WO2000021821A1

WO2000021821A1 - Self tracking mechanism for axle assemblies

Info

Publication number: WO2000021821A1
Application number: PCT/AU1999/000877
Authority: WO
Inventors: Brian Mckee
Original assignee: Raleigh Corporation Pty. Ltd.
Priority date: 1998-10-13
Filing date: 1999-10-13
Publication date: 2000-04-20

Abstract

A mechanism is disclosed for fitting to a rigid axle assembly (2) for mounting such an axle assembly (2) from a vehicle chassis (C) the mechanism including a support member (12) mounted to the chassis (C) so as to be pivotable about a generally horizontal first axis (23), connection means enabling connection of the support member (12) to a rigid axle of the axle assembly (2) at or near a mid point of the axle, the connection means permitting pivoting of the axle relative to the support member (12) about an upright castoring axis (24, 25) located either in front of or rearwardly of an axis of rotation (5) of wheels (4) operably mounted to opposed ends of the axle, the mechanism being such that only one of the castoring axes (24, 25) can be operational at any one time, and suspension means (8, 9) being provided to transfer vertical loads from the vehicle chassis (C) to the axle assembly (2).

Description

SELF TRACKING MECHANISM FOR AXLE ASSEMBLIES

The present invention relates to axle assemblies for multiple axle vehicles, including articulated vehicles, and in particular to improvements in tracking of undriven, non-steering axle assemblies. With large multi-axle vehicles it is desirable from the point of view of minimising tyre wear and maintaining vehicle adequate stability and control that the axis of rotation of each wheel maintains, at least approximately, perpendicularity to the line of travel of the wheel. This is here termed a "tracking" ability. With axle assemblies which are fixed in the sense of having no provision for tracking, cornering may involve wheel skid which significantly increases tyre wear and may reduce vehicle controllability. Arrangements which automatically maintain or partly maintain wheel tracking as close as possible to the ideal when cornering are known in the art. Such assemblies commonly include outboard stub axles to which the wheels are mounted hinged to a central main axle part permitting the wheel orientation to vary relative to the main axle part, and a linkage mechanism to align each wheel with its intended path. Arrangements of this kind are relatively complicated and expensive and moreover are not easily retrofitted to a vehicle that had fixed axle assemblies installed when initially built. Trailer parts of articulated vehicles, for example, commonly have fixed axles, but may be difficult and expensive to build or retrofit with assemblies of this kind.

In another arrangement known in the art, an axle assembly to which ground engaging wheels are fixedly mounted, i.e. with no ability to pivot in the way described above, is pivotable about a generally upright axis fixed in the vehicle chassis. The axis is located forward of a line between the wheel centres so that the wheels track by virtue of "castoring" of the entire axle assembly about the axis. In Australian Patent No. 680913, there is disclosed an axle assembly and suspension of this class, which is capable of use to modify existing fixed axle assembly vehicles in a comparatively inexpensive and simple manner.

In this earlier specification, a mechanism is disclosed which is adapted to be fitted to a rigid axle assembly of a multiple-axle vehicle, said mechanism comprising a support member adapted to be mounted to a chassis of said vehicle, first connection means enabling connection of said support member to a rigid axle of said rigid axle assembly at or adjacent a mid point of said rigid axle, said first connection means permitting movement of said rigid axle relative to said support member about a generally upright axis whereby opposed lateral ends of said rigid axle can move forwardly or rearwardly relative to the chassis of said vehicle, second connection means enabling pivotal connection of said support member to the chassis of said vehicle whereby said support member pivots relative to said chassis about one or more generally horizontal axes, and control means to control movement of said lateral ends of said rigid axle within predetermined limits in a forward or rearward direction. In the mechanism disclosed in Australian Patent No. 680913, it has been found, however, that the arcuate bearing arrangement located between the lateral ends of the rigid axle and the suspension means (such as leaf springs or lever-mounted air bags) are relatively complicated. Accordingly, it is one objective of the present invention to provide a further simplified form of self adjusting axle assembly of the type disclosed in Australian Patent No. 680913. Preferably such a self adjusting axle assembly will be adapted for use with an air bag suspension arrangement.

According to a further first preferred aspect, the present invention aims to provide an arrangement particularly suited to preventing self steering or automatic tracking adjustability when the vehicle is operated in a reversing mode.

According to a still further second preferred aspect, the present invention has as one of its objectives, the provision of a suspension and wheel tracking mechanism for an axle assembly of a vehicle which provides for resilient suspension action, castor-type tracking ability in both forward and reverse directions and which is at least reasonably suitable for fitting to existing vehicles as well as for use in new vehicles. By enabling castoring action of an axle assembly to take place while reversing, maneuverability, controllability and minimisation of tyre wear should be improved both in forward travel and when reversing.

According to a first aspect of the present invention, there is provided an axle mounting arrangement for use on a vehicle having at least one non-driven axle carrying at least one ground engaging wheel at either end and a suspension system including at least one elongate member extending in a fore and aft direction of the vehicle on opposed sides of the vehicle, each of said elongate members being adapted for connection to a respective end zone of the non-driven axle, said elongate members being mounted for at least limited movement in said fore and aft direction, said axle mounting arrangement including a support member adapted to be mounted to a vehicle chassis frame member by first connection means whereby said support member is pivotal about at least one generally horizontal pivot axis, second connection means enabling connection of said support member at or adjacent to a mid point of the non-driven axle whereby the non-driven axle is pivotable about a first generally upright pivot axis relative to said support member, and third connection means connecting opposed end zones of the non-driven axle to a respective said elongate member, said third connection means permitting pivotal movement between each said elongate member and said non-driven axle about second and third upright pivot axes spaced from but parallel to said first upright pivot axis. Preferably the first upright pivot axis is spaced forwardly of axes of rotation of adjacent said ground engaging wheel or wheels. Conveniently the second and third upright pivot axes intersect said wheel axes of rotation. In accordance with a preferred embodiment, each of the end zones of the non-driven axle carries a bearing support structure secured to the axle, the bearing support structure carrying bearing assembly means permitting rotational movement about one of the second or the third upright pivot axes and thrust force resistance in the direction of the second or the third upright pivot axes. Preferably the bearing assembly means includes a first bearing means permitting rotational movement about one of the second or the third upright pivot axes and a second bearing means capable of thrust force resistance in the direction of the second or the third upright pivot axes. The arrangement may further include clamping means arranged to clamp said bearing support structure and bearing assembly means to a said elongate member of the suspension system.

In accordance with a still further preferred embodiment, each of the end zones of the non-driven axle has a bearing support structure secured thereto extending upwardly and downwardly from the axle, each of said bearing support structures being adapted to carry bearing means co-axial with one of the second or third upright pivot axes, and at least one of said bearing support structures being adapted to carry thrust bearing means for resisting thrust loads in the direction of said second or third upright axes, an upper saddle member cooperable with the upwardly extending bearing structure and a lower saddle member cooperable with the downwardly extending bearing structure, and at least one clamping member positionable below the elongate member and cooperable with bolt means to secure said saddle members and said clamping member or members together with a said elongate member located between the lower saddle member and the clamping member or members.

According to another aspect of the present invention there is provided an axle mounting arrangement for use on a vehicle having at least one non-driven axle carrying at least one ground engaging wheel at either end and a suspension system including at least one elongate member extending in a fore and aft direction of the vehicle on opposed sides of the vehicle, said elongate members being adapted for connection to a respective end zone of the non- driven axle, said elongate members being mounted for at least limited movement in said fore and aft direction, said axle mounting arrangement including a support member adapted to be mounted to a vehicle chassis frame member by first connection means whereby said support member is pivotal about at least one generally horizontal pivot axis, second connection means enabling connection of said support member at or adjacent to a mid point of the non-driven axle whereby the non-driven axle is pivotable about a first generally upright pivot axis relative to said support member whereby the end zones of the non-driven axle are movable in a forward or rearward direction within predetermined limits, the or each said elongate member having a forward end pivotally supported by a rocker arm with a pivotal axis of each said rocker arm pivotal connection to a said elongate member coinciding with the at least one generally horizontal pivot axis of the support member in at least one position of use. Preferably the at least one position of use is a neutral position where the non-driven axle is at right angles to a forward direction of movement of the vehicle. According to a still further aspect of the present invention there is provided an axle mounting arrangement for use on a vehicle having at least one non-driven axle carrying at least one ground engaging wheel at either end and a suspension system including at least one elongate member extending in a fore and aft direction of the vehicle on opposed sides of the vehicle, said elongate members being adapted for connection to a respective end zone of the non-driven axle, said elongate members being mounted for at least limited movement in said fore and aft direction, said axle mounting arrangement including a support member adapted to be mounted to a vehicle chassis frame member by first connection means whereby said support member is pivotal about at least one generally horizontal pivot axis, second connection means enabling connection of said support member at or adjacent to a mid point of the non-driven axle whereby the non-driven axle is pivotable about a first generally upright pivot axis relative to said support member whereby the end zones of the non-driven axle are movable in a forward or a rearward direction within predetermined limits while the vehicle is moving in a forward direction, said axle mounting arrangement further including means for selectably locking the end zones of the non-driven axle in a neutral position at right angles to a direction of movement of the vehicle when the vehicle is moved in a reverse direction. According to yet another aspect of the invention, there is provided a mechanism adapted to be fitted to a rigid axle assembly for mounting the said assembly to a vehicle chassis, said mechanism comprising: a support member so mounted to the chassis as to be pivotable about a generally horizontal first axis; connection means enabling connection of said support member to a rigid axle of said rigid axle assembly at or adjacent a mid point of said rigid axle, said connection means permitting pivoting of said rigid axle relative to said support member about a generally upright castoring axis; and suspension means adapted to transfer vertical loads from the chassis to said axle assembly, wherein said connection means is adapted to enable selectable location of said castoring axis in a position ahead of or behind axes of rotation of wheels mounted to said rigid axle. As it may not always be desired to have castoring action of the axle assembly, it is preferred that the mechanism further include means operable by a user whereby pivoting of said rigid axle assembly about said castoring axis is preventable. In a particularly preferred embodiment, said connection means includes: a first member so secured to the support member as to permit relative rotation of the first member and the support member about a second axis ahead of the axes of rotation of the wheels mounted to said rigid axle, said second axis being generally upright; a second member secured to or formed as part of said rigid axle and so secured to the first member as to permit relative rotation of the first member and the second member about a third axis behind the axes of rotation of the wheels mounted to said rigid axle, said third axis being generally upright; and at least one locking means operable selectively either to prevent said relative rotation of the first member and the support member so that said castoring axis is said third axis, or to prevent said relative rotation of the second member and the first member so that said castoring axis is said second axis. The or at least one of said locking means may be operable by a user. The or at least one of said locking means may alternatively or additionally be automatically operable in response to output from a sensor adapted to sense whether said vehicle is travelling or configured to travel forward or backward, so that said second axis is said castoring axis for forward travel and said third axis is said castoring axis for backward travel.

Preferably, said support member includes an A frame with said connection means secured to an apex zone of said A frame and free ends of legs of said A frame including means whereby said support member is mounted to said chassis for pivotal rotation about said first axis.

Preferably, said suspension means includes suspension units secured to said chassis on opposing sides of said chassis and resilient in a generally vertical direction; and load transfer means each mounted between a lateral end zone of said rigid axle assembly and a said suspension unit.

In a preferred arrangement, each said load transfer means includes a first part secured to the suspension unit and a second part formed as part of or secured to the rigid axle, said first and second parts being capable of relative movement to accommodate rotation of the rigid axle unit about the castoring axis.

The second part may include a bearing support structure carrying bearing assembly means permitting rotational movement about, and thrust force resistance in a direction along, a generally upright axis and where said first part supports said bearing assembly means. Preferably, said bearing assembly means includes a thrust bearing assembly to provide said thrust force resistance and at least one bearing assembly to constrain said first and second parts for relative rotation.

At least one of the first and second parts of at least one of the load transfer means may include stop means to limit rotation of the rigid axle assembly about the castoring axis to a predetermined range.

The connection means may include stop means to limit rotation of the rigid axle assembly about the second axis and/or the third axis to a predetermined range.

The or each said suspension unit includes stop means to limit rotation of the rigid axle assembly about the second axis and/or the third axis to a predetermined range. In a particularly preferred form of the embodiments having suspension units, each suspension unit includes: an elongate member extending in a fore-and-aft direction of the chassis and mounted for at least limited movement in said fore-and-aft direction; and resilient means interposed between said elongate member and the chassis, and said load transfer means is secured to said elongate member.

Preferably, in each suspension unit a forward end of said elongate member is mounted to a rocker arm for pivoting movement about a generally horizontal lower axis in said rocker arm, and wherein said rocker arm is mounted to the chassis for pivotal movement about an upper axis which is a generally horizontal axis.

Preferably, with the axle assembly in a neutral position in which the rigid axle is at right angles to a forward direction of movement of the vehicle, the upper axis of each said rocker arm is at least approximately vertically above the lower axis thereof.

Also preferably, with said axle assembly in a neutral position in which the axle is at right angles to a forward direction of movement of the vehicle said lower axes in said rocker arms are at least approximately collinear with each other and with said first axis.

The said resilient means may be an air spring.

The said resilient means may be connected to said chassis via pivot means so that said resilient means is pivotable about said pivot means in response to fore-and-aft movement of said elongate member.

The elongate member may itself be a spring. It may be, for example, a leaf spring.

At least one of the suspension units, where these are included, may include further resilient means anchored directly or indirectly to the chassis and so arranged as to be deflected in response to castoring rotation of the axle assembly away from a straight-ahead position, and thereby to urge the axle assembly back to the straight-ahead position.

A forward end of each elongate member, where these are included, may be so mounted to the chassis that forward or rearward movement of the elongate member due to castoring rotation of the axle assembly away from a straight ahead position tends to raise the chassis, so that the weight of the chassis tends to urge the axle assembly back to the straight-ahead position.

According to the invention in another aspect, there is provided a mechanism adapted to be fitted to a rigid axle assembly for mounting the said axle assembly from a vehicle chassis, said mechanism comprising: a support member so mounted to the chassis as to be pivotable about a generally horizontal first axis in the chassis; connection means enabling connection of said support member to a rigid axle of said rigid axle assembly at or adjacent a mid point of said rigid axle, said connection means permitting castoring of said rigid axle relative to said support member about a selectable one of two generally upright axes, of which axes one is ahead of and the other is behind axes of rotation of wheels mounted to said rigid axle; elongate members extending in a fore-and-aft direction and on opposing sides of the chassis and mounted to the chassis for at least limited movement in said fore-and-aft direction; resilient means each interposed between an elongate member and the chassis; and load transfer means each secured to a said elongate member and to a lateral end zone of said rigid axle assembly, wherein each said load transfer means is adapted to permit relative rotation of the axle assembly and its respective elongate member about a generally upright axis in response to castoring of the axle assembly.

The present invention further anticipates providing a vehicle including at least one non-driven axle and an associated axle mounting arrangement as described in the foregoing paragraphs.

The terminology "vehicle" used throughout this specification is intended to identify both powered and non-powered vehicles such as trailers. Terminology such as "vehicle chassis" and "vehicle chassis members" are intended to identify both a main chassis structure or a sub-assembly forming a chassis part or section. The patent claims annexed hereto define preferred aspects and features of this invention and form part of this disclosure by way of this cross-reference thereto.

Several preferred embodiments of the invention will now be described in detail, but it is to be understood that these particular embodiments are a few out of many that are possible and that lie within the scope of the invention. Reference is made in the description to the attached figures, of which:

Figure 1 is a plan view (from above) of a mechanism according to one preferred embodiment of the present invention; Figure 2 is an isometric view showing preferred features according to one aspect which may be used in the embodiment of Figure 1 as well as other embodiments;

Figure 3 shows in schematic perspective view, one possible means of connecting the suspension arrangement shown in Figure 2 to a vehicle chassis; Figure 4 is a plan view (from above) of a further portion of the mechanism shown in Figure 1 ;

Figure 5 is a cross-section of the same portion of the mechanism shown in Figure 4, taken at "A-A" in Figure 4; Figure 6 is an exploded view of a load transfer unit of the mechanism of Figure 2;

Figure 7 is a further exploded view showing the features of Figure 6 together with further features of another preferred embodiment; Figure 8 is a perspective view of the embodiment of Figure 7 in an assembled condition;

Figure 9 is a front elevation of an alternative rocker arm and housing arrangement useable in a mechanism otherwise the same as the mechanism in Figures 1 and 2; Figure 10 is a cross sectional view of the rocker arm and housing arrangement taken at "B-B" in Figure 9;

Figure 11 is a side elevation of an alternative arrangement for a suspension unit useable in a mechanism otherwise the same as that shown in Figure 1 ; Figure 12 is a cross-sectional view of the arrangement shown in Figure

9, taken at "C-C" in Figure 11 ;

Figure 13 is a schematic plan view of an alternative form of the part of the mechanism shown in Figure 4;

Figure 14 is a schematic plan view of an alternative form of the part of the mechanism shown in Figure 4; and

Figure 15 is a schematic cross-sectional view taken at "D-D" in Figure 13;

There is shown in Figure 1 a mechanism 1 according to one preferred aspect of the invention. An axle assembly 2 includes a non-rotating axle 3 and ground engaging wheels 4 which are secured to the axle 3 and free to rotate about an axis 5. The axle assembly 2 partially supports a chassis of a vehicle (not shown) to which it is secured. Arrow F shows the direction of forward travel of the vehicle.

The axle assembly 2 is connected to the chassis by left and right suspension units 8 and 9, which are adapted to transfer vertical loads (being vehicle weight and vehicle load weight) from the chassis to the axle 3 via load transfer units 10, 11 and via wheels 4 to the ground (not shown). As further discussed below, the suspension units are adapted to provide resilient support for the chassis on the axle assembly 2 as required for travel on rough surfaces. The axle assembly 2 is also connected to the chassis by a support member 12 via connection means 7 including a connection link 13, in such a way that the axle assembly 2 is located laterally and longitudinally relative to the chassis. The support member 12 may be in the general form of an A-frame having legs 14 and 15 joined by upper and lower A-frame plates 16 and 17. At the ends of legs 14 and 15 are sleeves 18 and 19 for pivot pins 98 and 99 which are supported from hangers 20 and 21 secured to a chassis member 22 so that the support member 12 is free to pivot about an axis 23 which is horizontally and transversely disposed in the chassis.

When the vehicle is travelling forward (in the direction of arrow F), the axis 5 is perpendicular to the direction of travel. However, the axle assembly is able at the option of a user to be made free to pivot about a generally upright axis 24 ahead of the axis 5, so that when the vehicle negotiates a curved path the axle assembly 2 can pivot or "castor" relative to the chassis about the axis 24. The wheels 4 then travel curved paths without skidding or scrubbing, or with a lesser degree of skidding or scrubbing than would occur in the absence of castoring. When the vehicle is travelling backward, however, pivoting about the axis

24 is undesirable, as the axle assembly 2 would tend to pivot to, and then stay in, a position in which excessive scrubbing took place. Therefore the user may, for backward travel, either cause the axle assembly 2 to be locked in a position such that the axis 5 lies transversely across the chassis with castoring prevented, or enable castoring action about a second generally upright axis 25 which is on the opposite side of axis 5 from axis 24.

Figures 3 and 4 show that part of the mechanism 1 which enables control of the castoring action in this way. The axle 3 is shown as having a circular cross section, but may be of any convenient shape. To a mid-point position of the axle 3 is welded (or otherwise secured) a bracket 26. Mounted within the bracket 26 is a pivot pin 27, with a retainer 28 to hold the pivot pin 27 in position. The pivot pin 27 defines the axis 25 and is retained in upper and lower plates 29 and 30 of the bracket 26. The pivot pin 27 passes through a connection link 13 between plates 29 and 30, so that the connection link 13 can pivot about the axis 25. A locking pin 31 can at the option of a user be passed through the plates 29 and 30 and the connection link 13, thereby to prevent pivoting of the connection link 13 about pivot pin 27, or withdrawn to the position shown in Figure 4 so that such pivoting can take place. An actuator 32 is provided on the bracket 26 to move the locking pin 31 between its two positions. The actuator 32 is a pneumatic ram, but may be of another suitable type, such as a hydraulic ram or even an electrohydraulic linear actuator.

The connection link 13 also passes between upper and lower plates 16 and 17 of the support member 12. A pivot pin 35 passes through plates 16 and 17, is retained by a retainer 36 and defines axis 24. A locking wedge 37 between the plates 16 and 17 is able at the option of the user to be advanced into, or retracted from, a cooperating recess 38 in the connection link 13 by an actuator 39 mounted between the plates 16 and 17. The actuator 39 is a pneumatic ram, but may be of another suitable type, such as a hydraulic ram or even an electrohydraulic linear actuator. When the locking wedge 37 is in the position shown in figures 3 and 4, i.e. withdrawn from the recess 38, the connection link 13 may pivot relative to the support member 12 about the axis 24. When the locking wedge 37 is received within the recess 38, however, such pivoting is prevented. Pins 40 and 41 are provided at the top and bottom of the locking wedge 37 and run in slots 42 and 43 in the plates 16 and 17 so that, when the connection link 13 and the support member 12 are locked against relative pivoting by locking wedge 37, the actuator 39 need not transfer transversely-directed forces between the connection link 13 and the support member 12. Instead they pass through the pins 40 and 41. The locking wedge 37 and the recess 38 are tapered to enable locking even when the axle assembly 2 is not lying precisely transversely across the chassis.

The bracket 26 and the support member 12 are of course so designed as to enable enough angular travel of connection link 13 within them for the necessary degree of castoring. Actual contact between edges of the connection link 13 and either the support member 12 or the bracket 26, would limit the maximum degree of angular travel of the axle 3 due to castoring, but other approaches to this are preferable, and are described below. To enable castoring about the axis 24, actuator 39 is used to withdraw locking wedge 37 from recess 38 and locking pin 31 is pushed by actuator 32 through connection link 13 to lock the axle assembly 2 to the connection link 13. Alternatively, to enable castoring about the axis 25, locking wedge 37 is driven into recess 38 to lock the support member 12 to the connection link 13, and locking pin 31 is withdrawn from connection link 13. Appropriate interlocks may be provided, a straightforward matter for a person skilled in the art, to ensure that at no time is pivoting about both axes 24 and 25 enabled. By locking the connection link 13 to both support member 12 and bracket 26, the axle assembly 2 can be prevented from castoring if required. It is possible by means which also would be straightforward for a person skilled in the art to operate the actuators 32 and 39 responsively to sensors which can detect whether a forward or reverse gear is selected, so that the appropriate axis (24 or 25) is selected as the castoring axis. Referring now to Figure 2, in which the axle assembly 2 has been omitted for clarity, arrow F shows the direction of forward travel of the vehicle, and an axis 5 is the central axis of the axle 3. The left and right suspension units 8 and 9 are connected to the chassis via lugs 46, 47, 48 and 49 which are part of, or secured to, the vehicle chassis. Air bag suspension members 50 (i.e. air springs) are shown at rear ends of elongate members 51 (which are shown as simple beams but may also be leaf spring assemblies). The air bag suspension members 50 may themselves be conventional arrangements comprising an upper plate with a depending bag formation defining an internal recess to receive an upwardly extending piston like formation attached to a lower plate which in turn is attached to the elongate members 51. The upper plate member as shown is pivoted at 76 to chassis lugs 48, 49 either by a plain bearing defining axes 76' or more preferably an at least part spherical type bearing to provide some pivoting movement about axes perpendicular to the axes 76'. Conveniently the lower plate of the air bag suspension members 50 may also be mounted on pivot connections to the elongate members 51 such that they pivot about axes generally transverse to the elongate members 51. Again these pivot connections may preferably be such as to provide some pivot capability about an axis extending parallel to the elongate members 51. Conveniently tie rods connect the lower plate of the air bag 50 to the axle 3 with limited universal pivot connections at either end whereby the tie rod will limit pivoting movement of the lower plate relative to the members 51. On the elongate members 51 , load transfer units 10 and 11 are provided, and allow attachment of end regions of the axle 3 to suspension units 8 and 9. Front ends 44 of the elongate members 51 are mounted to rocker arms 70 and 71 for pivoting about horizontal axes 72 and 73 in the rocker arms 70 and 71. Rocker arms 70 and 71 depend from the lugs 46 and 47 and can pivot therein about axes 74 and 75. Figure 3 shows in schematic perspective view a possible alternate form of rocker arm assembly 96 for connecting the forward end of the elongate members 51 to the vehicle chassis. In this embodiment, rocker arm 97 is located within a rocker arm housing 98 secured to and depending from a vehicle chassis C. One such assembly is provided for each suspension arrangement secured to opposed ends of the axle 3. In each case, the rocker arm 97 is pivoted at an upper end via a pivot shaft providing a pivot axis 74, 75. A lower pivot shaft connects side plates of the rocker arm 97 and provides a lower pivot axis 72, 73. The pivot axes 72 and 73 are preferably coincident when the assembly is in the neutral position. In each case the lower pivot shaft pivotally connects a forward end 44 of one of the elongate members 51 (such as a parabolic spring) thereto such that this end 44 of the member 51 can move forwardly or rearwardly by the respective rocker arm 97 pivoting about its upper pivot axis 74, 75 but cannot move in an upward or downwards direction. Each rocker arm housing 98 also supports a leaf spring 85 with its rearward end fixed and its forward free end engaged with a transverse roller pin 86 of the rocker arm 97. The pin 86 is located rearwardly of the upper pivot axis 74, 75. Thus if the rocker arm 97 pivots rearwardly as a result of the elongate member 51 and the axle end having also moved rearwardly from the neutral position, leaf spring 85 is forced upwardly by the movement of the roller 86 and the spring 85 urges the roller 86 to return to the neutral position when external forces are sufficiently removed. Similarly if the rocker arm 97 associated with the axle end pivots rearwardly as a result of the elongate member 51 and the axle end having also moved rearwardly from the neutral position, then the leaf spring 85 on that side of the vehicle chassis is forced upwardly and provides a return spring force to the roller 86. Thus the two leaf springs 85 together tend to maintain the axle 3 in the neutral position in the absence of sufficient external forces applied by vehicle wheels to the ends of the axle 3. In addition a stop member 99 is provided with each of the rocker arm housings 98 and is selectably positionable to block forward movement of the lower pivot shafts from the neutral position. With both stop members 99 on opposed sides of the vehicle chassis located in such a blocking position, both ends of the axle 3 are prevented from moving in a forward direction and the axle 3 is thereby locked in the neutral position. As seen in Figure 3, the stop member 99 may be pivoted into or out of a blocking position by an actuator 99a. The actuator's 99a may be controlled from a position adjacent the vehicle driver by the driver, or alternatively may be moved to the locking position automatically upon the vehicle moving in a reverse direction. It is desirable for the axle 3 to be locked in the neutral position if the vehicle is to be backed but it should be free to self adjust or self steer when the vehicle is moved in a forward direction. When the vehicle is to be reversed, the axle will inevitably be in one of three positions, namely :- (i) a neutral position; (ii) one axle end being forward of the other axle end; or (iii) vice versa.

If the axle 3 is already in the neutral position, both stop members will be placed in the blocking position immediately by driver operation or by automatic actuation as a result of the vehicle reversing. In the case of (ii) above, the stop member 99 associated with the one axle end can be positioned in the blocking position but the other stop member 99 cannot be so positioned. However, upon reversing, the axle 3 will pivot such that the other axle end moves rearwardly until the stop member 99 associated with that axle end can be positioned in the blocking position. A similar but reverse effect is achieved in the case of (iii) above. Figure 5 illustrates, in particular, the load transfer unit 11 with the load transfer unit 10 being essentially the same. Each of the load transfer units 10 and 11 includes an upper saddle member 52 and a lower saddle member 53 secured to the axle 3 by welding or any other suitable means. The upper saddle member 52 carries an inner bearing shell 54 and provides a recess 55 for retaining a thrust bearing 56. The lower saddle member 53 also carries an inner bearing shell 57. There are also provided a lower or spring saddle member 58 and an upper bearing saddle member 59 each of which carries an outer shell bearing 60, 61 which respectively cooperate with the inner bearing shells 54, 57 when assembled. Clamping plate 62 with location projections 63 is positioned beneath an elongate member 51 and clamps the assembly together with the assistance of four bolts 64 (three of which are shown) passing through bores 65 in the saddle member 58 to be threadingly engaged in threaded bore holes 66 in the saddle member 59. The portion of vehicle and load weight shared by the axle assembly 2 is shared by the two thrust bearings 56 of respective load transfer units 10 and 11. The arrangement described permits the axle 3 to pivot relative to the elongate members 51 of the suspension units 8 and 9 about upright axes 67 coaxial with the bearing shells 54, 57, 60 and 61. This relative pivoting movement is, however, limited by the extent of the openings 68, 69 in the members 58 and 59 so that members 58 and 59 act as stops to prevent an excessive degree of castoring action. Preferably however, stops 90 and 91 are secured to the chassis adjacent to the rocker arms 70 and 71 , so that contact between rocker arm 70 and stop 90 or between rocker arm 71 and stop 91 serve to limit the maximum degree of angular travel of the axle 3 due to castoring.

In operation of the mechanism 1 , pivoting of the axle assembly 2 about either axis 24 or axis 25 (according to the direction of travel) leads to one of the elongate members 51 moving forward and the other moving rearward, the resulting change in angle between axle 3 and each elongate member 51 being accommodated by the load transfer units 10 and 11. The rocker arms 70 and 71 pivot about the axes 74 and 75 to accommodate such pivoting. Chassis and vehicle loads are transferred to the elongate members 51 through the air springs 50 and the rocker arms 70 and 71 , and thence via the load transfer units 10 and 11 to the axle 3. In the preferred embodiment, the axes 72 and 73 coincide with the axis 23 in the straight ahead position of the axle assembly, when the axle 3 is perpendicular to the direction of travel. The air springs 50 are mounted to the chassis on horizontal pivot pins 76 so that little or no distortion of the air springs is required to accommodate the fore-and-aft movement of the elongate members 51.

Torques applied to the axle 3 through braking (if the axle assembly 3 includes brakes) are resisted (i.e. balanced by forces applied to the mechanism 1 by the chassis) by the suspension units 8 and 9, with little or none of such torques being resisted through the support member 12.

Castoring of the axle assembly 2 causes slight lateral movements of the load transfer units 10 and 11 , and to accommodate this, some freedom of lateral movement may be provided at pivot pins 76 by means well known in the art such as rubber-mounted sleeve bearings (not shown) to support the pivot pins 76 or even ball joints. Similar provision may be made in the rocker arms 70 and 71 at axes 72 and 73.

Many modifications may be made to the mechanism 1 without departing from the scope of the invention. Figures 7 and 8, for example show one practical preferred embodiment utilising features described in the foregoing or in later described aspects. In this embodiment, like features have been given similar reference numerals. Figure 7 utilises a forward and reverse self steering system similar to that described hereafter with reference to Figures 13, 14 and 15. Figure 8 shows the support member arms 14, 15 connected at their forward ends to the chassis via upright shaft members but they include spherical or part spherical bearings that allow the ends of the arms 14, 15 to pivot about horizontal axes as with other embodiments. Two aspects of the invention will now be described, each of which provides a capacity for automatic centering of the axle assembly 2 additional to that provided by the castoring action described above.

Figure 9 and 10 show a rocker arm 80 within a rocker arm housing 81. A pair of such housings and arms may be provided as an alternative to the pair of rocker arms 70 and 71 and their mounting lugs 46 and 47, the housings being secured to and depending from the vehicle chassis C. The rocker arm 80 is pivoted at an upper end via a pivot pin 82 providing a pivot axis 83. A lower pivot pin 84 connects side plates of the rocker arm 80 and provides a lower pivot axis 34. The pivot axes 34 of each of the pair of rocker arms 80 are preferably coincident when the axle assembly 2 is in the neutral, straight-ahead position. Each lower pivot pin 84 pivotally connects to a forward end 44 of one of the elongate members 51 such that each elongate member 51 can move forwardly or rearwardly by its respective rocker arm 80 pivoting about its upper pivot axis 83 but so that the lower pivot pins 84 cannot move significantly in an upward or downwards direction. Each rocker arm housing 81 also supports a leaf spring 85 with its rearward end fixed to a pin 92 and its forward free end engaged with a transverse roller pin 86 extending between the two side plates 4 of the rocker arm 80. The pin 86 may include an outer sleeve 93 rotatable on the pin 86 which is located rearwardly of the upper pivot axis 83. The leaf spring 85 may engage against a fixed stop 94 located between the pins 86 and 92. Moreover retaining members for the pin 86 or the pin itself may be located in arcuate slots 95 formed in the plates 81 to allow pivoting movement of the rocker arm 80 about the axis 83 within absolute limits defined by the ends of the slots 95. Thus it if the rocker arm 80 pivots rearwardly as a result of the elongate member 51 to which it is connected moving rearwardly from the neutral position, leaf spring 85 is forced upward against the stop 94 by the movement of the roller pin 86 and the leaf spring 85 urges the roller pin 86 to return to the neutral position when external forces are sufficiently removed. Thus the two leaf springs 85 together tend to maintain the axle assembly 2 in the neutral position in the absence of sufficient external forces applied by vehicle wheels 4 to the axle 3.

The second of the two preferred aspects of the invention mentioned in the last-but-one paragraph will now be described. Figures 10 and 11 show a further arrangement 112 for supporting the front ends of the elongate members 51 which is an alternative to the rocker arms 70 and 71 and to rocker arms 80. Each of the suspension units 8 and 9 includes a hanger 100 secured to the chassis, the hanger having a pair of spaced-apart side plates 101 to each of which is secured one of a handed pair of bearing guides 102. Each bearing guide 102 has a slot 103, aligning with a slightly larger but matching slot 109 in its respective side plate 101 , the surfaces of the slots 103 (at least) being hardened. Sealed rolling-element bearings 104 reside within each slot 103 and are themselves mounted to the front end of one of the elongate members 51 by a pin assembly 105. In the neutral (straight ahead) position of the axle assembly 2, each bearing 104 rests in the central position shown in Figure 10, transmitting vertical loads from the elongate member 51 through the bearing 104 to the upper surface 106 of the slot 103 and thereby to the chassis. If the axle 3 castors about one of the axes 24 or 25, the bearings roll along the slots 103 towards the front or rear ends (107, 108, respectively) thereof. It will be appreciated that such movement tends to require a very slight lifting of the chassis, due to the sloping shape of the slots 103, so that the position shown in Figure 10 is a stable one for the bearings 104 and there is a resulting tendency for the axle to return to that stable position when castoring of the axle assembly 2 occurs. That is, a self centering action is obtained. Moreover, the ends 107 and 108 act as stops, limiting the maximum degree of castoring action. Preferably, the axes 111 of the two arrangements 112, when the axle assembly 2 is in the neutral, straight-ahead position, coincide.

As shown in Figure 11 , the pin assembly 105 is mounted in a rubber- supported sleeve 110 in the elongate member 51 , so that lateral movement of the front end of the elongate member 51 is accommodated, as previously described with respect to the rocker-arm-based embodiments.

Another possible variation to the invention is to provide a single arrangement, instead of the pair of arrangements described above, for selecting which of two axes is to be the castoring axis. Figures 12, 13 and 14 show such a variation as do Figures 7 and 8. It is to be understood that these particular figures are schematic, with some mechanical detail omitted or shown in simplified form, in either case in such a way that a person skilled in the art would still understand the principle of operation of the variation. In these three figures, there is shown an axle 203, a support member

212, a connection link 213, a bracket 226 and pivot pins 227 and 235, these components having the same functions in this embodiment as components 3, 12, 13, 26, 27 and 35 respectively in the embodiments described above. Arranged to move in a fore-and-aft direction along the connection link 213 are wedge blocks 301 which have front faces 302 and rear faces 303. To enable such movement, the blocks 301 are connected by a link 304 to a linear actuator 305 (such as a pneumatic ram) operable by a user or responsively to output from a sensor (not shown) able to detect whether the vehicle is travelling forward (in the direction of arrow F) or backward.

When the wedge blocks 301 are in a rearward position shown in Figure

12, they bear on the connection link 213 and on convergent faces 307 of the bracket 226 so that relative rotation of the connection link 213 and the axle 203 about a pin 227 and therefore about an axis 225 of the pin 227 is prevented. However, provided a wedge 237 is not received in a cooperating recess 238 in the link 213 (items 237 and 238 corresponding to items 37 and 38 in the embodiments described above) the connection link 213 may rotate about the pin 235 and its axis 224. Thus, in this position (Figure 12) of the blocks 301 , axis 224 is a castoring axis suitable for forward travel of the vehicle.

When the wedge blocks 301 are in a forward position as shown in Figure

13, they bear on the connection link 213 and on convergent faces 308 of blocks 309 secured to the support member 212, so that relative rotation of the connection link 213 and the support member 212 about the pin 235 and its axis 224 is prevented. However, relative rotation between the axle 3 and the connection link 213, about pin 227 and its axis 225 is possible, so that axis 225 is a castoring axis suitable for rearward travel of the vehicle. In this position of the wedge blocks 301 , the relative positioning of wedge 237 and recess 238 is irrelevant. That is, the wedge 237 and its actuator 239 are optional and may be left out altogether.

Apart from there being, in this embodiment, a single locking mechanism able to select one of the castoring axes 224 or 225, a further advantage is that a degree of initial misalignment between the axle 203 and the connection link 213, or between the connection link 213 and the support member 212 can be taken up automatically when the wedge blocks 301 are moved. Other mechanical details of this embodiment may be essentially the same as the analogous details of the embodiments described above.

The proportions of the mechanism 1 and its parts may of course vary in practice from those shown in the figures as required for different vehicles and different conditions. As but one example, the air springs 50 may be located differently along elongate members 51 from the position shown in the Figures to vary the proportions of total vertical forces transmitted through them and the rocker arms 70 and 71 (or 80).

It will also be appreciated by persons skilled in the art that, particularly where large deflections of the axle (3 or 203) relative to the chassis are to be allowed for, that variations are possible to the actual nature of the connections between parts without any departure from the scope of the invention. As but one example, to allow accommodation of large but uneven vertical deflections of the ground wheels, the axle (3 or 203) could be connected to the elongate members 51 by ball-and-socket joints (or mechanical equivalents thereof) instead of the load transfer units 10 and 11 , although in such an arrangement, the connection between the axle 3 or 203 and the support member 12 or 212 would have to be able to prevent rotation of the axle 3 or 203 under the influence of torque from operation of brakes on the axle, in contrast to the embodiments described above. It will of course be appreciated that the present invention is aimed at vehicles including trailers which commonly have numerous axle assemblies and the axle assembly according to the preceding description might be used for any non-driven axle assembly of such vehicles or trailers. The invention is particularly suited to modifying or retrofitting existing rigidly mounted axle assemblies but of course could equally be used as original equipment if desired. It will be further apparent that the previously described arrangements allow for self steering or castoring of the modified axle assembly in the forward direction with similar self steering or fixing of the modified axle assembly in a neutral position when reversing. Any feature or combination of features as disclosed in the foregoing may be used with any other feature to give this effect as desired. If further desired, the arrangements permit selectable fixing or locking in both the forward or reverse directions if desired. Other modifications or variations are anticipated within the scope of the annexed patent claims.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. An axle mounting arrangement for use on a vehicle having at least one non-driven axle carrying at least one ground engaging wheel at either end and a suspension system including at least one elongate member extending in a fore and aft direction of the vehicle on opposed sides of the vehicle, each of said elongate members being adapted for connection to a respective end zone of the non-driven axle, said elongate members being mounted for at least limited movement in said fore and aft direction, said axle mounting arrangement including a support member adapted to be mounted to a vehicle chassis frame member by first connection means whereby said support member is pivotal about at least one generally horizontal pivot axis, second connection means enabling connection of said support member at or adjacent to a mid point of the non-driven axle whereby the non-driven axle is pivotable about a first generally upright pivot axis relative to said support member, and third connection means connecting opposed end zones of the non-driven axle to a respective said elongate member, said third connection means permitting pivotal movement between each said elongate member and said non-driven axle about second and third upright pivot axes spaced from but parallel to said first upright pivot axis.

2. An axle mounting arrangement according to Claim 1 , wherein said first upright pivot axis is spaced forwardly of axes of rotation of adjacent said ground engaging wheel or wheels.

3. An axle mounting arrangement according to Claim 2, wherein the second and third upright pivot axes intersect said axes of rotation of the adjacent said ground engaging wheel or wheels.

4. An axle mounting arrangement according to any one of Claims 1 to 3, wherein each of the end zones of the non-driven axle carries a bearing support structure secured to the axle, the bearing support structure carrying bearing assembly means permitting rotational movement about one oi the second or the third upright pivot axes and thrust force resistance in the direction of the second or the third upright pivot axes.

5. An axle mounting arrangement according to Claim 4, wherein the bearing assembly means includes a first bearing means permitting rotational movement about one of the second or the third upright pivot axes and a second bearing means capable of thrust force resistance in the direction of the second or the third upright pivot axes.

6. An axle mounting arrangement according to any one of Claims 1 to 3, wherein each of the end zones of the non-driven axle has a bearing support structure secured thereto extending upwardly and downwardly from the axle, each of said bearing support structures being adapted to carry bearing means co-axial with one of the second or third upright pivot axes, and at least one of said bearing support structures being adapted to carry thrust bearing means for resisting thrust loads in the direction of said second or third upright axes, an upper saddle member cooperable with the upwardly extending bearing structure and a lower saddle member cooperable with the downwardly extending bearing structure, and at least one clamping member positionable below the elongate member and cooperable with bolt means to secure said saddle members and said clamping member or members together with a said elongate member located between the lower saddle member and the clamping member or members.

7. An axle mounting arrangement according to any one of Claims 1 to 6, wherein a forward end of the or each said elongate member is pivotally mounted from a pivoting rocker arm, said elongate member or members having a pivot axis with the associated rocker arm that coincides with the at least one generally horizontal pivot axis of the support member in at least one position of use.

8. An axle mounting arrangement according to Claim 7, wherein the at least one position of use is a neutral position where the non-driven axle is at right angles to a forward direction of movement of the vehicle.

9. An axle mounting arrangement according to any one of Claims 1 to 6, wherein a forward end of the or each said elongate member is pivotally mounted from a pivoting rocker arm, said rocker arms being selectably lockable in a position where the non-driven axle is disposed at right angles to a direction of movement of the vehicle.

10. An axle mounting arrangement for use on a vehicle having at least one non-driven axle carrying at least one ground engaging wheel at either end and a suspension system including at least one elongate member extending in a fore and aft direction of the vehicle on opposed sides of the vehicle, said elongate members being adapted for connection to a respective end zone of the non-driven axle, said elongate members being mounted for at least limited movement in said fore and aft direction, said axle mounting arrangement including a support member adapted to be mounted to a vehicle chassis frame member by first connection means whereby said support member is pivotal about at least one generally horizontal pivot axis, second connection means enabling connection of said support member at or adjacent to a mid point of the non-driven axle whereby the non-driven axle is pivotable about a first generally upright pivot axis relative to said support member whereby the end zones of the non-driven axle are movable in a forward or rearward direction within predetermined limits, the or each said elongate member having a forward end pivotally supported by a rocker arm with a pivotal axis of each said rocker arm pivotal connection to a said elongate member coinciding with the at least one generally horizontal pivot axis of the support member in at least one position of use.

11. An axle mounting arrangement according to Claim 10, wherein the at least one position of use is a neutral position where the non-driven axle is at right angles to a forward direction of movement of the vehicle.

12. An axle mounting arrangement for use on a vehicle having at least one non-driven axle carrying at least one ground engaging wheel at either end and a suspension system including at least one elongate member extending in a fore and aft direction of the vehicle on opposed sides of the vehicle, said elongate members being adapted for connection to a respective end zone of the non-driven axle, said elongate members being mounted for at least limited movement in said fore and aft direction, said axle mounting arrangement including a support member adapted to be mounted to a vehicle chassis frame member by first connection means whereby said support member is pivotal about at least one generally horizontal pivot axis, second connection means enabling connection of said support member at or adjacent to a mid point of the non-driven axle whereby the non-driven axle is pivotable about a first generally upright pivot axis relative to said support member whereby the end zones of the non-driven axle are movable in a forward or a rearward direction within predetermined limits while the vehicle is moving in a forward direction, said axle mounting arrangement further including means for selectably locking the end zones of the non-driven axle in a neutral position at right angles to a direction of movement of the vehicle when the vehicle is moved in a reverse direction.

13. An axle mounting arrangement according to Claim 12, wherein the selective locking means is actuated automatically upon the vehicle moving in said reverse direction.

14. An axle mounting arrangement according to any one of Claims 10 to 13, wherein said first upright pivot axis is spaced forwardly of axes of rotation of an adjacent said ground engaging wheel or wheels.

15. A vehicle including at least one non-driven axle and an associated axle mounting arrangement according to any one of Claims 1 to 14.

16. A mechanism adapted to be fitted to a rigid axle assembly for mounting the said assembly from a vehicle chassis, said mechanism including: a support member so mounted to the chassis as to be pivotable about a generally horizontal first axis; connection means enabling connection of said support member to a rigid axle of said rigid axle assembly at or adjacent a mid point of said rigid axle, said connection means permitting pivoting of said rigid axle relative to said support member about a generally upright castoring axis; and suspension means adapted to transfer vertical loads from the chassis to said axle assembly, wherein said connection means is adapted to enable selectable location of said castoring axis in a position ahead of or behind axes of rotation of wheels mounted to said rigid axle.

17. A mechanism according to claim 16 further including means operable by a user whereby pivoting of said rigid axle assembly about said castoring axis is preventable.

18. A mechanism according to claim 16 or 17 wherein said connection means includes: a first member so secured to the support member as to permit relative rotation of the first member and the support member about a second axis ahead of the axes of rotation of the wheels mounted to said rigid axle, said second axis being generally upright; a second member secured to or formed as part of said rigid axle and so secured to the first member as to permit relative rotation of the first member and the second member about a third axis behind the axes of rotation of the wheels mounted to said rigid axle, said third axis being generally upright; and at least one locking means operable selectively either to prevent said relative rotation of the first member and the support member so that said castoring axis is said third axis, or to prevent said relative rotation of the second member and the first member so that said castoring axis is said second axis.

19. A mechanism according to any one of claims 16 to 18 wherein the or at least one of said locking means is operable by a user.

20. A mechanism according to any one of claims 16 to 18 wherein the or at least one of said locking means is automatically operable in response to output from a sensor adapted to sense whether said vehicle is travelling or configured to travel forward or backward so that said second axis is said castoring axis for forward travel and said third axis is said castoring axis for backward travel.

21. A mechanism according to any one of claims 16 to 20 wherein said support member includes an A frame with said connection means secured to an apex zone of said A frame and free ends of legs of said A frame including means whereby said support member is mounted to said chassis for pivotal rotation about said first axis.

22. A mechanism according to any one of claims 16 to 21 wherein said suspension means includes suspension units secured to said chassis on opposing sides of said chassis and resilient in a generally vertical direction; and load transfer means each mounted between a lateral end zone of said rigid axle assembly and a said suspension unit.

23. A mechanism according to claim 22 wherein each said load transfer means includes a first part secured to the suspension unit and a second part formed as part of or secured to the rigid axle, said first and second parts being capable of relative movement to accommodate rotation of the rigid axle about the castoring axis.

24. A mechanism according to claim 23 wherein said second part includes a bearing support structure carrying bearing assembly means permitting rotational movement about, and thrust force resistance in a direction along, a generally upright axis and wherein said first part supports said bearing assembly means.

25. A mechanism according to claim 24 wherein said bearing assembly means includes a thrust bearing assembly to provide said thrust force resistance and at least one bearing assembly to constrain said first and second parts for relative rotation.

26 A mechanism according to any one of claims 23 to 25 wherein at least one of the first and second parts of at least one of the load transfer means includes stop means to limit rotation of the rigid axle assembly about the castoring axis to a predetermined range.

27. A mechanism according to any one of claims 18 to 26 wherein said connection means includes stop means to limit rotation of the rigid axle assembly about the second axis and/or the third axis to a predetermined range.

28. A mechanism according to any one of claims 22 to 26 wherein the or each said suspension unit includes stop means to limit rotation of the rigid axle assembly about the second axis and/or the third axis to a predetermined range.

29. A mechanism according to any one of claims 22 to 28 wherein each suspension unit includes: an elongate member extending in a fore-and-aft direction of the chassis and mounted for at least limited movement in said fore-and-aft direction; and resilient means interposed between said elongate member and the chassis, and wherein said load transfer means is secured to said elongate member.

30. A mechanism according to claim 29 wherein in each said suspension unit a forward end of said elongate member is mounted to a rocker arm for pivoting movement about a generally horizontal lower axis in said rocker arm, and wherein said rocker arm is mounted to the chassis for pivotal movement about an upper axis which is a generally horizontal axis.

31. A mechanism according to claim 30 wherein, with said axle assembly in a neutral position in which the rigid axle is at right angles to a forward direction of movement of the vehicle, the upper axis of each said rocker arm is at least approximately vertically above the lower axis thereof

32. A mechanism according to claim 30 or 31 wherein with said axle assembly in a neutral position in which the rigid axle is at right angles to a forward direction of movement of the vehicle said lower axes in said rocker arms are at least approximately collinear with each other and with said first axis.

33 A mechanism according to any one of claims 29 to 32 wherein said resilient means is an air spring.

34. A mechanism according to any one of claims 29 to 33 wherein said resilient means is connected to said chassis via pivot means so that said resilient means is pivotable about said pivot means in response to fore-and-aft movement of said elongate member.

35. A mechanism according to any one of claims 29 to 34 wherein said elongate member is a spring.

36. A mechanism according to any one of claims 22 to 35 wherein at least one of the suspension units includes further resilient means anchored directly or indirectly to the chassis and so arranged as to be deflected in response to castoring rotation of the axle assembly away from a straight-ahead position, and thereby to urge the axle assembly back to the straight-ahead position.

37. A mechanism according to claim 29 wherein a forward end of each said elongate member is so mounted to the chassis that forward or rearward movement of the elongate member due to castoring rotation of the axle assembly away from a straight ahead position tends to raise the chassis, so that the weight of the chassis tends to urge the axle assembly back to the straight- ahead position.

38. A mechanism adapted to be fitted to a rigid axle assembly for mounting the said axle assembly from a vehicle chassis, said mechanism comprising: a support member so mounted to the chassis as to be pivotable about a generally horizontal first axis in the chassis; connection means enabling connection of said support member to a rigid axle of said rigid axle assembly at or adjacent a mid point of said rigid axle, said connection means permitting castoring of said rigid axle relative to said support member about a selectable one of two generally upright axes, of which axes one is ahead of and the other is behind axes of rotation of wheels mounted to said rigid axle; elongate members extending in a fore-and-aft direction and on opposing sides of the chassis and mounted to the chassis for at least limited movement in said fore-and-aft direction; resilient means each interposed between an elongate member and the chassis; and load transfer means each secured to a said elongate member and to a lateral end zone of said rigid axle assembly, wherein each said load transfer means is adapted to permit relative rotation of the axle assembly and its respective elongate member about a generally upright axis in response to castoring of the axle assembly.