With any vehicle the load distribution amongst the vehicle wheels and the traction provided by the wheels is an important design consideration. This is particularly so with vehicles which are intended to have use "off road". For instance, vehicles such as agricultural tractors are often provided with a multi-wheel drive system (typically four-wheel drive). It is also known to provide such vehicles, with a tracked wheel system which may be a full-track or half-track in which only some of the wheels are tracked.

One problem that can occur with conventional wheeled and tracked vehicles is a loss of traction when the vehicle is used to tow a heavy load due to the front wheels (or a front part of the track) lifting from the ground. For instance, consider a traditional four wheel rear drive agricultural tractor towing a heavy load such as a tree trunk. The weight of the load can cause the front end of the tractor to rise (essentially pivoting about the rear axle) so that the front wheels are raised from the ground (the effective load will increase if the local meets an obstruction). If the tractor is a four wheel drive tractor any contribution to traction made by the driven front wheels will be lost. In addition, the increased load on the back wheels can cause the wheels to "bog down". Moreover, if the load is sufficiently high the tractor will simply tend to overturn rather then move forwards.

Similar problems can occur when a tractor is used to tow an implement such as a plough, there will be tendency for the front end of the tractor to lift as the implement meats an obstruction increasing the load at the back of the tractor. Modern tractors are conventionally fitted with a hydraulic system (described further below with reference to Fig. 1) designed to obviate this problem. Such systems generally work well but nevertheless if the load at the back of the tractor exceeds a certain maximum then the front end of the tractor will inevitably rise. The maximum resistance to lift provided by the tractor is limited by the mass of the tractor ahead of the rear axle about which the tractor will tend to pivot.

It is an object of the present invention to obviate or mitigate the above disadvantages.

According to the present invention there is provided a vehicle wheel mounting assembly comprising a beam provided with a wheel mount adjacent each end for mounting respective wheels thereto, pivotable mounting means intermediate the beam ends for mounting the beam to a vehicle so that it is free to swing about an axis substantially parallel to the wheel mount axes, wherein biasing means are provided to provide a selectively variable biasing force to resist swinging of the beam relative to the vehicle.

The biasing means is preferably adapted to selectively provide either a zero biasing force or a maximum biasing force sufficient to lock the beam in a fixed position relative to the vehicle.

Preferably the biasing means is adapted to selectively provide forces between said zero force and said maximum force.

In preferred embodiments of the invention the biasing means is adapted to provide a variable biasing force automatically determined as a function of a monitored load applied to the vehicle.

Preferably the biasing means is also operable to force the beam to pivot and thereby raise either one end of the beam relative to the vehicle to lift the respective wheel from the ground.

The biasing means is preferably hydraulic.

For instance the biasing means may include at least one hydraulic ram one end of which acts on the vehicle (for instance acting on a non-pivoting part of the wheel mounting assembly) and the other end of which acts on the beam.

The wheel mounts are preferably arranged to mount wheels in line with one another.

The assembly according to the present invention can be used to mount wheels to a vehicle in place of conventional driven or non-driven wheels, but is particularly advantageous where it replaces driven wheels. Thus, the assembly is preferably adapted for connection to the vehicle powertrain and includes means to transmit power from the said vehicle powertrain to at least one wheel mounted on the carriage.

Preferably the carriage includes means to transmit power from the vehicle powertrain to each wheel mounted on the carriage.

Preferably the mounting means is adapted for mounting the carriage to a vehicle axle assembly so that the carriage pivots about the vehicle axle axis. For instance, the carriage may be designed as a simple retro-fit assembly for replacing a wheel of an otherwise conventional vehicle.

For instance, in preferred embodiments of the invention the mounting means comprises an inner portion securable to the vehicle about the vehicle axle and an outer portion which is mounted on and pivots about said inner portion. The inner portion may preferably comprise a first generally cylindrical member for mounting concentrically about the vehicle axle and said outer portion may preferably comprise a second generally cylindrical member which is concentric about said first member and rotatable with respect thereto.

Since at least some of the wheels mounted to the carriage are preferably driven, the mounting means is preferably adapted for mounting the carriage about a driven axle of the vehicle.

In preferred embodiments of the invention the means for transmitting power to one or more wheels mounted on the carriage comprises a chain or belt drive between the driven vehicle axle and the or each wheel mount. For instance, each wheel mount may comprise an axle mounted for rotation within bearings housed within the carriage, and a sprocket or pulley wheel may be mounted to said axle to receive said chain or belt. The or each wheel mount sprocket or pulley wheel may then be driven by a chain or belt from a respective sprocket or pulley wheel mounted on the vehicle axle.

The chain drive or belt drive is preferably housed within a housing which supports said wheel mounts and which is configured as an arm which rocks about said vehicle wheel axle axis.

In one preferred embodiment of the invention in which the assembly is adapted for mounting about a driven axle of a vehicle, said axle being driven by a differential gear system, the means for mounting the assembly to the vehicle comprises an inner assembly including a first generally cylindrical member and means for attaching said first cylindrical member to the vehicle differential housing such that it is fixed in position relative thereto and concentric with the axle, and an outer assembly which includes a second generally cylindrical member which is concentric with said first cylindrical member and rotatable with respect thereto, and wherein said wheel mounts are provided in a portion of the carriage which is secured to said outer assembly such that it pivots about the inner assembly.

Although in the above summary of the invention, reference is made to wheels mounted to the beam, it will be appreciated that those wheels may be part of a track system. That is, the wheels may be fitted with a vehicle track encompassing all wheels mounted to the carriage. For instance, a conventional vehicle can be converted to a half-track vehicle in accordance with the present invention by replacing the standard rear wheels with carriages according to the present invention fitted with tracked wheels.

The present invention also provides a vehicle fitted with one or more vehicle wheel assemblies as defined above.

The present invention also provides a vehicle of the type fitted with a system for monitoring load applied to the vehicle rearward of the rear driven axle, comprising a wheel mounting assembly fitted to the rear driven axle, wherein the biasing means is adapted to provide a variable biasing force determined automatically as a function of the load applied to the vehicle as monitored by said monitoring system.

For instance the vehicle may be a tractor and said load monitoring system may be a hydraulic system linked to towing arms which monitors towing load and hydraulically raises and lowers the towing arms in response to the monitored load, wherein the biasing means produces a continuously varying biasing force proportional to the monitored load as it increases and decreases above a given threshold.

Similarly, the present invention provides a method of modifying a conventional vehicle by removing a standard vehicle wheel therefrom and replacing this with a wheel mounting assembly according to the present invention.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic illustration of a conventional prior art agricultural tractor; Fig. 2 is a front view of a vehicle wheel assembly in accordance with one embodiment of the present invention; Fig. 3 is a part-section through the assembly of Fig. 2 looking in the direction of arrow Fig. 4 is a section through Fig. 2 taken on the line 4-4; Fig. 5 is a view of part of the assembly seen from direction of arrow "B" of Fig. 3, showing some hidden detail; Figs. 6a and 6b illustrate an agricultural tractor according to the present invention; Figs. 7a and 7b illustrate a conventional tracked vehicle towing a heavy load; Fig. 8 is a further drawing of the tractor of Figs. 6a and 6b (only part of which is shown); and, Figs. 9a and 9b are examples of further vehicles modified in accordance with the present invention.

Referring first to Figure 1, this illustrates a conventional agricultural tractor 100 towing a plough 101. The tractor 100 is fitted with a conventional hydraulic system for attaching the plough 101. The plough is pivotably fixed to the ends of two lower towing arms 102 (only one of which is visible in Fig. 1) which are pivotably secured at their other ends to the tractor below the rear differential. An upper telescopic towing control rod 103 is pivotably connected between the plough and the tractor (forming the third mounting point of a three point mounting arrangement).

The height of the plough is controlled by the hydraulic system via lift arms 104 which are linked to each lower tow arm 102 by link arms 105 (only one each of the arms are shown).

As the tractor tows the plough, the plough will tend to dig into the ground increasing resistance which in a traditional tractor (not fitted with the illustrated hydraulic system) would tend to produce a moment about the back axle causing the front end of the tractor to lift. However, with the illustrated hydraulic system, as the plough resistance increases, the plough will tend to pivot about its connection to the lower tow arms 102 thus compressing the telescopic upper control rod 103. The control rod is linked to the hydraulic system so that as it compresses it operates a hydraulic valve which causes the lift arms 104 to raise the lower towing arms 102 and thus raise the plough. This in turn reduces the resistance and the telescopic arm 103 extends causing the hydraulic arms 104 to lower the plough once more. Thus, there is a constant adjustment of the height and resistance of the plough tending to maintain an equilibrium resistance (which can be set at a desired level by the vehicle operator) for the plough extending a given depth into the ground. The system works well but there is a limit to the load on the plough that can be compensated for by the hydraulic system. This limit is governed by the weight of the tractor ahead of the rear axle which contributes to a moment about the axle tending to resist lifting of the front end of the tractor. If, the load necessary to lift the plough exceeds this limit the front end of the tractor will lift from the ground. Any traction offered by the front wheels is thus lost.

A similar problem occurs on turning the tractor. To do this, it is necessary to completely lift the plough from the ground. However, given the weight of the plough compared to the weight of the conventional tractor, it is necessary to apply a hydraulic lifting force to both the front and back of the plough in order to prevent the front end of the tractor rising from the ground as it attempts to lift the ground.

The wheel mounting assembly according to the present invention obviates these problems. The particular embodiment described below is designed as a retro-fit assembly which may be fitted to the rear drive axle assembly of a conventional tractor such as that shown in Figure 1 in place of the standard rear wheels.

Referring first to Figures 2 to 5, the illustrated wheel mounting assembly comprises a pivoting beam 1; a mounting assembly, indicated generally by reference 2, for mounting the beam 1 to a vehicle; and a hydraulic ram 3 which is connected between the beam 1 and a non-rotating part of the mounting assembly 2. The arm 1 has an inverted "v" configuration and houses a wheel mount, indicated generally be references 3, at each of its ends. The mounting assembly 2 is located centrally between the wheel mounts 3 in the region of the apex of the inverted "v".

Each wheel mount 3 (Fig. 4 illustrates the wheel mount 3 not visible in Fig. 3) comprises an axle 4 which rotates on bearings 5 housed in respective bearing housings 6a and 6b in front and rear faces of the arm 1. The wheel mounts are adapted to receive conventional wheels (a conventional wheel hub is illustrated in chain-dot in Fig. 2 by reference 7). Each axle 4 is fitted with a chain sprocket 8 located within the arm 1. Each sprocket 8 forms part of a chain drive system housed within the arm 1 for delivering power to the wheel mounts 3 from the vehicle axle 10. The chain drive system further comprises sprockets 9a and 9b which when the assembly mounted to a vehicle are fixed to the vehicle axle 10 and connected to respective sprockets 8 by a chain (not shown). Since the sprockets 9a and 9b are mounted side by side the sprockets 8 of each wheel mount 3 must be offset slightly relative to one another to be in line with the respective sprocket 9a and 9b. This offset is evident by comparison of the wheel mount shown in Fig. 2 with that shown in Fig. 3.

The mounting assembly 2 comprises an inner cylindrical member 11 which is welded at one end to a cover member 12. The other end of the cylindrical member 11 terminates at (but is not fixed to) the arm 1 and houses a bearing 13 which is held in position by a bearing plate 14 which bolts to the cylindrical member 11. An outer cylindrical trunion 15 which supports the beam 1 (bolted thereto) is disposed concentrically around the inner cylindrical member 11 on bearings 16 such that it is rotatable upon the inner member 11. The trunion 15 is axially shorter that the inner cylindrical 11 and is closed at one end by a bearing cover plate 17 (which is fixed to the cover member 12) and at its other end by bearing cover plate 14. A bracket member 31 is welded to the cover 12 to provide a mounting location for one end of the hydraulic arm 30, the other end of which is mounted to the upper edge of one arm of the beam 1.

The above described embodiment of the invention is adapted for retro-fitting to the rear axle assembly of a conventional four wheel drive agricultural tractor (replacing each standard rear wheel). To mount the assembly to the rear axle in place of a standard rear wheel, the cover member 12 and inner cylindrical member 11 are placed over the axle 10 (which passes through the centre of cylindrical member 11 and through bearing 13) and is secured to the vehicle by bolting the cover member 12 to the differential housing (not shown) in place of the standard bearing cover plate.

The trunion 15 and bearings 14 are then assembled on the inner cylindrical member 11 and bearing plate 14 is bolted in position. The beam 1 is then bolted to the tumion 15 and the sprockets 9a and 9b are mounted to the end of the axle 10 which protrudes from cylindrical member 9 and drive chains are fitted. A bearing 18 is then fitted to the beam 1 at the end of the axle 10.

Once the assembly is mounted to the vehicle, the trunion 15, and beam 1 which is attached thereto, are free to pivot about inner cylindrical member 11. The driven axle 10 rotates within inner cylindrical member 11 on bearings 11 and 18 and in doing so rotates sprockets 9a and 9b which drive the wheels mounted to the arm 1 via respective chains (not shown) connected to the sprockets 8. Thus, by replacing a wheel of a conventional vehicle with the assembly according to the present invention, that wheel is replaced by two wheels, which, in this embodiment, are both driven.

Figs. 6a and 6b illustrate a tractor such as that shown in Fig. 1 fitted with wheel mounting assemblies according to the present invention to the rear drive axle, so the tow conventional rear wheels are replaced by 4 wheels; a front pair 32 not a rear pair 33. Fig. 6a illustrates the tractor in normal operating conditions with the hydraulic towing system functioning as described above to maintain the plough resistance at an equilibrium. However, with the invention, operation of the hydraulic ram 30 is linked to the load monitoring system of the conventional hydraulic towing arrangement (by a conventional hydraulic valving system). Under most conditions, the pressure in the hydraulic ram 30 will be effectively zero allowing the beam 1 to pivot freely about the rear axle. Power from the rear driven axle is then divided between the four wheels 32 and 33 (rather than two wheels) which provides an increased "foot print" contacting the ground giving improved traction and reduced pressure. However, should the monitored load approach a level which would otherwise be sufficient to raise the front end of the tractor, despite the operation of the conventional hydraulics, pressure in the hydraulic ram 30 is automatically increased to offer a certain amount of resistance to pivoting of the beam 1. The effect of this is best illustrated by first considering a situation in which the pressure in ram 30 is increased to such a level to effectively lock the beam 1 against pivoting. This would prevent the tractor from pivoting about the rear axle but rather in order for the front end of the tractor to lift the tractor must pivot around the axis of the rear wheels 33.

Since the pivot axis moves back this increases the weight of the tractor ahead of the pivot axis and therefore increase the moment which resists tipping of the tractor.

Thus, a greater load at the rear of the tractor would be required to lift the front end of the tractor and thus the tractor can accommodate high loads without the front wheels lifting. It should be noted, that in addition to the weight of the tractor body ahead of the wheels 33, the weight of the beam 1 and front pair of wheels 32 will also contribute to the moment resisting overturning of the tractor.

Thus, it will be seen that the towing load which the tractor can accommodate without tipping can be increased to a maximum by locking the beam 1 against pivoting, and this maximum load will be greater than that that could be accommodated by the unmodified conventional tractor. However, as the pivoting moment acts about the rear wheels 33 this will tend to increase the load on the those wheels and lighten the load on the wheels 32. It is, however, beneficial to maintain as even a load distribution as possible between the front wheels 32 and rear wheels 33 to maintain a large effective "foot print" providing traction.

If the maximum moment resulting from the towing load which the unmodified conventional tractor could accommodate without the front wheels lifting is designated m (which corresponds to the load required to tip the tractor modified in accordance with the present invention with the beam 1 completely free to pivot), and the moment resulting from maximum load which the modified tractor can withstand without tipping with the beam locked against pivoting is designated M, there will be a range of loads corresponding to moments between m and M which the modified tractor can accommodate. For instance, as the tractor tows the plough, provided the moment resulting from the towing resistance remains below m the conventional hydraulic system will be able to accommodate the changing towing load and the beam 1 can be left to pivot freely providing an even distribution of load between the wheels 32 and 33. However, for loads generating moments between m and M it is not necessary to completely lock the beam 1 to prevent the front end of the tractor lifting. Rather, the hydraulic ram 30 is linked to the monitoring system of the conventional hydraulic towing system so that pressure in the hydraulic ram is automatically increased to provide a biasing force providing resistance to pivoting of the beam 1 in proportion to the monitored load. This biasing force generates a moment about the tractor rear axle 10 which opposes lift of the front wheels. However, another effect of the resistance provided by hydraulic ram 30 is an increase of load on the rear wheels 33 with a proportional decrease of the load on the wheels 32. Thus, the system according to the present invention applies a minimum pressure to the hydraulic ram 30 required to provide the minimum biasing force necessary to prevent the tractor tipping and thereby minimising the transfer of load from wheels 32 to 33. Thus, as the tractor tows the plough the hydraulic ram 30 and pivoting beam 1 work in conjunction with the conventional hydraulic system to maintain the tractor level at loads which a conventional tractor would not be able to accommodate, whilst at the same time maintaining an optimum distribution of load between the front wheels 32 and rear wheels 33.

Should the load exceed the maximum load M, rather than lock the beam 1 (so that the tractor pivots around the rear wheels) as an alternative the beam 1 could be left free to pivot so that as the front end of the tractor rises all four wheels 32 and 33 remain in contact with the ground providing as larger foot print as possible. This is illustrated in Fig. 6b.

The wheels mounted 32 and 33 to the carriage can be conventional tired wheels, or can be fitted with tracks as illustrated in chain-dot in Figures 7a and 7b.

Such tracks could be simple re-enforced rubber bands or metal tracks and can be fitted around the standard wheel tires. For instance, the tires may be deflated to allow the track to be fitted in place, and then re-inflated to tension the track. The result is a half-tracked vehicle which has advantages over conventional track vehicles which behave as illustrated in Figs. 7a and 7b, 7b illustrates lift due to a load applied to the rear of the vehicle.

In addition to the operation of the system as described above, the hydraulic ram 30 can also be used to lock the beam 1 against pivoting when it is desired to lift the plough (or other implements which the tractor may be towing) from the ground in order to turn the tractor. For instance, with conventional tractors fitted with existing hydraulics, it is necessary to hydraulically lift both the front and the back of an implement such as a plough to avoid the tractor tipping. The present invention overcomes this.

A further advantage of the present invention is that the hydraulic ram 30 can be used to raise or lower either wheels 32 or wheels 33 from the ground so that the vehicle rides on wheels 32 or 33 only (together with the front wheels) for instance, by raising the rear wheels 33 so that the vehicle rides on wheels 32 the wheel-base of the vehicle is effectively shortened. This advantageously reduces the turning circle, and thus removability, the vehicle. Furthermore, alternate lifting of the wheels 32 and 33 can be used to help the vehicle "clime" over obstacles.

The wheel mounting system according the present invention also provides advantages during other uses. For instance, the pivoting beam 1 provides greater vehicle stability, and reduced bounce, and less heaving on uneven ground. Also, because the pressure is distributed to a greater number of wheels the vehicle can support a higher axle load and higher drawbar weight. The ability of the pivoting beam to reduce heave on uneven ground is illustrated in Fig. 8 in which it can be seen that the beam pivots to maintain the rear wheels 33 in optimum contact with the ground as the vehicle travels over uneven ground whilst at the same time reducing the disturbance to the vehicle body.

The wheel mounting assembly according to the present invention also provides advantages and the vehicle travels up an incline. That is, as a conventional vehicle travels up an incline the effective load tending to overturn the vehicle increases. With the present invention this increase and effective load can be counteracted by appropriate stiffening of the hydraulic ram 30, if necessary to the full extent that is required to lock the beam 1 against pivoting.

It will be appreciated that wheel mounting assemblies in accordance with the present invention could be fitted to a variety of different vehicles, and further examples of such vehicles are illustrated in Figs. 9a and 9b.

In addition, the wheel mounting assemblies could be fitted to both front and rear axles of a four wheel drive vehicle. Moreover, the chain drive system could be omitted and the carriage could be fitted to a non-driven axle and still provide advantages over conventional wheel systems.

It will be appreciated that many modifications could be made to the detail of the assembly described above, for instance to adapt the assembly for retro-fitting to different vehicles. For instance, the mounting means provided for mounting the beam to the vehicle may comprise a simple securing means fixed relative to the arm, the pivoting motion being provided by a pivoting or rotating part of the vehicle to which the carriage is mounted. In addition, the hydraulic ram could be filled between the beam 1 and another fixed part of the vehicle, not necessarily fixed to the mounting assembly 2. Also, the ram 30 could be replaced or supplemented by another ram, for instance connected to the other arm of the beam 1. As a further alternative the hydraulic ram could be replaced with other forms of biasing means.