source of fluid pressure, and control means for providing high pressure fluid to the actuators when reverse gear is selected.

Preferably, the vehicle further comprises a sensor for detecting a steering angle of a steerable axle of the vehicle, and the control apparatus is operable to apply different fluid pressures to the respective fluid elements of the actuator in order to produce steering movements in the self-steering axle which correspond to steering movements of the steerable axle. More preferably, the vehicle further comprises a sensor for detecting the vehicle's speed, and the degree to which the self-steering axle is moved in correspondence to a steering movement of the steerable axle is made dependent on the vehicle's speed as sensed by the speed sensor.

A further aspect of the invention provides a kit for converting a selfsteering axle of a vehicle to active control, the kit comprising a fluid actuator comprising a housing mountable to a vehicle and having an operating element connectable to the steering linkage of the selfsteering axle, the housing including opposed fluid elements operable to urge the operating element in opposite directions towards a median position, valve elements to selectively provide fluid pressure to the fluid elements, and control means responsive to a control input to operate the valve elements to provide a higher fluid pressure to the fluid elements when the control input is present.

Preferably the control input corresponds to the selection of reverse gear by the vehicle's driver, and may be sensed by a switch mounted to the vehicle's gearbox. The kit may further comprise a sensor mountable to the vehicle for detecting a steering angle of a steerable axle of the vehicle, the control apparatus being operable to apply different fluid pressures to the respective fluid elements of the actuator in order to produce steering movements in the self-steering axle which correspond to steering movements of the steerable axle.

A further aspect of the invention comprises a method of controlling a self-steering axle of a vehicle, the method comprising the steps of sensing the direction of movement or of intended movement of the vehicle, and operating an actuator to urge the self-steering axle toward a position in which the wheels are aligned with the longitudinal axis of the vehicle, wherein the force urging the self-steering axle toward the central position is dependent on the direction of travel or of intended travel.

A further aspect of the invention provides a positionable fluid actuator comprising an elongate housing, an output element extending out of the housing, a first fluid element situated within the housing and operable to urge the output element in a first longitudinal direction relative to the housing, and a second fluid element positioned in the housing and operable to urge the output element in a second longitudinal direction opposite from the first direction, the first and second fluid elements comprising air bags.

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 is a perspective view of a six-wheel tractor unit; Figure 2 is a view similar to Figure 1, showing the tractor unit with one axle lifted; Figure 3 is a schematic diagram showing a steering control system for a vehicle; Figures 3A and 3B show schematically two alternative valve systems for the control system of Figure 3; Figure 4 is a perspective view of a steering actuator; and Figure 5 is a detailed view of an example of the mounting of a lifting self-steering axle to a vehicle chassis.

Referring now to Figure 1, there is seen a tractor i.mit 1 having three axles. The front axle has mounted to it the front wheels 2, which are steered from the driver's cab 3 using a steering wheel 4 to control turning of the vehicle.

At the rear part of the tractor, there are two further axles. The rearmost axle is a fixed, non-steering axle carrying the rear wheels 5. Immediately in front of the rear wheels 5 are a further set of wheels 6, carried on a lifting self-steering axle.

In the tractor unit shown, a fifth wheel coupling 7 provides for the attachment of a load-carrying trailer. The invention is also applicable in vehicles such as buses and trucks which carry loads internally rather than by drawing trailers.

Figure 3 illustrates schematically the layout of the running gear of the tractor. The front wheels 2 are supported on stub axles, attached to steering arms 2a which are connected together by a track rod 2b. The front wheels 2 are steered by a conventional steering linkage from the steering wheel 4 in the driver's cab.

The rear wheels 5 are mounted to a fixed axle 5a, and are driven from a differential 5b powered by a propeller shaft 5c linked to the engine (not shown).

The wheels 6 are mounted on stub axles attached to respective steering arms 6a, which are connected by a track rod 6b so that the wheels 6 maintain proper alignment during self-steering. A steering actuator 7, seen in more detail in Figure 4, comprises an elongate housing 8, and an actuator element 9. The actuator element 9 is movable in the longitudinal direction of the housing 8, by means of first and second air bags l0a, lOb which are supplied with air pressure from a control unit 11 through lines 12 and 13. The housing 8 is mounted to the chassis of the vehicle, and the actuator element 9 is fixed to the track rod 6b of the self-steering axle. The actuator element may form part of the track rod 6b, or may be linked to the respective steering arms 6a by track rod halves or portions.

The control unit 11 controls the supply of pressurised fluid to the air bags ba and lOb by means of a valve system 14. In the valve system seen in Figure 3A, high pressure air from a reservoir 14a is led to a variable reducing valve 14b which is controlled by a signal from the control unit 11 to output air at a selected pressure. The output air is led by ducts 12 and 13 to the air bags 10 and lob, supplying both air bags with the same pressure.

In the alternative valve system seen in Figure 3B, high pressure air from a reservoir l4a is led to a pair of variable reducing valves 14c and l4d, each of which is controlled by a respective signal from the control unit 11 to output air at a respective selected pressure. The output air from valve l4c is led by duct 12 to the air bag bOa, and output air from valve 14d is led by duct 13 to the air bag lob. The air bags may be supplied with air at different or the same pressures, as determined by the control unit 11.

With the self-steering axle arrangement shown in Figure 3, when the vehicle is moving forward along a straight path, the castoring action built into the steering geometry of the self-steering axle maintains the wheels 6 in alignment with the wheels 5. When the driver turns the steering wheel and the vehicle enters a curved path, this castoring action turns the wheels 6 to a lesser extent than the front wheels 2 are turned, in order to avoid scrubbing of the rear wheels and 6 during cornering.

In a first embodiment of the invention, the valve system of Figure 3A is used to supply a low fluid pressure of, for example, two bar to the air bags lOa and lOb simultaneously, while the vehicle is moving forward. This low pressure in both of the air bags urges the actuator element 9 to a central position, thus tending to maintain the wheels 6 in fore-and-aft alignment with the rear wheels 5 when the vehicle is travelling in a straight line. The actuator also serves to damp oscillations in the steering linkage of the self-steering axle.

When the vehicle enters a curve, the castoring effect on the wheels 6 urges the track rod 6b laterally, against the biasing action of the air bags lOa and lob. When the air bags are pressurised at the low pressure, the force exerted by the air bags is insufficient to overcome that exerted by the castoring action and the wheels 6 will turn to follow their direction of travel, to avoid scrubbing.

When the vehicle is travelling in reverse, however, the castoring effect exerted on the wheels 6 is such as to cause them to turn to one or other extreme of their travel, even if the vehicle is being reversed in a straight line. In order to overcome this difficulty, a sensor 15 is provided on the gear selector gate iSa to detect when the driver engages reverse gear by selecting the "R" position in the gate with the gear selector lever 15b. When the control unit 11 detects the selection of reverse gear, the valve assembly 14 is operated to increase the pressure in both air bags l0a and lob to a high pressure of, for example, eight bar. This produces a much increased biasing action to return the track rod 6b to its central position. The vehicle can then be reversed without the castoring effect of the wheels 6 overcoming the bias of the air bags 10, and the wheels 6 are then maintained in fore-and-aft alignment with the wheels 5 throughout the reversing operation.

Since reversing is carried out generally over short distances and at low speeds, the wear on the tyres due to scrubbing is negligible.

In the first embodiment, the two air bags of the self- steering axle are operated at equal pressures, and are operated at low pressure (of typically 2 bar, but possibly from 1 to 5 bar) when the vehicle is travelling forward and at high pressure (of typically 8 bar, but possibly from 6 to 10 bar) when the vehicle is travelling in reverse. This arrangement enables the wheels of the self-steering axle to be locked in the "straight" Position during reversing, against the castoring effect which tends to turn the self-steering wheels to one or other extreme of their travel when the vehicle is reversed. The magnitudes of the "high" and "low" pressures applied to the air bags 10 may be made dependant on the loading state of the vehicle, for example by sensing pressure in a suspension air bag supporting the vehicle chassis, with higher pressures being used when the vehicle is heavily loaded as compared to the pressures used when the vehicle is unladen or lightly loaded.

In a second embodiment of the invention, also using the valve system of Figure 3A, the pressures in the air bags lOa and lOb may be maintained at the low level of from approximately 1 to 3 bar when the vehicle is travelling forward, but may be maintained at an intermediate level, for example from three to seven bar, when the vehicle is reversing in order to allow some steering movement of the wheels 6.

In a further embodiment of the invention, a steering angle sensor 20 may be provided to give a signal dependent on the angle to which the front wheels 2 are turned. The sensor output is provided to the control until 11, which then controls a valve assembly 14 such as that shown in Figure 3B to provide differential pressures to the air bags ba and lob in order to urge the track rod 6b in the appropriate direction to set the wheels 6 in correct alignment for reversing along a curved path without scrubbing. In the embodiment shown, the wheels 6 of the self-steering axle are turned in the same sense as the steered wheels 2. If the self-steering rear axle is mounted behind the fixed rear axle, then differential pressures may be applied to turn the wheels 6 of the self-steering axle in the opposite sense from the steered wheels 2.

In a further embodiment, the action of the self- steering axle when the vehicle is moving forward may be assisted by using the steering angle sensor 20 to detect turning movement of the front wheels 2, and by arranging the control unit 11 to control the valve assembly 14 to provide differential pressures to the air bags ba and lOb in order to set the wheels 6 in the correct alignment to execute a turn, depending on the degree of turning of the front wheels 2. A further refinement of this embodiment provides a speed sensor 21 to detect the speed of movement of the vehicle, and provides the speed input to the control unit 11 so that the outputs to the air bags lOa and lOb may be made different at low speeds from the outputs at high speed. For example, at low forward speeds, the air bags lOa and lOb may be provided with different pressures in order to actively steer the wheels 6 in unison with the turning of the steering wheels 2. At higher speeds, for example above 50km/h, the control unit 11 may provide equal pressures to the air bags l0a and lob in order that the steering action of the self-steering axle 6 should revert to a passive self-steering action driven by the castoring effect of the wheels 6.

In a further alternative, the wheels of the self- steering axle may be actively steered when the vehicle moves forward, by applying different pressures to the air bags 10 as described above, and may be held in their "straight" position for reversing by applying the same "high" pressure to both air bags 10.

Any of the above embodiments may be provided on a lifting self-steering axle, which is lifted off the ground when the vehicle is lightly loaded, and is placed in contact with the ground to bear a part of the weight of the vehicle when the vehicle is heavily laden. A sensor (not shown) may determine when the axle is raised and lowered, and may provide a signal to the control unit 11 so that control of the air bags ba and lob is only effected when the axle is lowered and in contact with the ground. When the axle is lifted, the control unit 11 may provide equal low pressures to the air bags 10 in order to maintain the wheels 6 in a fore-and-aft alignment. Alternatively, the self-steering axle may not be of the liftable type and may remain permanently in ground contact.

The control unit may comprise a microprocessor programmed with a table of pressure values to be applied to the respective air bags ba, lob in dependence on sensor inputs such as from the speed sensor, steering angle sensor, an axle lift detector, suspension air bags and a reverse gear selection detector, and may provide control signals to the variable reducing valves 14c and l4d to apply the appropriate pressures to the air bags lOa and lob.

Alternatively, the control means may calculate the required fluid pressure for each airbag on the basis of steering angle, sensed direction of travel, speed and vehicle laden weight.

In an alternative, manual, application the driver may be provided with a dashboard switch to enable selection of "low" or "high" pressure in both airbags 10, using the valve system of Figure 3A. The driver will then select "low" pressure when the self-steering action of the axle is required, and "high" pressure when the self-steering action is to be disabled, such as when reversing. The dashboard switch may be provided as an alternative to the control unit 11 and sensor 15 or in addition thereto or as an override facility.

Figure 5 illustrates a particular mounting arrangement for a lifting and self-steering axle. In Figure 5, there are shown two parallel chassis beams 30 which extend longitudinally of the vehicle. A mounting bracket 31 is attached to each chassis beam 30 (only one beam shown) to support a self-steering axle 32 beneath the vehicle chassis. The self-steering axle 32 comprises a fixed axle beam 33 to which pivotable stub axles are mounted. The stub axles are attached to steering arms 34 which are connected by a track rod 35. The axle beam 33 is held in position relative to the chassis by a swinging arm 36 which is generally "L" shaped and has one limb extending forwardly from a pivot point 37 and another limb extending upwardly from the pivot point. A lifting air bag 38 is positioned between the upwardly-extending limb and the mounting bracket 31, such that inflation of the lifting air bag 38 will cause the limb 36 to swing upwardly toward the chassis beam 30, lifting the self- steering axle 32 clear of the ground.

When the axle is placed in contact with the ground by deflation of the lifting air bag 38, a suspension air bag 39 acts directly between the chassis beam 30 and the axle 33 to support the weight of the vehicle on the axle 33.

Mounted at a point intermediate the length of the track rod 35 is an actuating element 9, which extends into the housing 8 of the steering actuator between two air bags lOa and lob contained within the housing.

The steering actuator is shown in greater detail in Figure 4, and has been described above. An actuating element 9 may be fitted to a conventional self- steering axle to convert the axle to use the control system of the present invention by removing a section of the track rod equal in length to the length of the actuating element, and attaching the cut ends of the track rod parts to the actuator element, and mounting the housing 8 of the actuator to the vehicle.

The actuating element 9 may be used as a positionable linear actuator in other devices than self-steering axles. The position of the actuating element 9 is controllable by regulating the pressures applied to the respective air bags, displacement of the element 9 from the median position corresponding to the pressure differential between the air bags, and the "firmness" of the positioning corresponding to the magnitudes of the pressures.

Claims

Claims: 1. A control apparatus for a self-steering axle of a vehicle, the

apparatus comprising a fluid actuator having first and second opposed fluid elements each operable to urge a self-steering axle toward a central position, valve means to supply fluid pressure to the fluid elements at a first, higher, pressure or a second, lower pressure, sensor means to detect reversing movement or intended reversing movement of the vehicle, and control means responsive to the sensor means and operable to control the valve means to provide the first higher fluid pressure to the fluid elements when reversing or intended reversing movement is detected and to provide the second lower fluid pressure to the fluid elements when reversing is not detected.
2. A control apparatus according to claim 1, wherein the sensor means detects selection of reverse gear by the driver of the vehicle.
3. A control apparatus according to claim 2, wherein the sensor detects the position of a gear selector lever of the vehicle in the position for selecting reverse gear.
4. A control apparatus according to any preceding claim, wherein the valve means comprises a variable reducing valve.
5. A control apparatus according to any preceding claim, wherein the valve means comprises a regulator valve operable to provide one of a plurality of reference pressures.
6. A control apparatus according to any preceding claim, wherein the fluid elements are air bags.
7. A control apparatus according to claim 6, wherein the air bags act in opposition on a component in the steering linkage of the self-steering axle.
8. An active control apparatus for a self-steering axle of a vehicle, the apparatus comprising a steering angle sensor for detecting the position of steered wheels of the vehicle, a first fluid actuator operable to urge the self-steering axle toward one end of its steering movement, a second fluid actuator operable to urge the self-steering axle toward the other end of its steering movement, valve means for supplying pressurised fluid independently to the first and second fluid actuators, and control means operable to control the valve means on the basis of the output of the steering angle sensor to provide different fluid pressures to the first and second fluid actuators in dependence on the sensed steering angle.
9. An active control apparatus according to claim 8, wherein the fluid actuators are air bags.
10. An active control apparatus according to claim 8 or claim 9, wherein the fluid actuators may be independently provided with continuously variable fluid pressures from respective regulator valves.
11. An active control apparatus according to claim 8 or claim 9, wherein the fluid actuators may be independently provided with a selected one of a number of different fluid pressures, and the control means is operable to select a first fluid pressure for the first actuator and a second fluid pressure for the second fluid actuator in dependence on the sensed steering angle.
12. An active control apparatus according to claim 11, further comprising a sensor for sensing the speed of the vehicle, and wherein the control means is operable to select and apply fluid pressures to the first and second actuators in dependence on the sensed speed and the sensed steering angle.
13. An active control apparatus according to claim 12, wherein control means is operable to select the magnitude of one of the fluid pressures on the basis of the vehicle speed, and to select the difference between the fluid pressures to correspond with the sensed steering angle.
14. An active control apparatus according to any of claim 12 or claim 13, wherein the control means is operable to provide different pressures to the respective actuators only at vehicle speeds below a predetermined threshold speed, and is operable to provide equal pressures to the respective actuators when the vehicle speed exceeds the threshold.
15. An active control apparatus according to any of claim 12 or claim 13, wherein the control means is operable to provide different pressures to the respective actuators only at forward speeds below a threshold speed, and is operable to provide equal pressures to the respective actuators when the vehicle's forward speed exceeds the threshold and when the vehicle is reversing
16. A self-steering axle assembly for a vehicle, comprising a pair of stub axles pivotally mounted for steering movement from a first end position to a second end position, a track rod linking the stub axles, a first fluid actuator operable to urge the stub axles away from the first end of the steering movement, a second fluid actuator operable to urge the stub axles away from the second end of the steering movement.
17. A self-steering axle assembly according to claim 16, further comprising valve means for providing fluid pressure to the fluid actuators.
18. A self-steering axle assembly according to claim 16 or claim 17, wherein the fluid actuators are air bags.
19. A self-steering axle assembly according to claim 18, wherein the air bags are mounted in opposition in a housing and act on an operating element fixed in relation to the track rod.
20. A self-steering axle assembly according to any of claims 16 to 19, wherein the axle assembly further comprises a sensor for detecting the direction of rotation of a wheel mounted to the axle, and a control means responsive to the sensor output to operate the valve means to provide high or low pressure to the fluid actuators depending on the rotation direction.
21. A vehicle having a self-steering axle according to any of claims 16 to 20, and further comprising a control apparatus including a sensor to detect selection of reverse gear, a source of fluid pressure, and control means for providing high pressure fluid to the actuators when reverse gear is selected.
22. A vehicle according to claim 21, further comprising a sensor for detecting a steering angle of a steerable axle of the vehicle, and wherein the control apparatus is operable to apply different fluid pressures to the respective fluid actuators in order to produce steering movements in the self-steering axle which correspond to steering movements of the steerable axle.
23. A vehicle according to claim 22, further comprising a sensor for detecting the vehicle's speed, and wherein control means is responsive to the detected speed and controls the valve means so that the degree to which the self-steering axle is moved in correspondence to a steering movement of the steerable axle is made dependent on the vehicle's speed.
24. A kit for converting a self-steering axle of a vehicle to active control, the kit comprising a fluid actuator comprising a housing mountable to a vehicle and having an operating element connectable to the steering linkage of the self-steering axle, the housing including opposed fluid elements operable to urge the operating element in opposite directions towards a median position.
25. A kit according to claim 24, wherein the fluid elements are air bags.
26. A kit according to claim 24 or claim 25, further comprising valve elements to selectively provide fluid pressure to the fluid elements, and control means responsive to a control input to operate the valve elements to provide a higher fluid pressure to the fluid elements when the control input is present.
27. A kit according to claim 26, further comprising a sensor mountable to the vehicle for detecting a steering angle of a steerable axle of the vehicle, the control means being operable to apply different fluid pressures to the respective fluid elements of the actuator in order to produce steering movements in the self-steering axle which correspond to steering movements of the steerable axle.
28. A method of controlling a self-steering axle of a vehicle, the method comprising the steps of sensing the direction of movement or of intended movement of the vehicle, and operating an actuator to urge the selfsteering axle toward a position in which the wheels are aligned with the longitudinal axis of the vehicle, wherein the force urging the selfsteering axle toward the central position is dependent on the direction of travel or of intended travel.
29. A positionable fluid actuator comprising an elongate housing, an output element extending out of the housing, a first fluid element situated within the housing and operable to urge the output element in a first longitudinal direction relative to the housing, and a second fluid element positioned in the housing and operable to urge the output element in a second longitudinal direction opposite from the first direction, the first and second fluid elements comprising air bags.
30. A self-steering axle assembly substantially as herein described with reference to Figure 3, Figure 3A, Figure 3B or Figure 5 of the accompanying drawings.
31. A vehicle substantially as herein described with reference to Figure 1 or Figure 3 of the accompanying drawings.
32. A fluid actuator substantially as herein described with reference to Figure 4 of the accompanying drawings.
33. A method of controlling a self-steering axle substantially as herein described.