GB2320473A - Trailer towbar assembly with steering control mechanism - Google Patents

Abstract

A towbar assembly U allows the towing vehicle V to vary in pitch attitude to the towbar assembly U, whilst turning angles made between the two will steer the road wheels A of the assembly to the required positions up to a 90 degree trailer X to towing vehicle V angle, as shown in Fig.2. The towbar assembly U complete with road wheels A, can be attached via front springs or otherwise, to the forward end of the body or chassis of a trailer X having separate rear wheels, to produce a stable and easily reversible trailer. A rotable member M positioned at the pivot L between the towing vehicle and trailer, drives a chain N which is conected to the track rods E of the steered wheels A via links P, Q and plate G, whilst a cam plate T maintains tension in the mechanical links P, Q and chain N. The mechanical relationship between towing vehicle turning angle and the steered wheels is tailored for different sizes of trailer by selection of a suitable cam plate T installed in the assembly. Provision can be made for allowance of small misalignments of the two vehicles, the effect on the steering being illustrated in Fig.3.

Description

A Trailer Towbar Assembly with a steering control mechanism.

The present invention relates to a towbar assembly for a trailer having a wheelbase sufficiently long to provide longitudinal stability independent of the tow vehicle, and which requires a means of steering the front wheels. In this specification the term trailer is used to denote a wheeled structure which can be towed behind a vehicle.

Large trailers, of a type that has a towbar able to swing vertically to avoid downloads on the tow vehicle, are commonly steered by mounting the front wheels and the towbar on a turntable, attached pivotally to the trailer body. During forward travel, this turntable steers the front axle by rotating according to the angle of the pulling force applied via the towbar from the hitch point of the tow vehicle. However, since this force will change to a pushing force when reversing, or under some conditions of hard braking, the effect then is to steer the wheels in the wrong direction, causing reversing round curves to be almost impossible, and in some situations causing dangerous instability when hard braking. In addition, when cornering or negotiating steeply cambered surfaces during forward travel, any momentary slide of the trailer front wheels due to grease or ice on the road can be catastrophic.

These problems, in particular the reversing difficulty, are of sufficient magnitude to have caused manufacturers increasingly to produce the type of trailer with a fixed towbar, and roadwheels in the centre, as seen on caravans. However, the centre-wheeled type has virtually no inherent stability, and is prone to "snaking" unless the hitch point is positioned virtually on the rear axle of the tow vehicle. This, together with the lack of longitudinal stability, makes unhitching when loaded impractical. Worst of all for heavy trailers, the load is limited because of the potentially high downloads that can be imparted to the tow vehicle.

Various attempts have been made therefore to design a long wheelbase trailer where the front wheels are steered by the relative angle between trailer and tow vehicle, rather-than by the forces between the two moving the towbar laterally. This avoids all the problems outlined above for long-wheelbase trailers, and makes reversing similar to the centre axle type, although slightly easier because with a long wheelbase, the fixed axle is generally further from the hitch point than for the former.

However, designs produced so far have had limited practical use because all have suffered from one or more of the following drawbacks: *Steering angle of trailer wheels limited to less than 90 degrees.

*Complicated mechanical or electrical control of steering, causing high maintenance and/or risk of catastrophic failure.

*Inaccurate steering angles for individual wheels causing high tyre wear.

*No ability for system to tolerate small lateral movements between the two vehicles as road undulations are followed.

According to the present invention, there is provided a towbar assembly comprising a hitch device attached to a rotatable element, a mechanism drivingly operated by rotation of this element, road wheels steered appropriately by this mechanism, and means for attachment of the whole assembly to a trailer. The hitch device allows the tow vehicle to move freely relative to the towbar assembly in both pitch and roll, but yaw movement, (tow vehicle turns about a vertical axis) will turn the rotatable element, and operate the steering mechanism.

Thus, a towbar assembly is provided for use as a component part of a trailer to be towed behind a vehicle, whereby the sensing of the angle that the assembly makes with the towing vehicle, the necessary mechanical drive to steer the roadwheels, and the roadwheels themselves on their respective pivot points, are all part of the assembly, and this assembly can then be attached to the chassis or body of a trailer at pivot points to allow vertical movement of the hitch point. These pivot points can be of a flexible type to allow some small lateral movement of the hitch point relative to the body of the trailer.

Road springs can be incorporated either between these pivot points and the trailer body or chassis, or can be incorporated between the pivot points and the towbar assembly if required.

An example of an assembly constructed in accordance with the present invention will now be described with reference to the accompanying informal drawings in which: Figure 1 illustrates, in plan view, a typical mechanism contained within the towbar assembly, which is operated by the angle that this assembly makes with the tow vehicle.

Figure 2 illustrates the same mechanism when a 90 degree angle of tow vehicle to trailer is realised.

Figure 3 illustrates the effect on the steering of a small lateral movement of the hitch, and shows how the road wheels are affected in such a way as to correct the condition.

The angles involved are greatly exaggerated in this illustration for increased clarity.

In Figs. 1 and 2 a towing vehicle V is shown connected pivotally to a trailer towbar assembly W by a horizontal bar J or similar, such as to allow pitching movement of the tow vehicle relative to the towbar assembly W. On the connection bar J a telescopic sliding joint K is mounted, similar to that used on conventional trailers, which also allows for relative rolling movement of trailer to towing vehicle. The telescopic movement of K operates a suitable mechanism (not shown) for operating the trailer brakes when required. If the trailer brakes are to be operated directly from the tow vehicle braking system, as in the case of an HGV with air brakes, the telescopic movement is limited according to requirements.

The hitch and telescopic sliding joint can pivot about the vertical axis L iocated in the towbar mechanical housing U, which is shown in typical outline. The vertical axis carries an input drive wheel M, toothed to drive a matching belt or chain N, such that the belt or chain N is drawn round M positively in proportion to the tow vehicle angle of turn relative to the towbar assembly W. Each end of the chain or belt is connected to a respective pair of iinics P, Q, which are in tension from this belt or chain and which terminate at points S on a plate G, pivoted on another generally vertical axis within the mechanism housing U at H. The two links P are constructed such as to allow one to pass through the other at the crossover point.

So far described, the arrangement would cause the plate G to rotate in the opposite direction to any rotation of drive input at M, but chain or belt tension would be lost as angle of rotation increased, and the ratio of input to output would not be accurately controlled. To achieve constant tension of the belt or chain N, and control the exact relationship of input to output angle as necessary, each connection of a pair of links P and Q is provided with a roller or guide R, which is caused to tour one edge of cam plate T, fixed to the chassis U. The cam plate T is shaped to deflect the pairs of links P, Q as the rollers move along it, to provide the exact non-linear relationship required between input and output items M and G. The cam plate T can be readily changed to accommodate the different relationships required for trailers of different dimensions, and can be positioned over or under an aperture to enable identification of the cam type without demounting. In the illustration the single cam plate deflects the rollers apart, but a similar effect can be attained by using two cam plates, one either side, that deflect the rollers R inwards. The crossover point of the links can also be made after the cam, by crossing links Q instead of links P.

On the example shown, the output plate G has two pivoted connections F to operate trackrods E. The trackrods, which alternatively can be connected at a single point to plate G, steer roadwheels A on pivot points B by means of forward arms C. The plate G is shown as one component for clarity, but if a mechanical seal is required against the ingress of dirt or water into the mechanism, the single plate G is replaced by two components, one mounted inside the sealed enclosure, and connected to the links Q at points S as shown, which drives the shaft H. This shaft passes through a sealed bearing in the mechanism enclosure to operate the second component, which is the arm carrying the track rod connections F The trailer X is mounted to the towbar assembly by pivot points allowing rotation of the assembly W round a horizontal lateral axis close to the front of the trailer.

The location of these points would normally be near the king pins B. The pivot points can also allow some small movement other than this rotation, if they are the type that incorporate rubber-mounted bushings or the like. If this type of pivot is used, then small misalignments of the trailer to the tow vehicle can be tolerated as road undulations are encountered, avoiding large lateral forces being exerted at the hitch. This improves the ride for the occupants of the tow vehicle, and can also improve road adhesion of both vehicles.

Figure 2 shows the same arrangement of parts as Fig. 1 but at a position where a 90 degree angle is being made between trailer X and tow vehicle V. The centre of turn of the pair of vehicles in this illustration is therefore located approximately at the position of the rear right wheel of the trailer (not shown) and the right front wheel therefore is turned correctly some 90 degrees.

Figure 3 shows in an exaggerated form the effect of some misalignment of trailer X to tow vehicle V, caused by road undulations or side wind, when both are travelling in a straight line which can be tolerated when rubber-mounted bushings or similar are used to attach the assembly W to the trailer X. In this situation, the towbar assembly W is angularly rotated within the tolerance of the mounting employed, and this causes the input at the hitch to be detected as a turn situation. The mechanics therefore operate the front wheels to steer accordingly, but because the entire assembly is not in line with the trailer body, the overall effect is to produce a small steering angle of the front wheels in a direction such as to move the trailer back slowly to its rightful position, aided by the mechanical forces given by the flexible mounting.

Figure 4 shows a side view of the typical mounting of the invention to a sprung trailer. In this case the pivot points are directly on the assembly, and are mounted on the front road springs. In other versions the front suspension can be mounted to the assembly, and the pivot points then attached to the trailer chassis directly. For some applications, specially designed rubber-type bushings can be used for the pivot points, such that they provide enough vertical movement to avoid the use of any other front suspension system.

Claims (9)

Claims

1. A towbar assembly comprising a hitch device attached to a rotatable element, a mechanism drivingly operated by rotation of this element, road wheels steered appropriately by this mechanism, and means for attachment of the assembly to a trailer.

2. A towbar assembly according to claim 1, wherein the mechanism includes means for adjusting the degree of steering of the wheels relative to the angle of the assembly with the towing vehicle.

3. A towbar assembly according to claim 1 or 2, wherein the mechanism includes a camplate, which controls the particular non-linear relationship between the input, (the angle between tow vehicle and the towbar assembly), and the output, (the resultant degree of steering of the attached wheels).

4. A towbar assembly as defined in claim 3, including means for changing the cam plate controlling the input/output relationship, thus adapting the assembly for use with differing sizes of trailer.

5. A towbar assembly as defined in any of claims 1 to 4, including means for externally identifying the cam plate installed without demounting of the cam plate.

6. A towbar assembly as defined in any of claims 1 to 5, including means for mounting a trailer chassis or body with flexible couplings or mounts, in such a way as to allow limited sideways movement of the hitch relative to the trailer longitudinal centre line.

7. A towbar assembly as defined in any of claims 1 to 6, including means for mounting a trailer chassis or body in such a way as to allow some forward and rearward limited movement of the hitch relative to the trailer body or chassis.

8. A towbar assembly as defined in any of claims I to 7 wherein the roadwheels are steered by individual track rods attached to respective operating arms.

9. A towbar assembly as defined in any of claims 1 to 8, substantially as described herein with reference to figures 1 - 3 of the accompanying drawings.