CA2428302C - Multi-axle running gear for automotive trucks, tractors, trailers, and semi-trailers - Google Patents

Abstract

The invention provides multi-axle running gear for automotive transport trucks and trailers, a minimum requirement being that the gear has two closely spaced parallel axles each carrying two spaced running wheels. One axle is mounted in a support housing for transverse movement along its longitudinal axis and will make such movement under scrubbing forces applied thereto by the wheels as the vehicle negotiates a curve or turn. The resultant slip steering axle provides a relatively simple structure equivalent to a self-steering axle, the slip steering movement reducing transverse scrubbing of the tires and yaw instability of the vehicle. The preferred extent of movement of the axle from its neutral position is equivalent to a steering angle of at least 20 degrees. Numerous applications are possible for multi-axle trucks and trailers, comprising such slip-steering axles in combination with one or more fixed, lifting and self-steering axles.

Description

Multi-Axle Running Gear for Automotive Trucks, Tractors, Trailers, and Semi-Trailers Field of the Invention This invention is concerned with multi-axle running gear for automotive transport trucks, tractors, trailers and semi-trailers, namely running gear having at least two longitudinally spaced axles mounted parallel to one another, one relatively closely behind the other, with each axle mounting at least two transversely spaced running wheels.

Review of the prior Art There is, and always has been, intense commercial interest in maximizing the load-carrying capacity of an automotive transport unit to that permitted by the regulatory authorities, and also to that possible with the use of commercially available axles and tires, and accordingly there have been a considerable number of prior proposals disclosing different arrangements for this purpose. The simplest proposals with a unitary vehicle for increasing its load-carrying capacity are first to double the number of rear wheels to two pairs on a single axle, thereafter to provi_de two or more longitudinally spaced parallel axles one closely behind the other with each axle mounting two transversely spaced running wheels, or mounting two transversely spaced pairs of wheels. It is usually impractical to mount more than two wheels side by side at the end of an axle, and t:he provision of more than two longitudinally spaced axles presents problems of sideways scuffing of the tire treads as the vehicle is steered around curves and corners; for this reason if such a third axle is provided it is common, where permitted, to make it of the lifting type so that it can be disengaged from the road when not needed for load bearing, the semi-trailer then pivoting on the remaining axles.

The tractor/semi-trailer combination, with or without an additional trailer attached to the semi-trailer, is now employed almost universally for large loads in view of the superior manoeuverability and flexibility of operation which is possible. Increasing the load carrying capacity of such semi-trailers has been restricted by regulations governing the distances between the axles, and consequently between the units of the vehicle and their overall length. With this has grown the proliferation of multi-axle semi-trailers and trucks, thus increasing the continuing commercial interest in steering systems for the running gear, provided running cost savings are possible to offset the higher initial cost.
Multi-axle semi-trailers are already somewhat detrimental. to the stability of the tractor, since the side forces generated by the large weights involved and combination of fixed axles can cause the tractor to lose directional stability, tending toward the possibility of a jackknife under certain conditions, especially with slippery road surfaces. This situation has brought about a growing demand for longer wheelbase tractors with extended Bumper to Back of Cab (BBC) lengths, which has tended to exacerbate the problem of off-tracking (i.e. too wide turns). The same considerations for increasing load carrying capacity that apply to the running gear of such combinations, and their separate components, also apply to a unitary vehicle.

Operators interested in maximizing load-carrying capacity usually wish to employ a semi-trailer unit that is as long as possible, but many jurisdictions have regulations as to the maximum length of a single tractor/semitrailer combination that is allowed on the highways, which typically is about 18 metres (60 feet), but in some jurisdictions may be as long as 23 metres (75 feet). Another practical limitation on maximum length is imposed by the need when turning corners for the rear wheels of the unit to "off-track" within prescribed minimum and maximum limits. An increase in length when all of the axles of the running gear are fixed automatically results in an increase in swept area, but it can be decreased by making one or more of the axles self-steering, or by lifting one of the axles from the road while turning. At this time there is a pressure in North America for the adoption of regulations limiting the "swept area" on turning of an inner radius of 6.4 metres (21 feet) and an outer radius of 15 metres (49 feet).

As mentioned above, single tractor/semi-trailer combinations are already somewhat unstable, especially with slippery road conditions, owing to the presence of the articulating pivot and the yaw that it permits, but the occasional jack knifing and overturning that occurs is accepted because of the inherent overall flexibility, practicability, and economy of operation of such units. A
so-called scuff ratio can be used to indicate the magnitude of the scuffing problem, and this ratio is calculated in the case of a semi-trailer as the bogie wheelbase length (WL-see Figure 1), namely the longitudinal spacing between the front and back axles of the bogie, divided by the forward length (FL), namely the distance between the front pivot (the king pin) and the effective turn axis of the rear bogie. As the value of WL
increases and/or the value of FL decreases the ratio increases and ideally the value of the scuff ratio is kept as low as possible For example, in the case of a three axle bogie in which the three axles are equally longitudinally spaced, the effective turn axis coincides with the axis of the centre axle. An increase in scuff ratio results in an increase in the overall yaw rate of the unit and corresponding increased yaw instability, and vice versa if the scuff ratio can be decreased. There is therefore a need in the industry fo:r new automotive transport units providing such increased load carrying capacity without, if possible, increasing any of the problems outlined above. There is also a need, if possible, owing to the inherent conservatism of the transport industry toward radically nex designs, to make as much use as possible of established designs of ru*+ning gear for tractors, trailers and semi-trailers Mhen designing new units.

Definition of the Invention It is therefore the principal object of the invention to provide neM multi-axle running gear for autamotive trucks, tractor units and/or semi-trailer units and/or trailer units.

It is another principal object to provide a new axle for multi-axle r+,*+**irg gear for autos-ot.ive trucks, tractor units and/or semi-trailer units and/or trailer units that can be used in combination with existing axles to form a readily steerable, reversible azle that by its steerinq capability will provide a reduced scuff ratio, as c=pared to the equivalent fixed azle, with consequent improved stability and manoeuverability.

It is a more specific object to provide such new multi-aale running gear for autos~ative trucks, tractor units and/or semi-trailer units and/or trailer units wherein one of the axles of the multi-axl.e running gear is mounted for substantial but limited transverse movement along its axis of rotation so that it can move transversely as the running gear negotiates a curve or corner under the applied transversely-actinq force to thereby provide an equivalent of pivoting of the wheels, reducing scrubbing of the associated tires and reducing the overall sideways yaw of the rnnni.ng gear.
In accordance with the present invention there is provided multi-axl.e r++nr+ ng gear for automotive transport tractors, trailers and seei.-trailers, such T*+*+*++*+q gear having at least two longitudinally spaced aales mounted parallel to one another, one closely behind the other, with each axle having a corresponding longitudinal axis of rotation disposed transverse to and at right angles to the directions of movement of the running gear, each axle mounting at least two transversely spaced *++*+*++ng wheels for engagement with ground over which the zunnIng gear passes, wherein one of the axles is a longitudinally movable azle mounted for transverse movement along its longitudinal axis under transverse forces applied thereto by the engagmnent of its running wheels with the ground as the running gear negotiates a curve or corner to thereby reduce transverse scrubbing of the rraaning wheels on the ground, while the other axle or axles of the r++*+*+irg gear is fixed against transverse movement along its longitudinal axis, or their respective longitudinal axes.

Description of the Drayings Automotivre transport trucks, tractors, trailers and semi-trailers incorporating mnlti-axle running gear of the invention nhich are particular preferred embodiments of the invention, will now be described, by way of example, with reference to the accompanying diagram atic drawings, wherein:
Figures 1 and 2 are side elevations of two tractor and semi-trailer ccssbination as at present more commonly used in the trucking industry, wherein at least one of the axles thereof is in accordance with the invention;
Figure 3 is a plan view from above of a combination as,in Figure 1, shown negotiating a turn and showing the change in axial position of the axle of the invention in relation to the other axles of the combination;
Figure 4 is an end elevation view of an axle of the invention as mounted on a semi-trailer chassis and showing the axle in its neutral or central position;

Figure 5 is an end elevation of the axle alone shown 30 removed from its supporting frame;
Figure 6 is a plane cross section through the axle of Figures 4 and 5 taken on the line 6-6 in Figure 4; and Figure 7 is a Table showing examples of the different configurations of running gear, and different types of vehicle, that are possible incorporating an axle of the invention.

Description of the Preferred Embodiments Figure 1 shows in side elevation a typical automotive tractor/semi-trailer combination as currently used comprising a tractor chassis 10 having at its front end the usual single axle steering wheel assembly represented by steering wheels 12 and raised driver's cab 14. Tandem steering axle assemblies are sometimes used instead when required to meet wheel loading requirements, and such tandem assemblies can also be used in the tractor units of the present invention. The unit illustrated is provided at its rear end with a single axl.e drive assembly comprising resiliently mounted drive wheels 16 mounted on respective drive stub axles, behind which is provided an additional axle and wheels 18 in order to meet wheel loading requirements. Although in this embodiment only a single axle drive assembly is illustrated a tractor unit of the invention may instead comprise two or more tandem drive axles and their associated driving wheels. There are many varieties of configurations and wheel suspensions used in the industry for both steering and running wheels, for example as specified by the designer and purchaser, and the specific choice thereof is of no concern in the implementation of the present invention, the specific requirements for any particular unit being apparent to those skilled in the art, so that specific description thereof beyond those described below with reference to Figures 1 and 4-6 is not required.

The tractor unit is provided with a fifth wheel assembly 20 to which is attached a semi-trailer comprising a semi-trailer chassis 22 supporting a load-receiving box container 24 provided with the usual forward disposed landing gear and rear end access doors (not shown'). The chassis is provided at its front end with the usual king-pin for cooperative engagement with the fifth wheel assembly 20. A tri-axle bogie 26, namely a bogie with three longitudinally spaced axles 28, 30 and 32, is mounted at the rear end of the chassis 22, each of the axles mounting a respective set of running wheels, in this embodiment the three axles being spaced equidistantly longitudinally from one another. The foremost axle 28 is a standard axle as commonly used in such bogies, while the middle axle 30 (indicated here and elsewhere by a combined vertical and inclined cross)is an axle of the invention, usually referred to herein as a "slip-steer" axle;
the rearmost axle 32 can again be a standard axle, but is shown as being a self-steering axle of any of the types now commonly available, this type of axle being indicated here and elsewhere by an inclined cross. Such self-steering axles are, for example, sold by Bergische Achsenfabrik Fr. Kotz &
Sohne of Wiehl, Germany or Asse Sansavini of Torbole Casagua, Italy.

' The combination shown in Figure 2 differs from that of Figure 1 in that a body 34 with chassis 36 rests on the tractor chassis 10 and is retained thereon by engagement of a king pin on the body with a fifth wheel 38 on the tractor chassis, the king pin and fifth wheel performing only this retaining function, since the body does not pivot relative to the tractor chassis as with the trailer body 22, 24 of Figure 1. The tractor has a drive axle 16 and load support axle 18, and a two-axle D-train bogie 40 is attached to the rear end of the tractor chassis 10, the tractor and bogie chassis being rigidly fastened together. Forward axle 42 of the bogie 40 is a slip-steer axle, while its rear axle 44 is a standard "fixed" axle. It is this D-train bogie that carries the fifth wheel assembly 20 to which the semi-trailer 20, 24 is attached for pivoting movement as the vehicle negotiates curves. The tandem axle bogie 26 at the rear end of the semi-trailer 22, 24 is provided with a front slip-steer axle 30 and a rear standard fixed axle 32.

Referring now more specifically to Figures 4-6, the slip-steer axle comprises an axle member 46 of square transverse cross section mounted in a fixed support frame 48 for sliding movement along its longitudinal axis 50 without the possibility of horizontal back and forth movement transverse to that axis, but with the possibility of a small amount of vertical movement. The axle member can be of other transverse cross sections, for example rectangular or oval; preferably it is of a cross section such that it will not rotate about its longitudinal axis to facilitate locking it in its neutral.
position, as described below. The frame 48 is mounted on the chassis 22 via a conventional suspension, which in this embodiment comprises an "air-ride" type pneumatic suspension, comprising a pair of main suspension arms 52 pivoted to the bogie frame at pivot 54 and respective transversely spaced air springs 56. A shock absorber 57 is provided for each arm, connected between the arm and the chassis 22. The frame 48 is attached to the suspension beams by brackets 58. Bolts 60 pass through the lower ends of the brackets and have a dual function in holding those lower ends at the required spacing and supporting rollers 62, the bolt and roller assembly retaining the axle in the frame 48 when the bogie is lifted off the ground. A bearing 64 consisting, for example, of a layer of "Fibreglide" (Trademark) a PTFE material applied to a rigid backing, or a linear roller bearing, is interposed between the frame and the axle. The axle has spindles 66 at its ends, each of which mounts a respective single running wheel 68 engaging road surface 70. The movement of the axle is limited by stops 72 thereon which in its two extreme positions engage the respective adjacent ends of the frame 48.
The axle is urged to the neutral position, as shown in Figure 4, by two oppositely operating springs 74 attached at their inner ends to the frame 48 and at their outer ends to the axle 46. In this embodiment these springs are pneumatically operated, but they can instead be hydraulically operated, or of metal, or a combination system, all of which can be sensed and controlled electronically. The axle can at any time be locked in the neutral position by the driver from the cab by remote operation of a pneumatic or hydraulic locking device 76 mounted on the frame 48 and comprising a pneumatic or hydraulic motor that moves a plunger (not shown) into and out of engagement with a cooperating bore 78 (Figure 5) in the axle.

The operation of the slip-steer axle is most clearly seen from consideration of Figure 3 showing the tractor/semi-trailer assembly of Figure 1 making a sharp left turn. The fixed front axle 28 tends to continue straight ahead and the semi-trailer may be considered as pivoting about this axle from a centre point 80, which lies to the left of the combination on an extension of the rotational axis of the front axle, which because of the presence of the slip steer axle 30 now constitutes the rear effective axis in calculating the scuff ratio of the running gear, instead of the axis of the centre axle 30. The rotational axis of the nearer tractor steering wheel 12 and the nearer wheel of self-steering rear axle 32 also meet at this point 80. The centre slip-steering axle is, as a result of this turn, subjected to a sideways acting scuffing force in the direction of arrow 82, owing to drag between the tire tread and road surface 70, causing it to move longitudinally for whatever distance is required to reduce this force to zero, if that is possible, also in such case reducing the sideways drag and scuffing of the tire surface to a minimum, preferably zero, or until the axle has moved to its maximum extent and has engaged the respective end stop 72, at which point the sideways drag and scuffing of the tire surface is at the minimum available amount. As soon as the combination begins to move straight ahead the axle is restored to its neutral position by the action of the centering springs 74. Upon a right turn the axle moves to the opposite side, again until the sideways scuffing force is zero, or until it is at a minimum set by engagement with the respective stop 72.

In the absence of any steering action, as is provided by the slip-steer axles of the invention, any wheel mounted on a fixed axle is subject to such lateral scuffing, with its consequent contribution to wear of the running gear and instability of the vehicle. Their replacement with a slip steer axle not only prevents wear of their own tires, but also improves the life of all of the other tires of the bogie or vehicle chassis, the other axles and all other parts of the running gear, as well as reducing the off-tracking of the vehicle as it turns. The axles of the invention are of relatively simple construction, as compared for example to a typical self-steering axle, with a minimum of moving parts, that can be made robustly without excessive weight, so that they are relatively inexpensive to manufacture, install and maintain. The strategic location of a slip-steer axle in a multi-axle bogie therefore assists in maintaining directional stability under the side forces generated as the vehicle steers around a turn, reducing as a result the yaw stress placed on the tractor, particularly at the king pin, thereby minimizing yaw instability and assisting against the possibility of jack knifing. The use of a slip steer axle as the rear axle reduces the value of WL to the distance between the axes of the front and centre axles, reducing the scuff ratio by a corresponding amount; the same effect is obtained by use of a conventional self-steering axle but at greater capital and running expense. In the embodiment described with a self-steering rear axle and a slip-steering centre axl-e the effective rear axis is coincident with the axis of the front axle and the term WL effectively becomes zero and the scuff "ratio" is at its absolute minimum for this bogie.

It is a most important feature of the slip-steer axles of the invention that they are always reversible in direction without the need to lock them in neutral position while the vehicle is reversed. In practice there is always the possibility of error whereby the driver forgets to lock the axle before reversing; reversing a heavily loaded vehicle without the axle locked can quickly cause damage to the tires, if not to the whole axle, which may be such that the tires, and/or the entire axle, which is a relatively very expensive item, must be replaced. It is also important that, as i.s seen from the description both above and below, the axles are readily incorporated into existing and established designs of tractors, trailers and semi-trailers without need for any substantial modification or redesign of the existing running gear and bogies.

The extent of the longitudinal sliding movement to be permitted to the axle 46 is best determined by using the value of the maximum effective steering angle to be provided, and can be calculated using a geometric method as shown in Figure 4. An arc 84 is struck from a point 86 that is exactly centred between the tires in their neutral position, the radius of the arc being equal to half the distance between the two axles of a tandem bogie, or between the centre and rear axles of a tri-axle bogie, the topmost point of the arc lying on the axle axis 50. Two equal angles 88 are laid out between a vertical line 90 from the point 86 and respective radii lines 92, which are extended beyond the arc until they intersect the axis 50. The distance that the axle 46 is allowed to travel is the tangential distance along its axis 50 from the vertical line 90 to the intersection of lines 92 with the axis 50. The maximum steering angle that is permitted by the self-steering axles currently available is about 20 , and this value is also satisfactory as a practical maximum for the slip-steer axles of the invention and therefore for the angles 88; the angles can of course be greater if required for some special design. A practical minimum for the steering angles is about 10 , since the benefit to be obtained with any angle less than this is unlikely to justify the additional cost of replacing a fixed axle. In a standard North American tri-axle bogie in which the three axles are spaced equidistantly from one another at 1.5 metres (60 inches) the tangential.
distance equivalent to a 20 steering angle is 25cm (10-inches), so that the axle 46 is capable of a total longitudinal movement of 50cm (20 inches). The longitudinal transverse movement required for other layouts can easily be calculated. With an axial movement of this extent it usually will be necessary to employ two single high load capacity wheels, as shown in Figures 1 and 3, in place of the two sets of dual wheels that are common, and that otherwise are employed with the other axles 16, 18, 28 and 32, since otherwise the wheels may protrude sideways out from the trailer during turning by an unacceptable amount, or to an extent that is not permitted in some jurisdictions. If nevertheless dual wheels are provided it may be found that they will need to extend into the trailer body, and this usually is unacceptable because of the problems that result in providing a clear pathway for loading and unloading the trailer. If it is possible to narrow the supporting chassis of the bogie, or of the tractor or trailer frame, to a sufficient extent that such protrusion inward and/or outward does not occur, then of course conventional dual wheel sets can be employed.

Figure 7 is a chart which shows a number of examples of the different configurations that are used in commercial vehicles, and with which the slip-steer axle of the invention can usefully be employed. Line A shows the standard tractor/semi-trailer combination with a tandem axle bogie on the semi-trailer, while line 1 illustrates the standard closely spaced tandem axle bogie in which both axles are fixed. Line 2 shows a tandem axle bogie with increased spacing, and with consequent increased scuff ratio, which is effectively reduced to less than that of the bogie of line 1 by making the front axle a slip-steer axle. Line 3 illustrates a tandem axle bogie of maximum axle spacing (a so-called Ohio bogie) to minimize road loading, and in which making the front axle a slip-steer axle is even more advantageous. It will be apparent that instead the rear axle could be a slip-steer axle while it is the front axle that is fixed, but this is not as advantageous as the two combination illustrated.

Line B and lines 4 through 9 illustrate a standard tractor/semi-trailer combination employing a tri-axle bogie for the semi-trailer. In all of these the slip-steer axle is shown in its preferred position, namely as the centre axle, although its use as the front or rear axle is also possible.
Lines B and 6 show the arrangement shown and described with reference to Figures 1 and 3 in which the bogie rear axle is a self-steering axle. Line 4 illustrates the case in which the front axle is a lift axle, while line 5 shows the most basic (and least expensive) arrangement in which both the front and rear axles are fixed. Line 7 illustrate an arrangement in which the front axle is a lift axle, maximum turning capacity being obtained by lifting the axle, although this generally is undesirable and may contravene some regulations since it increases the load on the other axles while it is lifted.
Line 8 shows an arrangement in which the rear axle is fixed while the front axle is spaced from the centre axle and lifts, and line 9 shows another arrangement with this axle spacing in which the front axle is self-steering.

Line C shows a common arrangement employed with semi-trailers for heavy bulk loads (such as cement) employing a rear tandem bogie according to line 2 and a centre axle which lifts.

Lines D and E and 10 through 14 show combinations employing a quad axle (four closely spaced axles) for the semi-trailer, that shown in line D corresponding to the bogie of line 10, where the two centre axles are both slip-steer axles, and that shown in line E corresponding to the bogie of line 13, in which the two centre axles are slip-steer axles, the front axle lifts and the rear axle is self-steering. In the bogie of line 11 only the front one of the two centre axles is a slip-steer axle, while the other is fixed, as is the front axle. In the bogie of line 12 both the rear axle and the rear of the two centre axles are fixed, while the front axle is more widely spaced forward and lifts, while in that of line 14 the rear axle lifts, the rear of the two centre axles is fixed, and both the front centre and front axles are slip-steer axles; a fifth forward lifting axle is also provided apart from but relatively closely spaced to the rear bogie.

Lines F and 15 through 18 are intended to illustrate various combinations applicable to trucks not employing a trailer or semi-trailer, and to tractors, even though line F
does show a vehicle with a trailer in tow. Line 15 shows both the front and rear sets of running gear are provided with a respective slip-steer axle, while in that of line 16 the front running gear is also provided with a self-steering axle as its rear axle. Lines 17 and 18 illustrate the invention as applied to tractors, the front axle shown therein being the tractor steering axle; the slip-steer axle when provided can be about midway between the steering axle and the two rear fixed axles (see line 17) and is also made to be lifted; in such an arrangement the slip-steer axle is still regarded as closely spaced from a fixed axle and instead it can be closely spaced with the two rear axles (see line 18). There will be other less common combinations available to designers to meet the special requirements of other vehicles as required by their customers.

List of Reference Signs 10. Tractor chassis 12. Steering wheel assembly 14. Driver's cab 16. Tractor drive axle and wheels 18. Tractor additional axle and wheels 20. Tractor fifth wheel assembly 22. Semi-trailer chassis 24. Semi-trailer container 26. Tri-axle semi-trailer bogie 28. Tri-axle bogie front axle 30. Tri-axle bogie middle axle 32. Tri-axle bogie rear axle 34. Container body on Tractor chassis 36. Chassis of container body 36 38. Second fifth wheel assembly 40. Two-axle D-train bogie 42. Front axle of bogie 40 44. Rear axle of bogie 40 46. Slip-Steer axle member 48. Slip-Steer axle supporting frame 50. Slip-Steer axle longitudinal axis 52. Slip-Steer axle suspension arms 54. Slip-Steer suspension arm pivot 56. Slip-Steer axle suspension air springs 57. Shock absorbers between arms 52 and chassis 22 58. Slip-Steer axle brackets 60. Connecting bolts for brackets 58 62. Supporting rollers on bolts 60 64. Sliding pad bearing for axle member 46 66. Spindles at ends of axle 46 68. Single wheel on each spindle 66 70. Road surface 72. Axle longitudinal stops 74. Centralizing springs between axle member and housing 76. Axle locking device 78. Hole of locking device receiving locking plunger 80. Pivoting action centre point 82. Arrow showing direction of scuffing force 84. Construction arc for calculating axle member movement 86. Origin point for arc 84 88. Steering angles 90. Vertical from arc origin point 86 92. Lines establishing angles 88 from vertical 90

Claims (15)

1. Multi-axle running gear for automotive transport tractors, trailers and semi-trailers, such running gear having at least two longitudinally spaced axles mounted parallel to one another, one closely behind the other, with each axle having a corresponding longitudinal axis of rotation disposed transverse to and at right angles to the directions of movement of the running gear, each axle mounting at least two transversely spaced running wheels for engagement with ground over which the running gear passes, wherein one of the axles is a longitudinally movable axle mounted for transverse movement along its longitudinal axis under transverse forces applied thereto by the engagement of its running wheels with the ground as the running gear negotiates a curve or corner to thereby reduce transverse scrubbing of the running wheels on the ground, while the other axle or axles of the running gear is fixed against transverse movement along its longitudinal axis, or their respective longitudinal axes.

2. Running gear as claimed in claim 1, wherein the transverse movement of the longitudinally movable axle from a neutral position is equivalent to a steering angle of at least degrees.

3. Running gear as claimed in claim 1 or 2, wherein the transverse movement of the longitudinally movable axle from a neutral position is equivalent to a steering angle of 20 degrees.

4. Running gear as claimed in any one of claims 1 to 3, wherein the longitudinally movable axle comprises an axle member mounted for such longitudinal movement back and forth along its longitudinal axis in a housing adapted to be fixed to a vehicle, spindles at each end of the axle adapted to receive respective road wheels rotatable about the axle longitudinal axis, and stop members adjacent each end of the axle member engageable with respective ends of the housing to limit the extent of movement of the axle member from a central neutral position.

5. Running gear as claimed in claim 4, and including two opposed spring means connected between the axle member and the housing and urging the axle member to the neutral position to restore it to that position upon lessening of any sideways scrubbing force to a value below that of the spring means.

6 Running gear as claimed in claim 4 or 5, and including locking means operable between the axle member and the housing under the control of an operator to lock the axle member in the neutral position.

7. Running gear as claimed in any one of claims 1 to 6, wherein the longitudinally movable axle is the front axle of a pair of axles.

8. Running gear as claimed in any one of claims 1 to 6, wherein the longitudinally movable axle is the centre axle of three closely spaced axles.

9. Running gear as claimed in claim 8, wherein the rear axle of the three closely spaced axles is a self-steering axle.

10. Running gear as claimed in claim 8, wherein the front axle of the three closely spaced axles is a self-steering axle.

11. Running gear as claimed in claim 8, wherein the front axle of the three closely spaced axles is a lifting axle.

12. Running gear as claimed in any one of claims 1 to 6, wherein the longitudinally movable axle is the front centre axle of four closely spaced axles.

13. Running gear as claimed in claim 12, wherein the two centre axles of the four closely spaced axles are longitudinally movable.

14. Running gear as claimed in claim 13, wherein either the front or the rear axle of the four closely spaced axles is a self-steering axle.

15. Running gear as claimed in claim 13, wherein either the front or the rear axle of the four closely spaced axles is a lifting axle.