AU2007247777A1 - Operation of self-steering axles on semi-trailers - Google Patents

Description

WO 2007/128073 PCT/AU2007/000611 OPERATION OF SELF-STEERING AXLES ON SEMI-TRAILERS 5 Field of the Invention This invention concerns the use of self-steering axles on semi-trailers of heavy goods vehicles used for road transport. It is particularly applicable to vehicles such as prime mover and semi-trailer combinations, and to heavy goods vehicle applications that 10 feature multiple semi-trailers, including medium- and long-combination vehicles; more commonly referred to as multi-combination vehicles or road trains. In this specification the following explanations apply to certain terms: 15 a) A "semi-trailer" is a towed vehicle unit of a heavy goods combination vehicle whose means of attachment to the preceding vehicle unit (a prime mover, another semi-trailer, or a converter dolly) results in some of its vertical load being imposed on said preceding unit through a tow coupling and whose rear is supported by a single axle or multiple axle bogie (tandem, tri, quad, 20 etc.) located towards the rear. b) A "converter dolly" (also called a "dolly") is a towed vehicle unit of a heavy goods combination vehicle whose tow coupling to the preceding vehicle unit (a rigid truck, a prime mover or another semi-trailer) results in little or 25 none of its vertical load being imposed on said preceding vehicle unit through the tow coupling and whose rear is supported by a single axle or multiple axle bogie (tandem, tri, quad, etc.) located at or near to the centre of the main load carrying area The main load bearing or carrying area of a converter dolly usually incorporates a tow coupling device so that the dolly may be mounted 30 beneath the front of a semi-trailer body, thereby converting the semi-trailer into a trailer with two axle groups of which the front axle group is steered by connection to the preceding vehicle unit. Internationally, a converter dolly is WO 2007/128073 PCT/AU2007/000611 2 also known as a pivoting bogie, a dolly truck or a special track. A converter dolly may also be adapted to carry goods. c) A "trailer" in a heavy goods combination vehicle is a towed vehicle unit 5 that can be either a semi-trailer, or a combination of a converter dolly plus a semi-trailer, or a converter dolly adapted to carry goods. d) A "motor vehicle" in a heavy goods combination vehicle is a vehicle unit which is motorised for propulsion and is built to tow one or more trailers. 10 One of the trailers may be a powered trailer. A motor vehicle can be a prime mover or a rigid truck. e) A "powered trailer" in a heavy goods combination vehicle is a trailer having a largely self-contained means of generating.propulsion through one or more 15 of its wheels. In a combination vehicle one or more of the trailers may be a powered trailer. f) A "prime mover" is a vehicle unit of a heavy goods combination vehicle built to tow a semi-trailer or a converter dolly. A prime mover may also carry 20 a load. g) A "rigid truck" is a non-articulated motor vehicle built to carry a load and, in a heavy goods combination vehicle, tow a trailer. 25 h) A "vehicle unit" is either a trailer, a semi-trailer, a converter dolly, a motor vehicle, or a powered trailer. Thus a combination vehicle has a plurality of such vehicle units connected by an articulated coupling between each adjacent vehicle unit. 30 i) A "heavy goods combination vehicle" is a heavy goods or road transport vehicle with at least one articulation point.

WO 2007/128073 PCT/AU2007/000611 3 Background to the Invention Self-tracking axles are well known for use on heavy goods vehicles, such as prime mover and semi-trailer combinations having semi-trailer design axle loads in the 5 order of 10 to 27t. Self-tracking axles are also known as self-steering axles, self-steer axles and castor-steering axles. They operate by the wheels being allowed to alter their alignment, by means of a castor action, in response to a turning movement of the vehicle. 10 Transport safety regulations in most countries require the use of self-tracking axles in many situations in order to reduce tyre scrubbing and consequent damage to the pavement. Use of a self-tracking axle allows a spreading of the vehicle weight among more axles without the limitations of manoeuvrability that would be encountered by using a conventional-non-steered or otherwise fixed axle arrangement. 15 However, the use of self-tracking axles does cause deterioration in the stability and handling of.vehicles in turning manoeuvres performed at medium to high road speeds, particularly speeds above 50km/h, and especially above 80km/h. It is known that there is a significant degradation of high-speed offiracking where the towed vehicle 20 unit tracks outboard of the towing vehicle unit during high-speed manoeuvres. The High-Speed Offtracking (HSO) phenomenon occurs in transient manoeuvres, such as a lane change (where it is more commonly referred to as high-speed transient offtracking), and during steady turn manoeuvres, where the vehicle follows a constant radius path at a constant steady speed (where it is more commonly referred to as high 25 speed steady-state offtracking). But sophisticated analysis and mathematical modelling by the present applicant has now revealed that there is also a substantial degradation in the rollover stability of a semi-trailer incorporating one or more self tracking axles, and that in many high speed turning manoeuvres the rollover stability, as reflected in the Static Rollover Threshold (SRT) performance measure, may be 30 exceeded before the allowable HSO limits'are reached.

WO 2007/128073 PCT/AU2007/000611 4 An aim of the present invention is to provide a controlled self-tracking system. whereby in a turning manoeuvre both the SRT and the HSO of a vehicle having one or more self-tracking axles may be improved. 5 Summary of the Invention In one aspect the invention provides a method of operating a semi-trailer being towed as part of a heavy goods vehicle by a motor vehicle, where the semi-trailer has at least one self-tracking axle which has road wheels and castor-steering, said method 10 comprising: (a) at road speeds above a first predetermined speed in the forward direction, applying a restoring moment to the castor-steering action of one or more of said at least one self-tracking axle to cause alignment of its wheels, when measured relative to a reference alignment the wheels would take when travelling along a is straight path on a flat level surface, to be moved away from an alignment which would be adopted if the castor steering action was unrestrained and toward the reference alignment; and (b) at road speeds at least above a second predetermined speed in the forward direction, actively controlling the magnitude of the restoring moment so that said 20 alignment of the wheels is dependent upon the road speed and upon a sideways acceleration measurement being made on the heavy goods vehicle. Said first and second predetermined speeds may be the same, or said second predetermined speed may be greater than said first predetermined speed. 25 For some road speeds above said first predetermined speed, said restoring moment may be applied by holding or locking alignment of tjhe wheels into a straight-ahead position. 30 Said application of the restoring moment may be achieved by applying a controlled force to the wheels to cause alignment of the wheels to be moved away from an alignment which would be adopted if the castor steering action was unrestrained.

WO 2007/128073 PCT/AU2007/000611 5 For some road speeds above said second predetermined speed and some sideways or lateral acceleration levels, a controlled force may be applied to cause the steer angle of the wheels (steer angle being taken to mean the alignment angle of the wheels 5 measured relative to the alignment the wheels would take travelling along a straight path on a flat level surface) to be decreased below that steer angle, or taken or held over centre and the steer angle increased in the other steer direction, which would be adopted if the castor steering action were unrestrained, but such a controlled force application may be made dependent upon said lateral acceleration measurement 10 falling within a predetermined range. At low road speeds in the forward direction, the semi-trailer may be towed with the castor-steering action of said at least one self-tracking axle enabled and unrestrained. Alternatively, at road speeds below a third predetermined speed in the forward 15 direction, said restoring moment may be applied to an extent sufficient to cause the alignment of the wheels of said at least one self-tracking axle to be aligned at a steer angle which is greater than the alignment which would be adopted if the castor steering action were unrestrained. 20 Said measurement of lateral acceleration may be made on the semi-trailer, but is preferably made on said motor vehicle. In multi-combination vehicles said measurement of lateral acceleration may be made on the vehicle unit immediately preceding the semi-trailer on which the self-tracking axle to be controlled is installed. 25 Said restoring moment may be controlled in order to achieve a predetermined size of the moment, or may be controlled in order to achieve a predetermined wheel alignment or amount of steer angle. The magnitude of said restoring moment, or steer angle directly, may be controlled in 30 response to a yaw rate measurement being made on said semi-trailer, and/or on other preceding vehicle units. The magnitude of said restoring moment, or steer angle directly, may be controlled in response-to measurements being made of wheel speeds WO 2007/128073 PCT/AU2007/000611 6 on said semi-trailer and/or said motor vehicle and/or preceding vehicle unit. The magnitude of said restoring moment, or steer angle directly, may be controlled in response to a measurement of a lateral force being applied at a towing coupling between the semi-trailer and the preceding vehicle unit, and/or between other 5 preceding vehicle units. A greater braking force may be applied to at least some of the wheels on said semi trailer which are on that side of the semi-trailer towards which the heavy goods vehicle is travelling. 10 Brief Description of the Drawings In order that the invention may be more fully understood there will now be described, by way of example only, preferred embodiments and other elements of the invention 15 with reference to the accompanying drawings where: Figure 1 is a view of a prime mover coupled to a semi-trailer to which one embodiment of the present invention is applied; Figure 2 is a rear view of the vehicle in Figure 1 shown in the process of rounding a curve in a road at relatively high speed; 20 Figure 3 is a plan view showing the vehicle further into the curve than the position shown in Figure 2; Figure 4 is a layout schematic of the quad axle group at the rear of the semi trailer in Figure 1; Figure 5 is a view of the quad axle group in Figure 4 but shown with the wheels 25 on the self-tracking axles being steered towards the right; Figure 6 is a view of a triple axle group incorporating a further embodiment of the present invention; Figure 7 is a view of a quad axle group incorporating a further embodiment of the present invention; 30 Figures 8, 9 and 10 are diagrams illustrating forces acting during certain manoeuvres on a semi-trailer incorporating the quad axle group shown in Figure 7; WO 2007/128073 PCT/AU2007/000611 7 Figure 11 is a plan view of a self-steer wheel shown (in dotted lines) on the left hand side of the vehicle rounding a right-hand curve in a road at relatively high speed as in Figure 2, superimposed on a view (in solid lines) of the wheel in its straight-ahead position; and 5 Figure 12 is a plan view of a self-steer wheel on the left-hand side of the vehicle rounding a curve in a road at relatively high speed as for the wheel in Figure 11, but this time with the wheel (in dotted lines) steered in-the direction of the turn in accordance with an embodiment of the invention, and again superimposed on a view (in solid lines) of the wheel in its straight-ahead position. 10 Description of Examples of the Invention The heavy goods vehicle 10 shown in Figures 1 to 3 has a prime mover 12 towing a semi-trailer 14. The prime mover forms a motor vehicle for the combination. The 15 prime mover is connected to the semi-trailer by means of a turntable 16 which forms a towing coupling. The semi-trailer 14 has a quad axle group 18 at its rear. The leading two axles 20 and 22 are of conventional non-steered design and incorporate road wheels 34 to 37. But 20 the rear two axles 24 and 26 in the quad set are self-tracking axles. These incorporate road wheels 38 to 41. The quad axle group shown has a wide single tyre on each end of each axle set 20, 22, 24 and 26 but the invention is also applicable to configurations having multiple wheels on the ends of axles and other sequences of fixed and self-steer axles. 25 The prime mover providing the driving means in this example has two steering axles 28 and 29 towards its front, and two non-steering driving axles 30 and 31 towards the rear. The semi-trailer 14 has its front supported, via the turntable 16, by the rear of the prime mover 12. 30 As seen in Figures 2 and 3, when the vehicle 10 navigates a high-speed curve in a road, the rear of the semi-trailer tracks wide. This is the HSO described earlier in this WO 2007/128073 PCT/AU2007/000611 8 specification and is partly induced by the four wheels 38 to 41 on the self steering axles being turned towards the left due to their castor action. The line 44 is a locus of the path followed by the outer edge of the front left hand wheel 21 on the prime mover, and line 46 is a locus of the path of the outer edge of the rear left hand wheel 5 40 on the semi-trailer. Referring now in particular to Figures 4 and 5, the axles 24 and 26 each have the same configuration, so like components have been numbered alike. Axle 26 includes a pair of steering arms 50 connected together by a tie rod 52. The wheels 40 and 41 10 thus turn in either direction in unison and adopt a steer angle 0. A hydraulic linear actuator 54 has its body 53 attached to the axle 26 and its actuating arm 55 attached to the tie rod 52 whereby the tie rod may be pushed in either direction relative to the axle 26 which is left or right to the direction of travel. The wheels 40 and 41 have a natural castor tendency due to the geometry of the self-steering axle assembly, but the 15 actuator 54 provides a means whereby the castoring can be counteracted by a controlled moment. The actuator 54 is fed by a servo valve 56 which draws its pressurised fluid from a power source (hydraulic pump) 58 via a feed line 60. Fluid exiting the servo valve 56 20 flows via a check valve 62 to a sink 64 comprising a fluid reservoir which feeds the pump 58. The servo valve 56 is controlled by means of an electronic signal sent by an electronic controller 70 via a connection 66 to a solenoid 68 on the servo valve 56. The 25 controller 70 takes inputs from a sensor 75 measuring lateral acceleration, a sensor 76 measuring the yaw rate, a sensor 77 measuring the road speed of the vehicle, and sensors 78 measuring individual wheel speeds of the wheels 34 to 41 on the axle group 18. The controller 70 uses these inputs to determine the direction and magnitude of the force or displacement desired to be applied by the arm 55 of the 30 actuator 54 to the tie rod 52, and sends an appropriate signal to the solenoid 68. A sensor on the actuator 54 measures the position of the actuator arm 55 (and thus indicates the turning angle or steer angle 0 adopted by the wheels 40 and 41) and WO 2007/128073 PCT/AU2007/000611 9 provides a position feedback signal, via connection 72, to the solenoid 68. In other words, the hydraulic system would provide the required restoring moment to position the 'self-tracking' wheels at the required steer angle as determined by controller 70. 5 Single integrated units combining the servo valve 56, solenoid 68, actuator 54 and connection 72 are widely available commercially from many manufacturers and a suitable unit may be readily selected from the suppliers' literature. At high road speeds, and in a basic form of the invention, the restoring moment or 10 command steer angle signal from the controller 70 to the servo valve 56 responds to lateral acceleration and yaw rate through a simple gain (multiplier) such that a lateral acceleration and yaw rate response associated with a tum to the right would cause the wheels to be steered to the right. Because the lag ii the build-up of tyre side force depends upon speed, and the magnitude of the side force developed depends on steer 15 angle, the gain multipliers are set to be sensitive to speed. At low road speeds the system reverts to a standard self-steer castor system. However, a further benefit of the system can be realised by also using the controlled steering arrangement at low speed to reduce the tendency for semi-trailers to track 20 inwards .on tight bends, such as when turning at road intersections. Steering the wheels away from the direction of the turn, by the application of a restoring moment or steer angle directly, further than would occur under the normal castoring wheel action significantly reduces the level of inboard offtracking. 25 The lateral acceleration sensor 75 may conveniently be placed on the semi-trailer 14. While a measurable improvement of HSO can be achieved in this way, a much greater improvement can be achieved by positioning the sensor 75 on the prime mover 12, and in particular at the prime mover's front steer axle 28. 'It is thought that the improvement is because when entering a curve, the prime mover experiences the 30 lateral acceleration before the semi-trailer, so measuring it at the prime mover allows the restoring moment, or steer angle directly, to be applied to the axles 24 and 26 earlier than otherwise.

WO 2007/128073 PCT/AU2007/000611 10 This strategy works better than one where the restoring moment, or steer angle directly, applied to the axles 24 and 26 responds to lateral acceleration of the semi trailer centre of gravity because the preferred strategy provides the semi-trailer with a 5 degree of preview, setting it up for the turn in advance. Performance may be improved if the applied steer torque restoring moment is controlled in response to the measured lateral acceleration. Further improvement may be achieved if the controller 70 determines and causes' the separate restoring moments or steer angles to be different for wheel pairs on different axles. 10 The hydraulic based system shown for actuating the tie rods in Figures 4 and 5 may be replaced by a pneumatic or electric based system which activates each relevant tie rod using a linear actuator or separate steering arms using rotary actuators. 15 The advantages of controlling the wheel alignments using the system described is that, at low speed, the advantages of self-tracking axles are available and can be improved upon, namely there is a reduced tendency for the semi-trailer to track inwards on tight bends such as when turning at road intersections, whereas at high speed operation, the HSO is greatly reduced from what would be experienced with 20 conventionally set up self-tracking axles and the SRT is improved as will now be described with reference to Figures 11 and 12. An advantage of the present invention relates to improvements in rollover stability. Rollover stability is sensitive to the ratio of the height of the sprung mass centre of 25 gravity to the offset distance between the vehicle centre line and the centre of the tyre contact patch. The plan view in Figure 11 shows (in dotted lines) a steer rotation of a left-side castoring wheel 40 on a self-steer axle in the right hand turn under a normal castor steer, as depicted in Figure 2. As shown in Figure 11, the forward location of the self-steer axis 84 and, to a lesser extent, the inboard location of that axis, leads to 30 a nett inward migration of the tyre contact patch from 86a to 86b when the wheel steers. The inward displacement is shown as "A" in Figure 11. This nett inward movement of the tyre contact patch reduces the effective track width of the vehicle on WO 2007/128073 PCT/AU2007/000611 11 the heavily loaded side causing a reduction in the vehicle's rollover stability. As seen in Figure 12, when the restoring moment applied to the self-steer axle by the actuators in accordance with the present invention is sufficient to stee- the wheel in the direction of the turn, the contact patch migrates laterally outwards by a displacement 5 "B" from 86a to 86c to increase the effective track width thereby improving vehicle rollover stability. The system described above may be incorporated into a multi-combination vehicle (ie one having multiple semi-trailers). In this case sensors (accelerometers) are placed at 10 various locations along the vehicle with each sensor feeding signals to a controller of each respective trailer. The embodiment of the invention shown in Figure 6 utilises only a single self tracking axle 124 at the rear of a triple axle group 118. The arrangement of hydraulic 15 actuator 54, servo valve 56, pump 58, and connection lines therebetween are the same as described in relation to Figures 4 and 5, but the input sensors are reduced to only a lateral acceleration sensor 75 and a road speed sensor 77, both mounted in the prime mover. This simplified system of controlling wheel alignment provides most of the dynamic improvement available from the more complex system described above with 20 reference to Figures 4 and 5. The quad axle group 218 shown in Figure 7 is the same as the quad axle group 18 shown in Figure 5 except that quad axle group 218 includes three additional features. One is that the electronic controller 270 also receives, and takes account of, a signal 25 from a sensor 279 which measures the lateral force being applied at the time to the towing coupling at the hitch point (turntable 16) of the semi-trailer.- The second feature is that, through electronic controller 270 and connections 273, the braking force applied to each wheel may be controlled independently in response to the output detected from the wheel speed sensors 278 on each of the wheels 34 to 41. 30 The third feature is that sensors 280 are provided to measure either the steer angle of the prime mover 12, which forms the motor vehicle in this embodiment, or the WO 2007/128073 PCT/AU2007/000611 12 articulation angle between the respective trailer and the adjacent vehicle unit in front. This feature would be used during low speed operation. During low speed turns the signals from the lateral acceleration sensor 275 would be inadequate to accurately sense the motion of the vehicle so signals from the steer angle and/or articulation 5 angle sensors 280 would instead be used during low speed turns to control the restoring moment, or steer angle directly, applied to the self-steer axles 24 and 26. In some embodiments of the present invention, the alignment of the wheels in a self tracking axle is controlled in response to a braking of the vehicle. This provides a 10 particularly desirable outcome. An explanation will now be given with reference to Figures 8 to 10. Figure 8 is a free-body diagram (schematic and not to scale) which shows the forces Fl to F6 and moment MI acting on the semi-trailer 214 if the brakes are applied on 15 the wheels 35, 37, 39 and 41 on one side of the semi-trailer by an amount significantly more than along the other side. Alternatively the brakes could be applied along one side only. Forces F1 and F2 shown are the resultant tyre braking forces acting on each respective row of tyres. The braking at individual wheel positions is controlled with an anti-lock brake system (ABS). A suitable ABS system may be 20 chosen from a range of such systems available commercially. Inertia forces FS and F6 and a moment M1 act at the centre of gravity 286 of the semi-trailer. A nett result of this unequally distributed braking is a yaw inertia moment M1 that will resist the tendency to swing the semi-trailer to the right and a longitudinal force (the resultant of forces Fl and F2) that will try to slow the vehicle (the nett braking effect). The 25 reaction forces at the tow coupling 217 between the prime mover and the semi-trailer 214 resolve to a braking force F3 and a lateral reaction force F4 that will try to push the tail of the prime mover out to one side. This is undesirable. Figure 9 is a free-body diagram that shows the forces (again not to scale) acting on the 30 semi-trailer 214 when the wheels 38 to 41 on the self-tracking axles 24 and 26 are actively steered according to the present invention and without braking. Force F7 is the resultant of tyre forces at the steered axles 24 and 26 due to the steered tyres 38- WO 2007/128073 PCT/AU2007/000611 13 41. Inertia force F9 and moment M2 act at the centre of gravity 286.. During the initial part of the turn there is a reaction force at the tow coupling 217 between the semi-trailer and the prime mover that will try to push the tail of the prime-mover out of the turn. Figure 9 shows that the lateral reaction force F8 at the tow coupling is 5 opposite in direction to the lateral reaction force F4 in Figure 8 for the pure braking case. Force F8 is also undesirable but may be small enough in most circumstances to not cause major instability of a vehicle turning at high speed. Figure 10 is a combination of the situations shown in Figures 8 and 9. It shows the 10 front and centre-front wheels 35 and 37 on the right side braked (creating a braking force F10), and the centre-rear and rear axles 24 and 26 respectively steered (creating a lateral force F11). Force F12 is the reaction force at the semi trailer hitch point. Inertia force F13 and moment M3 act at the centre of gravity 286 of the semi-trailer. When the forces of the braking and the turning are considered together, the braking 15 force produces a clockwise moment about the centre of gravity 286 which counters the anti-clockwise moment about the centre of gravity resulting from the side force F1l from the steered axles. This means that if braking and steering are each controlled by just the right amount, two desirable outcomes may be achieved simultaneously, namely: 20 1) a nett yaw moment is applied to the semi-trailer that starts it rotating (yawing) in the direction of the turn, and 2) control is achieved over the undesirable lateral tow coupling forces (referred to above) that act on the rear of the prime mover. Accordingly, if steering and braking are controlled simultaneously the undesirable 25 'tow coupling lateral forces may be reduced to small and insignificant levels as described above with reference to Figure 10. The brakes may be applied on both the steered and the non-steered wheels but, for simplicity, in the Figure 10 example, the brakes are shown applied on the right side 30 front and centre-front wheels causing the semi-trailer to follow the prime mover without exerting any significant lateral forces at the tow coupling that may otherwise destabilise the prime mover or adversely affect its handling.

WO 2007/128073 PCT/AU2007/000611 14 The embodiment described above with reference to Figures 7 to 10 may be simplified by deleting the lateial force sensor 279 and the motor vehicle steer or articulation angle sensor 280. 5 Whilst the above description includes the preferred embodiments of the invention, it is to be understood that many variations, alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the essential features or the spirit or ambit of the 10 invention. It will be also understood that where the word "comprise", and variations such as "comprises" and "comprising", are used in this specification, unless the context requires otherwise such use is intended to imply the inclusion. of a stated feature or 15 features but is not to be taken as excluding the presence of other feature or features. The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that such prior art forms part of the -common general knowledge.

Claims (18)

1. A method of operating a semi-trailer being towed as part of a heavy goods 5 vehicle by a motor vehicle, where the semi-trailer has at least one self-tracking axle which has road wheels and castor-steering, said method comprising: (a) at road speeds above a first predetermined speed in the forward direction, applying a restoring moment to the castor-steering action of one or more of said at least one self-tracking axle to cause alignment of said wheels, when measured 10 relative to a reference alignment the wheels would take when travelling along a-. straight path on a'flat level surface, to be moved: (i) away from an alignment which would be adopted if the castor steering action was unrestrained, and (ii) toward the reference alignment; and 15 (b) at road speeds at least above a second predetermined speed in the forward direction, actively controlling the magnitude of the restoring moment so that said alignment of the wheels is dependent upon the road speed and upon a sideways acceleration measurement being made on the heavy goods vehicle. 20

2. A method according to claim I wherein for at least some road speeds above said first predetermined speed, said restoring moment is applied by holding or locking alignment of said wheels into a straight-ahead position.

3. A method according to claim 1 wherein said application of the restoring 25 moment is achieved by applying a controlled force to the wheels to cause alignment of the wheels to be moved away from an alignment which would be adopted if the castor steering action was unrestrained.

4. A method according to claim 1 wherein for some road speeds above said 30 second predetermined speed and some said acceleration measurements, a controlled force is applied to cause the steer angle of said wheels to be decreased, where said steer angle is taken to mean the alignment angle of the wheels measured relative to WO 2007/128073 PCT/AU2007/000611 16 the alignment the wheels would take travelling along a straight path on a flat level surface.

5. A method according to claim I wherein for some road speeds above said 5 second predetermined speed and some said sideways acceleration measurements, a controlled force is applied to cause the steer angle of said wheels to be held over centre in the other steer direction from the steer direction which would be adopted if the castor steering action were unrestrained. to

6. A method according to claim 5 wherein said controlled force application is made dependent upon said sideways acceleration measurement falling within a predetermined range.

7. A method according to any one of the previous claims wherein at low road 15 speeds in the forward direction, the semi-trailer is towed with the castor-steering action of said at least one self-tracking axle enabled and unrestrained.

8. A method according to claim 1 wherein at road speeds below a third predetermined speed in the forward direction, said restoring moment is applied to an 20 extent sufficient to cause the alignment of the wheels of said at least one self-tracking axle to be aligned at a steer angle which is greater than the alignment which would be adopted if the castor steering action were unrestrained.

9. A method according to any one of the previous claims wherein said first and 25 second predetermined speeds are the same.

10. A method according to any one of the previous claims wherein said second predetermined speed is-greater than said first predetermined speed.. 30

11. A method according to any one of the previous claims wherein said measurement of acceleration is made on said motor vehicle. WO 2007/128073 PCT/AU2007/000611 17

12. A method according to any one of the previous claims wherein said heavy goods vehicle is a multi-combination vehicle and said measurement of acceleration is made on the vehicle unit immediately preceding the semi-trailer on which said self tracking axle to be controlled is installed. 5

13. A method according to claim 1 wherein said restoring moment is controlled in order to achieve a predetermined size of the moment.

14. A method according to claim 1 wherein said restoring moment is controlled in 10 order to achieve a predetermined wheel alignment or amount of steer angle.

15. A method according to claim 1 wherein said magnitude of said restoring moment, or steer angle directly, is controlled in response to a yaw rate measurement being made on said semi-trailer and/or on other preceding vehicle units. 15

16. A method according to claim 1 wherein said magnitude of the restoring moment, or steer angle directly, is controlled in response to measurements being made of wheel speeds on said semi-trailer and/or said motor vehicle and/or preceding vehicle unit. 20

17. A method according to claim 1 wherein said magnitude of said restoring moment, or steer angle directly, is controlled in response to a measurement of a lateral force being applied at a towing coupling between the semi-trailer and the preceding vehicle unit, and/or between other preceding vehicle units. 25

18. A method according to claim 1 wherein a greater braking force is applied to at least some of the wheels on said semi-trailer which are on that side of the semi-trailer towards which the heavy goods vehicle is travelling.