The present invention provides a steering mechanism for use on, for example four wheeled trailers or caravans or recovery trailers which are adapted to support the front or rear wheels of a vehicle being towed. With such trailers caster wheels are preferably used to provide a self steering effect. When caster wheels are used in this manner it is desirable that the self steering wheels defining one axle of the vehicle are interconnected so that the steering angles of the two wheels are the same and to prevent the wheels from vibrating independently due to single wheel impacts, for example when one of the wheels is subjected to a discreet disturbance such as a pothole or kerb strike. Hitherto caster wheels on this type of vehicle have been interconnected by a mechanical link. Such a link will however require unobstructed access between the two wheels, which places restraint on the design of the vehicle.

According to one aspect of the present invention, a steering mechanism for a vehicle characterised in that a pair of self steering wheels defining one axle of a vehicle are interconnected hydrostatically, each wheel having a steering arm which is connected to a double acting ram, the ram associated with the wheel on one side of the vehicle being interconnected to the ram associated with the wheel on the other side of the vehicle by means of a fluid line, so that as one of the wheels pivots about its axis the movement is transmitted to the other wheel.

With the steering mechanism according to the present invention, the fluid lines, for example flexible hoses, may be routed to conform with the design of the vehicle. Moreover the steering mechanism may be used on vehicles with adjustable tracks or vehicles in which the axle may be folded for stowage purposes.

The hydrostatic linking of the wheels may also provide a damping effect, which may be adjusted by suitable selection of the size of the hoses or the fitting of restrictors, to control the rate of flow of fluid between the rams.

According to a further embodiment of the invention, an isolation valve may be provided between the rams in order to lock the wheels, when for example reversing.

The present invention is now described, by way of example only, with reference to the accompanying drawings, in which:- Figure 1 is a plan view of a trailer in accordance with the present invention, in its deployed configuration; Figure 2 shows a front view of the trailer illustrated in Fig. 1, in its stored configuration; Figure 3 shows a perspective view of the trailer in its stored configuration; Figure 4 shows a side elevation of the trailer shown in Fig. 1, illustrating a first stage of deployment of the trailer ; Figure 5 shows a plan view of the trailer shown in Fig. 1, illustrating a second stage of the deployment of the trailer ; Figure 6 shows a detailed perspective view of a wheel pan and wheel of the trailer illustrated in Fig. 1; and Figure 7 illustrates diagrammatically a hydrostatic steering linkage for the trailer illustrated in Fig. 1.

As illustrated in the drawings, a trailer 10 has a drawbar 12. The drawbar 12 is formed in two sections, a forward section 14 and a rearward section 16. The forward and rearward sections 14,16 are pivoted together about adjacent ends, by pivot pin 18 so that the sections 14 and 16 may be disposed end to end, or the rearward section 16 may be folded underneath the forward section 14. The end of the forward section 14 extends beyond the pivot pin 18 and has apertures therein, the apertures aligning with corresponding apertures in the rearward section 16 when sections 14 and 16 are disposed end to end, so that a locking pin 20 may be inserted through the apertures, to lock the drawbar in its extended position. A stop (not shown) is provided to prevent the sections 14 and 16 going past the position in which the apertures are aligned.

A pair of gas struts 22 are provided between the forward and rearward sections 14 and 16 of the drawbar 12 to assist and control relative movement of the sections 14 and 16, as the drawbar 12 is moved between its folded and extended positions.

A towing hitch, for example a ball joint cup 24 is secured to the forward end of position 14 of the drawbar 12, for attachment to a towing hitch 100 of the recovery vehicle. A pair of trunions 26 are also provided adjacent the forward end of the forward section 14 of drawbar 12, one extending to each side thereof.

A pair of arms 30 are pivotally mounted to a bracket 31 secured to the rear end of the rearward section 14 or the drawbar 12. The arms 30 are mounted by means of pivot pins 32, for movement in the plane of the rear section 16 of the drawbar 12. The arms 30 extend beyond the pivot pins 32 and a pair of links 34 extend from the end of each arm 30 to a block 36 mounted for axial movement on a lead screw 38. A drive formation 40 is provided on the lead screw 38 by which the lead screw 38 may be rotated to move the block 36 linearly, causing the arms 30 to pivot about pins 32, between a stowed position in which the arms 30 are disposed parallel and in juxtaposed relationship to the

rearward section 14 of the drawbar 12, as illustrated in Figs. 2 and 3; and a deployed position in which the arms 30 extend transversely of the axis of the drawbar 12, as illustrated in Fig. 1. In the deployed position, apertures at the end of arm 30 are aligned with apertures 42 in the bracket 31 so that locking pins 44 may be located through apertures 42, to lock the arms 30 in their deployed positions.

A pair of wheel pans 50 has tubular mounting means 52, which locate and slide one on each of the arms 30. The wheel pans 50 define a platform for the wheels of the towed vehicle. Spring loaded adjustable chocks 54 are provided at the rearward end of the pans 50, which will be depressed as the wheel of the vehicle is moved onto the pan 50 and will spring up to help retain the towed vehicle on the trailer 10. Additional fastening means, for example straps or chains are also provided for securing the towed vehicle.

Links 60 are pivotally connected between the pans 50 and projections 62 on the bracket 31, the links 60 causing the wheel pans 50 to slide outwardly on arms 30 as the arms are moved between their stored and deployed positions and to slide inwardly on arms 30 as the arms 30 are moved between their deployed and stowed positions. The links 60 are secured to the projections 62 by means of locking pins 64 which engage through aperture 66 in the ends of links 60 and projections 62. Several apertures 66 are provided in both the links 60 and projections 62, by which the effective length of the links 60 may be adjusted.

The separation of the wheel pans 50 when the arms 30 are in their deployed positions, may thereby be adjusted to suit the track of the towed vehicle.

A pair of self steering wheels 70 are mounted one on each of the wheel pans 50, the self steering wheels 70 being mounted on the outside of the wheel pans 50, when the arms 30 are in their deployed positions. The self steering wheels 70 are mounted on kingpins 72 which extend substantially vertically but are inclined at appropriate camber and castor angles to provide a self-steering effect.

Steering levers 74 are provided on each of the self steering wheels 70, the

steering levers 74 being connected to double-acting rams 76 mounted on the wheel pans 50. The rams 76 on either side of the trailer 10 are interconnected by flexible hydraulic hoses 78, the forward chamber of one ram 76 being connected to the forward chamber of the other ram 76 and the rearward chamber of one ram 76 being connected to the rearward chamber of the other ram 76. The connected chambers thereby form closed hydraulic circuits by which any steering movement of one wheel 70 will be transmitted to the opposite wheel 70. For example, when the left wheel 70 is steered to the left due to its self steering properties, the volume of the forward chamber 84 on the left ram 76 decreases forcing fluid to be transferred to the forward chamber 84 on the right ram 76 causing the right wheel 70 to be steered to the left. This steering motion causes the volume of the rearward chamber 86 of the right ram 76 to decrease and forces fluid into the rearward chamber 86 of the left ram 76, which is consistent with the applied steering motion. This fluid transfer works in the opposite sense to steer both wheels 70 to the right.

In this manner, the wheels 70 will remain in phase to minimise tyre scrub and steering shimmy, particularly when the trailer 10 is subjected to any uneven forces, for example, when one wheel 70 is subject to discreet disturbance such as a pothole or kerb strike. Furthermore, interlinking of the wheels 70 in this manner will resist and damp out any unwanted motion of the wheels 70. The amount of steering damping can be varied by adjusting the sizes of the hoses 78 or fitting restrictors 82, to control the rate of flow of fluid.

An isolation valve 80 may be provided between the rams 76, in order to lock the wheels 70 when, for example, reversing.

The use of flexible hoses 78 as described above allows for movement of the wheels 70 between the deployed and stowed positions of the trailer 10 and also permits adjustment of the track of the trailer 10.

In the stowed position, as illustrated in Fig. 2, the trailer is disposed substantially vertically in the rear of the recovery vehicle, the trunions 26 on the forward section 14 of the drawbar 12 engaging a pivot bracket 90 which is mounted to the floor of the recovery vehicle. The stowed trailer 10 may be locked in position in the recovery vehicle by, for example, means of the locking pin 20 which may engage apertures in a suitable locking bracket 92 mounted within the recovery vehicle.

A winch 94 may also be provided in the recovery vehicle, the winch cable 96 passing over pulley means 98 and being secured to the drawbar 12 adjacent the interconnection of the forward and rearward sections 14,16 thereof.

To deploy the trailer the locking pin 20 is released from the bracket 92 within the recovery vehicle and the forward section 14 of the drawbar 12 is permitted to pivot rearwardly about trunions 26, movement thereof being controlled by the winch 94. As the forward section 14 pivots rearwardly the rearward section 16 of the drawbar 12 pivots away from the forward section 14 under the influence and control of the gas struts 22.

When the wheel pans 50 and wheels 70 are clear of the recovery vehicle, as illustrated in Fig. 3, the locking pins 64 may be engaged through suitable apertures 66 in the links 60 and projections 62 on bracket 31, to provide an appropriate separation of the wheel pans 50 to match the track of the vehicle to be towed.

The lead screw 38 may then be rotated to pivot arms 30 outwardly, as illustrated in Fig. 5, until the arms 30 extend transversely of the longitudinal axis of the drawbar 12. Locking pins 44 are then located through the apertures in the ends of arms 30 and apertures 42 in the bracket 31 to lock the arms 30 in position.

The forward section 14 of drawbar 12 is then lowered further by means of the winch 94 until the forward and rearward sections are brought to the fully deployed position, when the locking pin 20 is located through the aligned apertures therein. As the forward section 14 of the drawbar 12 is lowered, small rollers 58 at the rear of the wheel pans 50 will first engage the ground permitting the wheel pans 50 to move backwardly until the self steering wheels 70 come into engagement with the ground.

After the sections 14, 16 of the drawbar 12 have been locked in the deployed position by pin 20, the front of the drawbar 12 may then be lifted, either manually or by means of the winch 94, from the mounting bracket 90 and the ball joint cup 24 engaged with the towing hitch 100 of the recovery vehicle.

Detachable ramps (not shown) are then attached to the trailing edge of the wheel pans 50 and the winch 94 is used to draw the vehicle being recovered onto the wheel pans 50 and the vehicle is located and secured to the trailer 10 using the adjustable wheel chocks 54 and restraining straps.

The reverse procedure is used to stow the trailer 10 after use.

Various modifications may be made without departing from the invention. For example while the invention has been described with reference to a two wheeled trailer designed to support the front or rear wheels of a towed vehicle, the invention also applies to four or more wheeled trailers having one or more pairs of self steering wheels.