The market for agricultural tractors is demanding higher speeds, and legislation is gradually being introduced to allow such vehicles to run at 50 kph and 60 kph. Most tractor manufacturers are attempting to meet this market by offering tractors having a rigid axle without any suspension system or springs and dampers as is used on cars and trucks. As a result of forced vibrations delivered to the system by the road surface, the out of round of the wheel assembly and the out of round of the tyres, the system is diable to arrive at a resonance frequency, with the consequence that the axle will start to bounce in an uncontrolled manner, with the result that the operator is unable to steer or stop the vehicle. This is clearly an unacceptable situation.

To enable such a situation to be avoided, both the tyre makers and the wheel makers are being pressed to produce increasingly accurate products. For the wheel maker by careful control of the manufacture of rim rolled rim and the disc it has been found possible to produce a wheel with a run out of 2. 5mm for front wheels and 3. 5mm for rear wheels in their factories. Wheels made to this tolerance have been found to be satisfactory for 50 kph use, but it is certain that even more accurate wheels will be needed for 60 kph.

When the wheel is designed with a fixed off-set, that is the disc is welded into the rim, this run out condition is guaranteed for the life of the wheel.

The difficulty for the wheel maker is to establish a design which will provide the same guarantee of run out on variable track wheels. A variable track wheel has a disc which can be detached from the rim by a farmer to allow one machine to be used for various crop row spacings.

Agricultural wheel makers already supply such wheels, and a number of design solutions are available. For high speed tractors the problem for the wheel maker is to find a form which will"by design"inevitably reproduce the run out condition which can be established when the disc is assemble in the wheel at the factory.

The most common design for adjustable wheels is to weld brackets to the inside of the wheel rim to which the disc is bolted. These brackets and the associated discs are designed to provide the farmer with 8 track widths to match the various crop row spacings, and it is accepted in the market that this is sufficient.

There is a further demand on the design for high speed wheels due to the much higher dynamic loads experienced at high vehicle speeds. For fixed track designs this can and is accommodated by using thicker rim material and/or higher strength steels, thicker disc material and/or higher strength steels. It is also common practice to make the disc a full circle providing 360° support to the rim to withstand the higher loads.

To provide a similar design for adjustable wheels, there are various offerings incorporating a shaped ring welded into the rim to which can be attached a disc, the required off-sets being obtained by having a stepped pressed ring (as the waffle) or using spacer washers (as GKN) to ensure accurate re-fitting of the disc by the farmer. Dowel pins in the ring or the disc which locate in accurately positioned holes in their mating parts have been proposed.

An object of the invention is to provide an improved wheel as compared to said abovementioned known arrangements.

According to the invention there is provided a vehicle wheel comprising a wheel rim and a wheel disc, the wheel disc being releasably securable within the rim in a predetermined position, the wheel rim having in its interior a multiplicity of fixing means for securing the wheel disc to the wheel rim, the wheel disc having respective locators arranged to co- operate or substantially to co-operate with the fixing means, thereby to position the wheel disc relative to the wheel rim in said predetermined position, and fixing elements for releasably securing the wheel disc to the wheel rim.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which : Figure 1 is a sectional side view through a wheel of the invention, Figure 2 is a fragmentary, schematic view on arrow A of Figure 1, Figure 3 is an enlarged fragmentary view of part of the wheel shown in Figure 1, being a sectional view on line B-B of Figure 4, and Figure 4 is a view on line C-C of Figure 3.

Figure 1 shows a vehicle wheel of the invention, this being a variable track or adjustable wheel, i. e. one having a non-fixed off-set. The disc can thus be detached from the rim and repositioned to provide the vehicle with a different track width. The invention is particularly concerned with such wheels intended for use on a high speed agricultural tractor. As used herein, the term'disc'includes both circular and non-circular plates, all of which are adapted for fitting the wheel to a vehicle axle/hub.

The wheel 10 is made up of a steel rim 11, of substantially conventional profile, and a steel disc 12 releasably secured therein. The disc has a central circular hole 13 and surrounding bolt holes 14 for mounting the wheel to a vehicle axle in the normal manner.

At at least two, normally diametrically opposed, positions, but in the illustrated embodiment at eight equi-spaced positions around the interior of the rim at the centre thereof, are steel lugs 15. Each lug is of generally elongated tubular form and its lateral cross-sectional shape can be seen from Figure 4, namely an inner semi-circular portion 16 from which respective opposite side arms 17,18 extend to the inner surface of the rim, each arm being outwardly concave curved, and the arms extending away from one another i. e. being splayed apart. The arms are welded both at their sides to the inner surface of the rim, as at 19, and also at both ends, as at 20. The portion 16 defines a part-circular bore for reception of a shank 21 of a fixing bolt 22 with a nut 23, for a purpose to be described.

The lug is of constant wall thickness.

As shown in Figure 2, the disc is not truly circular, but is, in this example octagonal, there being at a short straight section at the junction of each of the sides a steel locator 24, each being to mate with one of the eight lugs 15 described above. The disc could alternatively be wholly circular and the locators equi-spaced therearound.

Each locator 24 is in the form of a short part-cylindrical wall welded to a side of the disc adjacent the periphery thereof, as shown in Figures 3 and 4. The wall has constant wall thickness and its inner concave curvature matches the external curvature of the portion 16 of lug 15 so that it can matingly engage thereon. For clarity Figures 3 and 4 show an exaggerated clearance between the locators 24 and lug 16. Normally there would be a close fit, or actual engagement for at least some of the locators and lugs.

The wall is arranged on the disc so that when it mates with the external surface of a lug it is symmetrically disposed relative thereto, as shown in Figure 4, extending equally at either side of a line centrally through the lug, such a line corresponding to the section line B-B of Figure 4.

The disc has a circular hole 25 therethrough between the wall and the short straight peripheral surface of the disc, the cylinder of which the wall is part having the same centre as the hole 25. Moreover the hole 25 is aligned with the part-circular bore defined by the portion 16 so that the shank 21, which substantially fills the hole 25, can extend through the hole 25 and lug 15, as shown in Figure 3, with a head 25 of the bolt engaging against the disc which itself abuts an end of the lug. With the nut 23 tightened as in Figure 3, the disc is securely attached to the rim, the attachment being the same at all the lugs provided.

As shown in Figure 3, an identical wall is also welded as a locator at the opposite side of the disc at the same relative position. This enables the disc, which, as shown in Figure 1, is not symmetrical radially, to be fitted, in the same orientation, at either end of a lug, thereby altering the axial position at which the disc is fixed in the rim, the bolt and nut fixing being reversed if necessary.

Accordingly the locator provides a positive location onto the external profile of the lug, which profile need not be arcuate but could be square, rectangular, pentagonal etc. , or even merely a straight surface to mate with the locator surface. The profile is preferably formed by forging.

Accurate repositioning of the disc when it is remounted in the rim is thus ensured. The locators can themselves be accurately positioned and welded into position using appropriately designed jigs. By providing the locators on both sides of the disc, all track positions are catered for within the single disc and lug design. There is no need for pins to be used as locators, and there is thus no possibility of a farmer or other using mis- assembling the disc by removing the pins. The embodiment illustrated is particularly advantageous in that the partially circular shape of the locator not only provides accurate relocation, but is also favoured strength-wise, being difficult to damage either on purpose or by accident. Any dirt which will accumulate therein or thereat can easily be cleaned out or off by virtue of its open nature.

Instead of there being holes in the disc for the bolts, the bolt heads could instead merely engage an outer surface area of the disc to hold it in abutment with an end of a lug. Moreover the bore for the shank provided by the lug could be such as to provide a tight fit for the shank, particularly if there is no hole through the disc for the shank to pass through.

Naturally the locators could be provided at one side only of the disc if full versatility of variable tracks was not required.