GB2462482A - A drawbar assembly for trailers - Google Patents

Abstract

A drawbar assembly of a trailer or other towable vehicle which is connectable by means of a coupling to a towing vehicle, including a pivotable or otherwise displaceable support system for the drawbar wherein a load bearing part of the support system of the drawbar includes a hydraulic piston-cylinder strut 21' or other load responsive/sensing means, and wherein the coupling height is either fixed or adjustable within a range limited such that the load and therefore hydraulic pressure in the cylinder or load on the other load responsive means has a constant or substantially constant relationship with the vertical load at the coupling, and pressure indicating means for indicating, in use, the hydraulic pressure in the cylinder or on the other load responsive means.

Description

A DRAWBAR ASSEMBLY FOR TRAILERS

The present invention relates to a height adjustable drawbar assembly for trailers or other towed vehicles wherein means are provided to indicate the vertical load iniposed by the trailer coupling on the towing vehicle coupling and such a vehicle incorporating the drawbar assembly.

The measurement of the load imposed by a trailer coupling on the vehicle coupling is an important aspect and which the present invention enables. Often trailers are used to transport small excavators or diggers and such are driven onto the trailer via a rear ramp and then the driver needs guidance onto the correct position for safety in transporting. To illustrate how important and critical the need for load measurement is, the following is an example based on typical trailer weights and dimensions: trailer unladen weight 500Kg, loaded weight 2700 Kg (therefore weight of load 2200Kg), unladen imposed load at trailer coupling 60Kg, maximum legal imposed load at coupling (limited by coupling design) 100Kg, minimum imposed load for safe towing of laden trailer 70Kg, distance from centre of axle group to coupling 2.9m. To keep the coupling load within the desired range the operator has to position the centre of gravity of the machine he is carrying within {(30/2200) x 2900}/2 = 20mm. Within 20mm of the optimum position -and the operator has no idea where that optimum position is or, for that matter, where the centre of gravity of the large machine is. In practice it is believed possible that vehicles and trailers may travel along roads with imposed loads of 400Kg or more. The practical maximum for ball-type 02 couplings is 150Kg.

In our copending UK Patent Application 0807636.6 filed 28 April 2008, a height adjustable drawbar assembly is disclosed and having a similar parallelogram like linkage..

The assembly of the present invention is not for use with semi-trailers where the wheels are located at the rear end of the trailer but rather for trailers having a wheel or preferably at least two wheels located either side and substantially centrally of the length of the load bed of the trailer.

A load measuring drawbar assembly according to the present invention is as defined in claim 1 and subsequent claims and preferred features are as described herein in the specification and as defined in the claims.

Also according to the invention, a drawbar assembly of a trailer or other towable vehicle which is connectable by means of a coupling to a towing vehicle, including a pivotable or otherwise displaceable support system for the drawbar (preferably said support system is mounted at the rear ends on a trailer part and at front ends on a coupling member or mounting therefor and forming a form of or functioning as a parallelogram linkage mechanism), and inclined support means for the linkage comprising a pivotafly mounted load sensing and load-bearing means (23) connected in series to an adjustment member via a locus link member (22) or other movement path control/guide means, said adjustment member being adjustable in its supporting length so as to raise or lower said load sensing and load-bearing means (23) and thus the drawbar linkage such that there exists a constant or substantially constant relationship with the vertical load at the coupling.

The function of the locus is central to the objective of achieving said substantially constant relationship. In further explanation, if a hydraulic strut forms the load-sensing and load-bearing means, and if it were vertical it would generate equal and opposite loads in the link arms. As the height changes, linkage arms would incline up or down. ].f the loads in the linkage arms (in response to a vertical load applied at the coupling) were equal and opposite on account of the strut being constantly vertical and the linkage arms being the same length and parallel, then at all inclinations these loads would cancel out and the net vertical component of the link arm loads would be zero -and there would be nothing adding or subtracting from the load in the strut.

However to achieve a practical structure there is realized the need to incline the strut. Therefore there is a horizontal component to the strut load. This is reacted unequally by the link arms -so the link arm loads become unequal.

In the particular case of the link arms being exactly horizontal, this inequality still produces no net vertical load. However at all inclinations above or below the horizontal the inequality in the link arm loads (even though they are parallel) produces a net vertical load -which varies as their inclination changes i.e. as the coupling height changes. The net vertical component of the link arm loads can only be reacted by the strut and therefore for the same vertical load at the coupling different loads will result in the strut due to the changing inclination of the link arms -if the strut remains at the same angle.

By inclining the strut towards the vertical, the load can be reduced in it for any given coupling load; by inclining the strut away from the vertical the strut load is increased for any given coupling load. Thus if the inclination of the strut is changed as the height is changed, (therefore as the link arm inclination changes), we can make one effect balance out the other. It can be shown (referring to Figure 10) that, to an acceptable approximation the relationship between vertical load at the coupling (V) and strut load (Fs) is represented by the formula V = Fs (sinB + sinA.cosB) and thus that the desired constant relationship is achieved if the strut inclination (B) is made to change in response to the link angle (A) such that the function (sinB -sinA.cosB) remains constant. The locus of the strut lower pivot is defined by virtue of it being connected to the coupling mounting member (pivot point 23B).

Therefore the inclination of the strut is effected by controlling the locus of the top pivot point (23A). The required locus to achieve the inclination which will generate the constant relationship referred to above (assuming a fixed length strut) is shown diagrammatically in Figures 7, 8 and 9 as a series of circles representing different positions for (23A). The locus of point (23A) is not a circular arc but, within a very small margin of error can be approximated to a circular arc. The swinging link/locus link locating the lower pivot point of the height adjustment strut and the upper pivot point (23A) of the hydraulic strut to a fixed pivot point generates the desired circular arc providing this pivot is positioned to give the best fit of the circular arc to the theoretically correct locus of (23A).

Whilst a drawbar assembly according to the invention may use a fixed support strut or an adjustable one secured at a fixed length and including load responsive means associated with the strut, it is preferred to have an adjustable length support connected in series with a load responsive means as mainly described herein especially as such gives a usefully accurate coupling load indication over a large range of height adjustment this being the essence of the difference between the preferred embodiment of the invention and the simpler variants.

The drawbar assembly of the invention is described in combination with a trailer although protection for such per se is also intended.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: -Fig. 1 is a front perspective view from above and one side of an adjustable load-measuring drawbar assembly forming a preferred embodiment of the invention; Fig. 2 is a rear perspective from below and one side of the assembly of Fig. 1; Fig. 3 is a side elevation of the assembly of Figs. 1 and 2 in a low height setting position; Fig. 4 is an elevation of the assembly of Figs. 1 and 2 in an elevated or high height setting position; Fig. 5 comprises nine elevation views of the assembly of figs. 1 and 2 in different position of height adjustment; Fig. 6 is a rear perspective view from above and said one side of a modified assembly with a trapezium-shaped lower link frame member and without the pivot bolt couplings thereof and with the height adjustment strut in this modification (which is a hydraulic piston-cylinder arrangement as in Figs 7 to 10) and bucket rest being omitted and its support arm modified accordingly; Fig. 7 is a schematic side elevational view showing the locus arc; Figs. 8 to 10 are similar schematic side elevations illustrating the required locus of the top pivot point of the hydraulic load sensor cylinder where it connects with the locus link in different positions of adjustment; Fig. 11 is a schematic elevation of the assembly with hydraulic piston-cylinder indicating angles A and B; Fig. 12 is a perspective view of a trailer incorporating the drawbar assembly of Figs. 1 to 10; Fig. 13 is an enlarged fragmentary detail of the inclined cantilever support frame of figs 1 to 5 and which carries one end of the load measuring apparatus and showing the pressure load indicating gauge with central safety-loading zone; and Fig. 14 is a schematic force diagram of a simplified embodiment according to the present invention and illustrating the main components of the assembly of Figs. I to 12 but wherein the adjustment strut (21), locus link member (22), piston-cylinder and hydraulic load measuring strut (23) of the earlier embodiment are replaced by a single adjustable strut (25) pivotally mounted at each end between cantilever extension (19) and the coupling mounting member (5) a securing strut is of fixed length or adjustable in length secured at such and incorporates a load measuring device.

Embodiments of a drawbar assembly according to the invention are illustrated in Figs. ito 13 wherein there is a parallelogram-like linkage similar to the parallelogram-like linkage of Figs. 1 to 6 of UK Patent Application 0807636.6 but differs mainly in that load measuring apparatus or components are included.

The drawbar assembly of figs. 1 to 12 is very similar to that of Figs. 1 to 6 of our co-pending application mentioned above and is intended to form part of a trailer (as shown in Figs. 12, 13) having two wheels 3 mounted on either side intermediate the front ends and a loading/unloading ramp 4 at the rear.

The drawbar assembly 1 has a coupling mounting member 5 pivotally connected to link members readwardly. Coupling member 5 is generally U-shaped and formed by a generally horizontal lower base plate 6 and two vertically extending side plates 7 connected thereto and further interconnected at the rear by a tubular cross member 8 and has hitch/coupling receiving means 9. Mounting member 5 has mounted thereon, as shown in Fig. 11, a ball-socket towing hitch 10, a hand-brake 11 and override wire 12 and a jockey wheel 13 and jockey wheel jack 14.

The drawbar assembly 1 includes an upper and generally triangular frame member 15 and a lower double-triangle frame link member 16 (Figs. 1 to 3) (or a generally trapezium-shaped in plan view frame link member 17 in Fig. 7) formed mainly of circular-sectioned steel tubes the frames are of circular section tube which is more efficient in compression and facilitates production. Frame link member 15 comprises two equal length side members 15A and a rear side transverse tube i5B forming the upper link member and is pivotally mounted at its two rear corners 15C on lugs 18A of a hollow torsion box beam 18 which forms the front of or is connected to the front of the trailer platform (see Fig. 12). Link member 15 is pivotally mounted at 15C at its forward apex via lug 15D on a central extension lug 8A of tubular cross member 8 of coupling mounting member 5.

Lower link member 16 in Figs. I and 2 almost has the appearance in plan of two adjacent triangular frames on a connection base (whilst in the modification of Fig. 7 when viewed in plan has the appearance of a truncated triangle or trapezium). Link member 16 comprises a transverse or rear tube 16A pivotally mounted at its ends on tugs 18A of the box beam 18 and about an axis parallel to the pivotal axis of tube 15B. Two side tubes 16B extend forwardly from the ends of tube 16A at equal angles and are pivotally mounted at 16C on appropriate sides of mounting member 5 pivotable about the same axis which is parallel to the other three pivotal axes. Bracing members 16C extend from the middle of tube 16A to a forward region of each side tube 16B to complete the respective triangular portions. In the modification of Fig. 6, side tubes 16B' are connected at their front ends by crosspiece 16F to form a trapezium shaped frame member when viewed in plan and is pivotally mounted on forwardly extending tugs at 16C at opposite sides of 6.

The rear tube 16A of link member 16 is of equal length as upper link member rear tube 15B and pivotally mounted therebeneath on the torsion box and pivotable about an axis parallel to the pivotal axis of 15B. The two diverging/converging bracing struts or bars 16C each extend from the bottom link base tube 16A to side tubes 16B to stabilize such. Member 16 is pivotally mounted at its rear corner 16D to lug 18A at lower part of the box beam 18 beneath the mounting points for the upper frame member 15. The upper and lower links 15, 16 are of the same length in the direction of travel and the top and bottom pivots are staggered/offset (unlike in Figs. 1 to 6 of the copending application where respective pivots are vertically one above the other).

An inclined cantilever support arm 19 extends upwardly and forwardly from the middle of the torsion box 18 and is formed by the spaced apart cross-braced shaped plates 19A, 19B and which arm 19 carries an excavator bucket rest plate 20 at its upper end.

Thus a parallelogram linkage is in effect formed by the pivotal connection of the upper and lower link members 15, 16 to the torsion box 18 and to the coupling mounting member 5. The parallelogram linkage is maintained in an elevated position with the towing coupling member or equivalent part of the coupling mounting member 5 in a horizontal or substantially horizontal disposition by means of an inclined height adjustment strut 21 which in the embodiment is telescopic and adjustable in length by engaging a retaining pin or other retaining member (not shown) extending through apertures in the outer tubular sleeve 21A and inner tube 21B slideable therein to maintain the coupling assembly at a desired elevation.

(Alternatively, a hydraulic displaceable piston/cylinder arrangement 21 may be used for the adjustment member as in Figs. 6 to 13). The height adjustment strut 21 is pivotally mounted at 21C at its upper end between the shaped plates 19A, 19B of the cantilever arm 19 and pivotally mounted at 210 at its lower end intermediate the ends of an inclined locus link bar 22 which comprises an arm formed of two spaced plates pivotally mounted at 22A at its lower end between the plates on the cantilever arm 19. A load sensing member in the form of an inclined hydraulic strut 23 is pivotally connected at 23A at its upper end to the upper end of the locus link 22 and at 23B at its lower end is pivotally mounted on the cross-tube 8 of the coupling mounting platform or member 5 intermediate in elevation between the parallel axes of mounting of the upper and lower Unks on the coupling mounting member and rearwardly thereof and with its axis of rotation parallel to the other pivotal axes. All of the pivotal axes of the assembly so far described are parallel.

Thus the telescopic or otherwise adjustable height adjustment strut 21 connects via the locus link member 22 with the hydraulic strut 23 and is thus in effect in series therewith in its connection between the cantilever support arm 19 forming a fixed trailer part and the mounting member for the coupling.

The locus link 22 controls the position and angle of the hydraulic strut 23.

Fig. 3 is a side elevation of the assembly of Figs. 1 and 2 showing a "low" setting of the drawbar assembly 1 whilst Fig. 4 is at a "high" setting.

Figs. 6 to 11 are various schematic views showing various positions of adjustment and indicating the principles of operation. The locus of the top pivot 23A of the locus link 22 is a circular arc with a centre at the lower axis 22A of the rotation of the locus link 22 on the torsion box 18 and thus is considered sufficiently accurate even though slight variations occur at extremities.

In Fig. 11 angles A and B are indicated. Angle A being the desired height adjustment link angle and angle B being the generated support angle.

Fig. 5 illustrates the assembly at nine different height positions of setting wherein the load may be unloaded.

Figs. 12 and 13 show the assembly modified as in Fig. 6, on a trailer with a hydraulic pressure gauge 24 which is in connection with the interior of the hydraulic strut 21' to indicate the pressure therein and its dial has two lateral sections/red zones indicating a light load or too heavy a load on the coupling, and an upper central section/zone (possibly green zone) indicating an acceptable load on the coupling. More specifically where a pressure gauge is used to measure strut load it might be arranged so that, instead of reading units of pressure, it has a red zone from zero up to a pressure corresponding to the minimum desirable imposed load (typically 60 Kg), a green zone up to the maximum allowable coupling load (100 or 150 Kg depending on coupling type and trailer gross weight), a red zone extending to perhaps twice the allowable imposed load. If the pressure gauge is positioned so that the driver of a machine being loaded on to the trailer can see it during the loading operation then the driver, who will likely have limited skills in using instrumentation, simply has to position the machine so that the needle is in the green zone. As mentioned the indication of hydraulic pressure to the operator is preferably by means of pressure gauge 24 having a maximum pressure reading equal to about 150% of the maximum coupling imposed load. The visible dial of the pressure gauge is preferably comprised of red and green segments such that there is a red segment at all pressures below that corresponding to 50% of the maximum imposed load and another red segment at all indicated pressures above that corresponding to 100% of the maximum imposed load. Between these two red segments is a green segment. The dial of the pressure gauge is preferably orientated so as to be clearly visible to an operator loading cargo on to the trailer, particularly an operator loading an item of construction equipment, such as an excavator, from the rear. Therefore to get the cargo loaded in such a position that thtere is generated a safe imposed coupling load, the operator has only to watch the needle on the pressure gauge: if it is in the lower red zone he has to move the cargo forward until it is in the green zone, if it is in the upper red zone then he has to move the cargo rearwards until the needle returns to the green zone.

The coupling loads and associated hydraulic pressures so far referred to are in the static condition. Dynamic loads that occur as the trailer is being towed along the road can be many times higher. If the pressure gauge were to be specified such that its maximum safe pressure was high enough to survive the maximum dynamic pressure the green zone corresponding to the required static load would be a very small segment at the lower end of the scale. In these circumstances the usefulness to the trailer operator would be severely reduced. To overcome this problem there is preferentially inserted into the hydraulic connection between the hydraulic cylinder and the pressure gauge a pressure protection valve the function of which might be as follows.

The first time the dynamic pressure exceeds a predetermined level corresponding to the safe maximum pressure of the pressure gauge the pressure protection valve disconnects the pressure gauge from the hydraulic cylinder. This disconnection remains until the operator manually resets the system. In practice the operator is required to press a reset button every time he wants the system to function to assist loading but the disconnection of

II

hydraulic cylinder from pressure gauge so as to protect the pressure gauge from potentially damaging dynamic pressure in transit is automatic.

The gauge 24 is located between the plates 1 9A, 1 9B of the cantilever arm 19 and between the top pivot 19C of the adjustment strut 21 and the torsion box 18 and facing rearwardly so as to be visible to the driver of an excavator when located on the trailer to facilitate adjustment of the correct positions of the excavator.

A simplified embodiment is illustrated in Fig. 13 wherein the height adjustment member (21 or 21'), the locus link bar (22) and the hydraulic strut 23 are replaced by a single strut 25 pivotally mounted between the trailer and coupling mounting member 5 and including a load sensitive means which may comprise the strut 25 being in the form of a hydraulic piston-cylinder arrangement with closed circuit and a fluid pressure gauge used to indicate the pressure thereon representative of the load on the coupling (not shown in detail) (or may be a load cell or other load indicating means) and pressure indicating means (not shown) are also provided such as in the form of an hydraulic pressure indicating gauge (not shown). The position or inclination of the hydraulic strut is controlled or such that there is within a range of height setting at which the device is designed to operate, a substantially constant relationship between the coupling vertical load and thus the hydraulic pressure in the strut. Preferably the dial or indicator of the gauge is visible from the platform of a trailer so that when positioning a vehicle such as an excavator thereon, the driver can readily view and adjust the position thereof.

The suitable pressure gauge position on the trailer enables the operator to readily understand the visual indication and use such in determining as to where to position the cargo so as to achieve an acceptable improved load on the towing vehicle. Alternatively a gauge may be located on the trailer showing height setting against pressure connection factor so that the operator can read the pressure indicated, read the graph and combine the two to collate the actual coupling load.

The embodiment of Fig. 14 according to the invention is provided to enable the load which will be exerted at the towing hitch of the towing vehicle to be ascertained. Thus by presenting the user of the trailer, as he is loading his machine, with a clear visual indication of the pressure in the strut, and therefore the vertical imposed load at the coupling, he is enabled to position his machine correctly.

Thus, fig. 14 represents two possible variants:- 1) a fixed height device, similar to the AD device, giving an accurate load indication but a single fixed height, and 2) a virtually identical device giving a less accurate indication of coupling load but allowing a limited range of height adjustment.

A further simple embodiment of the invention may be achieved by providing a drawbar assembly having a conventional A-frame drawbar, pivoted at the rear, with rigid attachment at the front bulkhead replaced by hydraulic cylinder(s) -which could indicate coupling load. By allowing the hydraulic cylinder(s) some variation in length a degree of coupling height adjustment will be achieved by inclining the drawbar up or down. However inclining a conventional drawbar means inclining the coupling device attached rigidly to its front so the extent of the height adjustment will be limited by the requirement to run the coupling device very close to the horizontal (typically within 3 degrees). In practice there may well be additional difficulties, including instrusion into load-space, associated with the coupling height adjustments needed to reach higher truck coupling heights -i.e. adjustments above the horizontal.

This preferred embodiment described previously does not suffer these inherent limitations.

Claims (16)

1. CLAIMS1. A drawbar assembly of a trailer or other towable vehicle which is connectable by means of a coupling to a towing vehicle, including a pivotable or otherwise displaceable support system for the drawbar wherein a load bearing part of the support system of the drawbar includes a hydraulic piston-cylinder strut or other load responsive/sensing means, and wherein the coupling height is either fixed or adjustable within a range limited such that the load and therefore hydraulic pressure in the cylinder or load on the other load responsive means has a constant or substantially constant relationship with the vertical load at the coupling, and pressure indicating means for indicating, in use, the hydraulic pressure in the cylinder or on the other load responsive means.
2. 2. A height adjustable drawbar assembly as claimed in claim 1, in which the support system is based on a parallelogram mechanism and in which a main load bearing component in reacting the vertical load applied at the coupling is a pivotally mounted load bearing means, (e.g. a hydraulic strut or other load bearing member), the position and inclination of which is controlled such that there is, within the range of height settings at which the assembly is designed to operate, a substantially constant relationship between the coupling vertical load and therefore the hydraulic pressure in the strut or pressure on the other load responsive means.
3. 3. An assembly as claimed in claim 1 or 2, in which a pressure gauge or other load measuring device and/or indicator is positioned on the trailer such as, in use, to give the operator a ready visual indication where of the load resulting from the position of a cargo so as to enable position adjustment to achieve an acceptable imposed load on the towing vehicle.
4. 4. A height adjustable drawbar assembly as claimed in claim 1 or 2, in which the hydraulic strut having the substantially constant relationship with vertical load at the coupling, is of substantially fixed length, and the height adjustment is achieved by a second and adjustable-in-length supporting strut connected thereto by guide means in series with said hydraulic strut and where the guide means connecting the two struts is also the means of positioning and inclining the first strut so as to achieve/maintain the said constant relationship.
5. 5. An assembly as claimed in any of claims 1 to 4, in which said connecting guide means is a locus link member pivotally connected at its rear end to the vehicle and at its front upper end to the upper end of the hydraulic strut or other load responsive/sensing means and an end of an adjustable-in-length/height, and pivotally mounted adjustment strut (21) is maintained on the height adjustment strut (21).
6. 6. An assembly as claimed in any of claims 1 to 5, including a rearward crosspiece (18) forming part of the trailer or other towed vehicle, and link members (15,16) pivotally mounted on the cross-piece and extending forwardly from the crosspiece (18) to a coupling member (10) (which is connectable to a towing hitch) or to the coupling mounting member (5) for receiving a coupling member (10), and pivotally mounted thereon (5) such as to create the movement control of a parallelogram-like linkage (18, 15, 16, 5), and such as to ensure said coupling member (10) or coupling mounting member (5) moves vertically whilst maintaining a horizontal or substantially horizontal disposition during desired adjustment of the height thereof.
7. 7. An assembly as claimed in claim 6, in which the adjustable-in-length supporting strut (21) is mounted on an extension (19) of the crosspiece (18) which crosspiece extends inclinedly forwardly and upwardly, and such supporting strut (21) extends inclinedly forwardly and downwardly.
8. 8. An assembly as claimed in claims 5 or 6, in which the supporting strut (21) is a telescopic strut with one member movable in and out of the other tubular member, and a plurality of through-holes are provided in said members and extending along their length and being alignable to receive a locking pin or like extendable thereth rough in a desired position of adjustment, of the strut, or in which the adjustable supporting strut is an adjustable hydraulic piston-cylinder arrangement, or in which the supporting strut is a screw-jack.
9. 9. An assembly as claimed in any of claims 5 to 7, in which the control/guide arm and mounting arrangement is such that the top pivot of the substantially fixed length hydraulic load sensing cylinder (23) moves such that the desired height adjustment is achieved and, at the same time, there is generated a strut angle (B) which keeps the function (sinB -sinA.cosB) constant or substantially constant and therefor (the relationship between V and Fs constant according to the formula V = Es (sin B -sinA.cosB) achieves) an acceptable level of accuracy for the relationship between the vertical imposed load at the coupling (V), and the load in the hydraulic strut (Fs).
10. 10. An assembly as claimed in any of claims ito 9, in which the assembly and arrangement are such that locus of the top pivot (23A) of the hydraulic cylinder (23) is a circular arc.
11. ii. An assembly as claimed in any of claims ito 10, in which the load bearing responsive part forming the load sensing means, such as a hydraulic piston-cylinder, is inclined to the vertical or received on inclined force transmission element such as to produce a horizontal component of the strut load which is acted upon unequally by the link arms,
12. 12. As assembly as claimed in any of claims ito 11, in which all the pivotal axes are parallel.
13. 13. An assembly as claimed in any of claims 1 to 12, in which the upper pivot axis of the height setting means and at the upper pivot axis of the load responsive/sensing means be above the rearward pivotal axes of the pivotable support system.
14. 14. A drawbar assembly of a trailer or other towable vehicle which is connectable by means of a coupling to a towing vehicle, including a pivotable or otherwise displaceable support system for the drawbar, and inclined support means for the linkage comprising a pivotally mounted load sensing and bearing means (23) connected in series to an adjustment member via a locus link member (22) or other movement path control/guide means, said adjustment member being adjustable in its supporting length so as to raise or lower said load sensing bearing means (23) and thus the drawbar linkage such that there exists a constant or substantially constant relationship with the vertical load at the coupling.
15. 15. A drawbar assembly as claimed in claim 14, in which said support system is mounted at the rear on a trailer part and at the front on a coupling member or mounting therefor and forms a form of or functions as a parallelogram linkage mechanism.
16. 16. An assembly substantially as herein described and illustrated in Fig. 7 having regard to Figs 1 to 13 or as modified by Fig. 14.