AU664919B2 - Vehicle steering apparatus - Google Patents

Description

I t 1 "VEHICLE STEERING APPARATUS" This invention relates to trailer steering apparatus.

This invention has particular but not exclusive application to trailer steering apparatus suitable for use on multi axle semi trailers, and for illustrative purposes reference will be made to such application. However, it is to be understood that this invention could be used in other applications, such as multiple bogey drives, rolling stock and the like, articulated or non-articulated wheeled or tracked vehicles whether for road use or otherwise.

In general, multi axle semi trailers have included parallel mounting o! the multi axles, resulting in excessive *o side scrubbing of tyres when cornering. This scrubbing action causes excessive tyre wear, road damage and excessive loading of trailer and prime mover suspension components.

There have been some attempts to overcc a the problems of parallel tracking axles, and these attempts have generally 0o fallen into two categories. These categories are active steering systems where positive steering forces are applied to steerable axles by, for example, hydraulic steering actuators, and passive steerable axles, where the axles are mounted on a turntable, generally provided with a degree of castor to ensure stable tracking at highway speeds. Active steering systems are inherently expensive to build and maintain due to their mechanical complexity. The permutations of desirable relative steering angles between axles are so numerous as to effectively render active steering impractical without the use of sophisticated, generally electronic, control systems, providing for a yet 2 further reduction in simplicity, increase in cost and increased likelihood of failure and downtime.

The prior art passive steering systems are generally inconvenient since the castor which provides stable tracking also makes the trailer difficult to reverse since the castoring action is set to operate only in the forwards direction. In a typical triaxle unit, and the leading and trailing axles are allowed to self steer by means of the *:Se internal forces acting on the tyres during cornering causing the axle to assume a position which is approximately normal to the instantaneous direction of travel at the axle centre.

The point about which the axle pivots is located ahead of the axle centre-line and horizontally-disposed hydraulic cylinders are generally required to provide damping of this 15 motion. In general the amount of castor that would be required to provide stability at speed would result in excessive steering forces being required. Accordingly, a minimum castor is generally assisted by a spring based self centering bias to a centre detent to prevent the axle developing speed induced oscillation and instability on the road. Of course, the spring bias tends to reduce the effectiveness of the steering action away from the centred position and with the damping adds to the mechanical complexity of the apparatus. For the foregoing reasons castoring multi axle semi trailers have not found commercial acceptance.

The present invention aims to substantially alleviate at least some of the above disadvantages and to provide trailer steering apparatus apparatus which will be reliable and d~ I I efficient in use. Other objects and advantages of this invention will hereinafter become apparent.

With the foregoing and other objects in view, this invention in one aspect resides broadly in a steerable trailer assembly of the type including:a trailer chassis; o an axle assembly, and a steering pivot disposed between said axle assembly and said trailer chassis, characterised in that the axle of said axle assembly is selectively locatable relative to said pivot between a position where the axle is ahead of said pivot and a position where said axle is behind said pivot.

Preferably, the axle assembly takes the form of a subframe bearing the axle and its suspension components such 105 as springs, links and the like such that the axle assembly is steerably moveable relative to the trailer chassis without undue complication being introduced in relation to the ancillary suspension components.

The pivotal interconnection between the axle assembly and the chassis may take any suitable form consistent with the function of permitting the axle of the axle assembly to be selectively located relative to the pivot. For example, there may be provided a pair of pivots such that selection of the operative pivot provides the requisite relative relationship between the axle and the pivot. The pair of pivotal connections between the axle assembly and the trailer chassis may for example comprise pins associated with one or both of the chassis and the axle assembly and adapted to cooperate with corresponding holes provided in the other -e 4 member. In such apparatus, the axle aosembly preferably includes a bearing surface adapted to cooperate with a corresponding bearing surface on the chassis of the trailer.

The bearing surface may take any suitable form with it being preferred to select the bearing surface, in semi trailer applications, from greased and greaseless turntables and the like. Where a greaseless arrangement is selected, I.t is preferred that the corresponding turntable components be coated or laminated with a low friction material such as polytetrafluoroethylene or other low friction polymeric substance.

In such dual pivot apparatus, the selection means may take any form consistent with the function of selecting a chosen pivotal connection to have effect in castoring the 15 steering of the axle to suit selectably forward or reverse operation. Preferably, the selection means, in semi trailer S..applications, includes an intermediate turntable plate interposed between the turntable surfaces of the chassis and axle assembly to form a turntable assembly, the intermediate turntable plate being adapted to passively engage one or the other of the pivotal connections in response to the direction of the applied force on the turntable assembly.

The intermediate plate may selectively engage the pivotal connections by any suitable means. For example, in one embodiment of the present invention, the chassis turntable surface is provided with a pivot pin corresponding to the forward pivotal connection and a curved slot corresponding to a part circumference described by a rear pivot pin about the radius drawn from the forward pin. In turn, the intermediate plate is provided with a pivot hole engaging the forward pivot pin, and a rear pivot pin adapted to track in the rear slot of the chassis turntable surface.

The rear pivot pin extends both upwards into the described chassis turntable slot and downward to engage a pivot hole OQOO0 S. provided in the axle assembly turntable surface. The front e.

e of this lower turntable is provided with a slot to permit Smovement of the front pivot pin which extends from the chassis turntable through both the intermediate and lower 5550 turntable plates.

If desired, the pins may be located and retained at their centred position by locking means to provide parallel 0 5 axles at highway speed, The locking means may take any suitable form with it being preferred to use flat plates with "f 5 curved pin engaging surfaces, adapted to run in millings in the turntables and engage and disengage the pins as required.

•co• The locking plates may be operated by compressed air from the trailer braking air supply. In the interests of safety, the plate locking the rearward pivot pin is preferably spring biased in the disengaged position, whilst for the same reason the forward pin is preferably biased to the engaged position This is because in use, the force of accelerating the trailer and wheel drag is tending to push the axle assembly rearwards relative to the chassis, and therefore forward castor is required. In the event of air failure, it is desired that the forward castor be locked in.

In an alternative arrangement, the axle assembly may be pivotally connected to the chassis by means of a single pivot, wherein a turntable assembly is mounted to the chassis I- 6 and the axle assembly is selectively movable relative to the chassis by sliding engagement of the axle assembly with the turntable assembly. Again, it is preferred that the axle assembly includes the suspension components of the axle such as springs, links and the like such that the axle assembly is moveable relative to the trailer chassis as a complete assembly.

The turntable assembly is in sliding engagement with the remainder of the axle assembly whereby forward motion of

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the trailer chassis causes the axle assembly to slide rearwards with respect to the turntable assembly and reverse motion of the trailer chassis causes the axle assembly to slide forwards with respect to the turntable assembly.

If desired, the selection of the position of the axle assembly relative to the turntable assembly may be provided by actuation means such as air or hydraulic actuating means.

If necessary, the relative movement the axle assembly relative to the chassis and turntable assembly and/or about the steering axis of the turntable may be damped by damping means. Where actuation and damping is desired, the actuator means and damping means may be integral and may for example include a laterally displaced pair of hydraulic dampers interconnected through an adjustable flow restrictor.

Preferably the hydraulic dampers are used to force the axle assembly forwards or rearwards as desired under active control whilst still permitting the pivoting action afforded by the castor mechanism hereinbefore described. Of course, the actuator means may be separate from the damping means and of alternative form such as threaded screw, electric motor, winch or such like.

Apparatus in accordance with the present invention is particularly suited to multi axle applications such as semi trailers where typically three axles are involved. Of particular advantage in the context of multi axle combinations is the possibility of the respective axles and the chassis to interact to provide passive steering of the trailer in both forward and reverse without having recourse to the actuating means described above. Accordingly in another aspect this invention resides broadly in steerable triaxle semi trailer apparatus including:- 'a trailer chassis; a pair of steerable axle assemblies pivotally mounted to respective ends of said chassis, the steerable axle of each 4*9 said axle assembly being selectively locatable between Spositions ahead of and behind its respective pivot axis; an intermediate axle assembly disposed between said steerable axle assemblies and movable fore and aft relative to said chassis; 20 link means interconnecting each of said steerable axle assemblies with said intermediate axle assembly such that said steerable axles are movable in concert with said intermediate axle assembly, and brake means operable on the wheels of said inte.rmediate axle assembly whereby selective operation of said brake means whilst said chassis is moving in forward or reverse effects selection of respective ones of said positions.

In the case of the common three axle semi trailers, it is preferable that the forward and rear axles both be of the steerable kind and that the middle axle be non steerable to provide a tracking datum relative to which the steerable axle assemblies may castor in both forward and reverse. An arrangement such a6 that described has the particular r Sadvantage of reducing the turning circle of the semi trailer and prime mover in assembly particularly when reversing, as Swell as the general advantages conferred by the use of *t *l S go apparatus in accordance with the present invention.

rr Preferably, the steerable axle assemblies of the three axle trailer are of the type described above and including a 40 turntable assembly mounted to the chassis and relative to which the axle assembly is movable in a fore and aft direction. The middle axle assembly is preferably slidably 9 e0 mounted to tche chassis on low friction bearings or surfaces.

The link means between the respective axle assemblies 0000 preferably comprises a single link functional in both compression and tension, it being advantageous to provide the link ends with articulation means whereby angular deviations effected by the steering action of the steerable axle assemblies may be accommodated.

The selective braking means may take any form consistent with the aforementiond objects and will generally take a form consistent with the existing braking systems in current use in respect of trailers. For example, the braking means may comprise a user operable air control adapted to release the air maintaining the middle axle spring brakes in their disengaged configuration. However, it is preferred that the brake operation be controlled by control means responsive to

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9 the operation of the reversing light or reversing hazard warning and the relative positions of the axles such that when the operator selects reverse gear the trailer automatically selects a brake-applied condition resulting in application of the correct castor for reversing. When the erg...

S• position of the steerable axle relative to the turntable So.

pivot axis has been achieved, the control means may then effect release of the brakes such that the trailer mey be reversed. When reverse gear is deselected the control means may automatically brake the middle axle again, forward motion then pulling the axles into correct castor for forward motion. When the appropriate castor is reached, the control

S

means may then release the brakes for forward travel of the trailer. Preferably, the control means is adapted to operate a brake on the wheels of the intermediate axle assembly and be responsive to both a reversing electrical circuit of the S. trailer and limit switches located at the limits to fore and aft movement of the intermediate axle assembly and/or said steerable axles relative to the chassis.

If desired, the control means may also function to engage locking means adapted to lock the respective axle assemblies in a operable position for reversing or forward motion. For example, the control means may be responsive to microswitches or air controls adapted to detect the movement limits of one or more of the axle assemblies, the detection serving to release the brakes and lock the axle assembly or assemblies in position for the particular direction of travel selected.

Preferably, the axle assemblies of trailers in accordance with all of the previously described aspects of the present invention are provided with suspension adapted to render the axle assemblies as free to undertake the respective movements which are preferred. Conventional suspension of road transport trailers and particularly heavy transport trailers such as prime movers and trailers has in the past been accomplished by the use of coil or leaf springs the action of which under load and shock is damped by use of *5 5* 4o gas or hydraulic dampers. Coil springs have an advantage over leaf springs in that the spring characteristics engineered into the spring are more consistent over the life of the spring than is the case with leaf springs, which are 4. o* subject to interleaf wear. Coil springs also take up less space and require fewer strengthened mounting points than do 5 leaf springs. However, coil sprung trailers, in common with leaf sprung trailers also require additional mounting points for the damping unit, unless the damper unit is mounted coaxially with the spring. Mounting the damper unit coaxially with the spring risults in undesirable lateral forces being placed on the damper as the spring conforms with a beam axle under differential loads. Both coil and leaf sprung suspensions are limited in that the spring rates of the springs are not readily adjustable in service to modify the damped behaviour of the suspension. In recent times, air bag suspension has been adopted in certain limited applications but on the basis of cost has not been universally employed. In any case the aforementioned spring and damping systems suffer from the disadvantage in the case of steerable trailer assemblies that the static and dynamic 11 loading over tht respective wheels on each side of the trailer varies greatly, to the extent that tracking of the trailer may be adversely affected.

Accordingly in a yet further aspect this invention resides broadly in a trailer suspension system of the type including axle locating means, spring means and motion damping means characterised in that said spring means and *O said damping means comprises a spring and damper assembly for 4.

each wheel of the trailer, said spring and damper assemblieoeO comprising:a housing having a bore defined therein; 0 a plunger having a chamber defined therein and sealingly 00:o slidable within said housing; *S S a port for hydraulic fluid to pass between said bore and 5 said chamber, and a movable member disposed in said housing or said chamber and defining a gas cavity substantially sealed from said port. Preferably, the bores of the said spring and damper assemblies of each side of the trailer apparatus are hydraulically interconnected.

The housing preferably takes the form of a cylinder having one open end and one closed end to define a bore within which the plunger may slide. The housing is preferably provided with mounting means to permit mounting of the apparatus to a chassis o- axle member or the like. In order to permit sliding fit of the plunger with a maximum of lateral stiffness, the open end of the housing is preferably provided with a reinforcing portion. The fit of the plunger is facilitated by use of a bush or the like configured to the outer dimension of the plunger, with it being preferred to use an oil impregnated bronze bush.

The plunger may take any form consistent with the function of being slidably mouited relative to the housing and having a chamber therein in fluid communication with the S housing. Preferably, the plunger comprises a substantially hollow, cylindrical body portion defining said chamber. One end of the plunger is preferably adapted to be sealably inserted into the housing, the sealing being effected by seal means preferably associated with the open end of the housing.

The port may take the form of an open end to the plunger. However, it is preferred that the port be provided as a drilling in a relatively long plug end to the plunger.

The length and diameter of the drilling is prefe-ably 5 selected to provide the desired rate of damping, which is brought about by non-ideal behaviour of hydraulic fluid as it rapidly passes through the restricting orifice of ~e port.

The end plug is preferably removable from the plus r to facilitate replacement thereof when necessary due to fluid erosion of the walls of the drilling.

The plunger is preferably provided with mounting means to permit mounting of the apparatus to a chassis nr axle member or the like, The movable member may take any form consistent with the function of defining a gas cavity substantially sealed from said port. Since the hydraulic fluid in the communicating spaces interconnected for fluid transfer bt- the port is substantially incompressible, the gas cavity provides an elastic means to accommodate the reduction in combined 13 volume of the housing and plunger in compression.

Accordingly, it is preferred that the movable member interact as little as possible with the housing or plunger to substantially avoid loss of spring characteristics of the apparatus. Preferably, the movable member is associated with the plunger so as to avoid impinging on the limited space Be within the housing lezt when relative movement of the housing

B

and plunger has been accounted for. Preferably, the movable Se* member comprises a free piston slidably and sealably adapted

*S

to the interior of the plunger and defining a gas cavity on the piston face remote from the port.

Preferably, the spring characteristics of the apparatus are modified by varying the pressure of said gas in OB the gas cavity. To this end, it is preferred to provide 5 means of introducing or releasing gas to and from the cavity.

For example, the cavity may be in fluid communication with a source of compressed gas and/or the atmosphere though a selectably operable non-return valve. The gas in the cavity may be of any type such as air, with it being preferred to use substantially inert gas such as nitrog n to avoid problems of oxidation of components.

Preferably, the hydraulic interconnection between respective suspension strut assemblies in grouped into manifold systems for each side of the trailer, to avoid a situation where differential loading to one side of the trailer invokes a lean to one side or the other. For example, in a typical tri axle trailer, it is advantageous to hydraulically link the three left hand suspension strut assemblies by means of a first manifold, and to link the 14 three right hand strut assemblies by a second manifold hydraulically isolated from the first manifold. Typically, the manifolds each comprise a rigid component such as metal tube associated with the trailer chassis, communicationg with their respective strut assemblies via lengths of flexible hydraulic conduit.

In order that this invention may be more easily 4 understood and put into practical effect, reference will now 4 be made to the accompanying drawings which illustrate a preferred embodiment of the invention, whereint- FIG. 1 is a plan and elevation of axle assemblies suitable for use in a tri-axle trailer in accordance with the present invention; FIG. 2 is a plan and elevation of the suspension details *15 of the apparatus of FIG. 1; FIG. 3 is a plan view of a trailer chassis adapted to mount the apparatus of FIGS. 1 and 2; FIG. 4 is a perspective view of a steerable axle assembly frame in accordance with the present invention; FIG. 5 is a diagrammatic representation of wheel loading ratios of apparatus according to the present invention compared with those .f the prior art, and FIG. 6 is a view in longitudinal cross section of hydraulic spring/damper apparatus suitable for use in apparatus according to the present invention.

In the figures, there is illustrated triaxle trailer apparatus including a trailer chassis 50 of the ladder type comprising C channel member 51 and rung members 52.

Fabricated to the trailer chassis at each end thereof is a pair of turntable support plates 53 each including a turntable aperture 54 and turntable limit slots In between the turntable support plates 53, the C channels 51 are adapted at 56 to form a captive sliding bearing for the upper frame portion 57 of a middle axle assembly 60. At each end of the trailer chassis and "corresponding to each of the turntable support plates 53 are 34 0 steerable axle assemblies 61, supported at their respective turntable support plates by a turntable upper member 62 bolted to the support plate 53 and a turntable lower member 63 mounted for rotation relative to the upper member 62 and S. affixed to a frame member 64. The frame member 64 is 4 slidably mountable in the channels 65 of an axle subframe member 66.

The upper frame portions 57 of the middle axle assembly are interconnected by cross members 67 to form a middle S axle subframe The respective axle assemblies 60, 61 may be commonly treated in respect of their unsprung components and like numerals refer to like components of the assemblies 60, 61.

Downwardly depending from each side of the axle subframe members 66 and the middle axle subframe 70 are suspension supporting struts 71, each having articulated thereto a beam axle 72 via a parallel linkage comprising an upper A-frame member 73 and a pair of lower links 74. Each end of the beam axle 72 supports a wheel Positioned across each of the subframe members 66, and located above the respective axles are transverse beams 76, the axles 72 bearing the weight of the trailer chassis 16 assembly on suspension strut members 77 located at each end of the axles 72. The suspension strut members are described in more detail hereinafter.

The three suspension strut members 77 on each side are interconnected by means of left side 79 and right side manifolds associated with the trai.er chassis 50, each suspension strut member 77 being connected its respective "a manifold 79, 80 by means of a flexible hydraulic pipe 81.

The axle assemblies 60, 61 are interconnected along the centre line of the chassis 50 by means of radius rods 82, the ends of which are articulated to the centre of the respective a. transverse beams 76 at mounting points 83.

Each end of the channel members 65 of the front and rear axle assemblies 61 are provided with microswitches (not shown) adapted to be operable in response to the movement forwards and backwards of the turntable frame members the microswitches are linked to a separate braking control adapted to apply and release the brakes of the middle axle assembly and functions in concert with the reversing light circuit of the trailer. The microswitches also include the function of operating solenoids (not shown) adapted to engage and disengage locking pins (not shown) serving to lock the frame members in their respective positions in use.

In FIG. 6, there is illustrated the spring and damper apparatus designated 10 above and comprising a cylindrical housing 10a fabricated from a length of precision steel section 11 and having one end closed by a welded steel plug 12. Recessed into the outer end of the plug 12 is a ball joint seat 13 adapted to receive a ball and stud 14, which is I retained in the joint by a securing plate 15. The stud portion of the ball and stud 14 is adapted to be affixed to either the axle 72 or transverse beam 76 the axle assemblies 61.

The open end of the housing 1C is threaded to engage and end cap 16, which serves to retain an oil impregnated bronze bush 17 and dust seal 20 relative to the housing Slidably mounted within the housing 10a and guided by the bush 17 is a plunger 21 having a threaded bore provided at its outer end to receive the threaded stud portion of a ball and stud 22, the ball and stud 22 forming part of a ball joint 23 adapted to be affixed to the other of either the axle assembly 60, 61 or the transverse beam 76 to that engaged by the ball and stud 14.

The plunger 21 is provided with a longitudinal bore 24, the open end of which is closed by a plunger end piece t" threaded to engage a corresponding threaded portion of the bore 24. The outer face portion 26 of the plunger end piece is adapted to function as a piston in the bore of the housing 1Oa and to this end is provided with a flange portion 27 adapted to sealingly engage the housing 10a via a seal To prevent the ingress or egress of materials to or from the bore 24 past the threaded engagement with the plunger end piece 25, there is provided an 0-ring seal 31.

The plunger end piece 25 is provided with a longitudinal transfer port 32, permitting fluid communication between the interior of the bore 24 and a fluid filled space 33 in the housing 10. The transfer port 32 has a diameter selected in combination with the dynamic properties of the 18 hydraulic fluid 34 and the length of the transfer port 32 to provide the required damping characteristics of the apparatus.

The bore 24 is provided with a gas filled cavity separated from the hydraulic fluid 34 by a free piston 36, slidable in the bore 24 and sealed thereto by sliding seals 37. The gas filled cavity 35 is charged with gas via a drilling 40 provided with a non return valve 41. Access to the non return valve 41 is provided by means of a cut away portion 42 of the ball joint 23. In the present apparatus 0**S the gas selected for use is dry industrial or technical grade nitrogen.

The fluid filled space 33 is hydraulically connected to the corresponding fluid filled spaces of the strut .0 assemblies on the same side of the trailer by means of an internally threaded spigot 43 adapted to receive the flexible pipe 81 and thereby providing communication to its respective manifold 79, 80. The plug 12 is relieved at 44 to ensure that the communication between the fluid filled space 33 and the bore 45 of the spigot 43 remains open for all travel of the piston To instal the gas strut the apparatus is interposed between the relatively moving members and secured thereto by the ball and stud 14 and the ball joint 23 respectively. The trailer weight is permitted to rest on the trailer suspension, causing some compression of the apparatus, forcing hydraulic fluid 34 from the fluid filled space 33 of the housing 10a through the transfer port 32 to the bore 24.

Since the fluid 34 is substantially incompressible, the fluid 19 34 drives the piston 36 along the bore 24 until the force exerted by the gas pressure on the face of the piston 36 opposes the force applied by that portion of the weight of the trailer bearing on the apparatus. The gas filled cavity 35 is then charged with gas via the non return valve 41 and the drilling 40 to trim the suspension to the appropriate combination of ride height and spring stiffness.

In use, the trailer assembly in accordance with the S. abovedescribed embodiment will move forward thereby causing 0':010 the centre of the turntable 62, 63 to be ahead of its axle 72 0:46 o*Go on the steerable axle assembly 61. As the frame member 64 contact the ends of channel 65, the microswitches effect the locking of the apparatus in this position by means of the *0 solenoid. When the trailer is stopped in preparation to reversing and the reverse gear is engaged, the reversing light circuit effects the unlocking of the steerable axle assemblies 61 and the simultaneous application of the brakes of the middle axle assembly 60. As the prime mover reverses the chassis 50 is pushed rearwardly relative to the linked assembly of the steerable axle assemblies 61 and the middle axle assembly 60 causing the centre of the turntable 62, 63 to pass behind the axles 72 thereby changing the camber to one suitable for reversing. As the frame member 64 reaches the limit of travel in the channel 65, the microswitches effect unlocking of the brakes of the middle axle assemblies such that the trailer may be reversed. If desired, the apparatus may be configured such that engagement of the forward gear effects locking of the brakes of the middle axle again such that the chassis is actively pulled forward -r relative to the linked axle assemblies to engage forward castor, the attainment of forward castor being again accompanied by locking of the frame members by solenoid and release (f the brakes of the middle axle assembly.

On the road, the suspension assembly described has each suspension strut member acting as a gas spring and hydraulic dashpot damper in a compact sealed structure that requires only two locating strong points. The apparatus reduces the unsprung mass of the trailer below that achieved by prior art apparatus, axle location requiring only relatively light weight locating rods or the like. Ball joint mounting of the apparatus at both ends conveys the further advantage of eliminating the lateral loads to which the concentric coil spring and damper combinations are prone, see due to variation of suspension geometry in use. The hydraulic interconnection of the struts provides for load levelling at both slow and road speeds.

The aforedescribed trailer was attached to a prime mover g. S and the behaviour of the trailer was observed during normal highway travel and during turns at moderate and at low speeds. Behaviour of the suspension when negotiating spoon drains was also observed, and the steering action when reversing was also observed.

It was observed that, when moving forward slowly and turning, the rearmost axle showed the greatest degree of castoring. This indicates that the effective turning centre of the axle group is forward of the centre axle, reducing the effective wheelbase and therefor reducing offtracking. When reversing, the leading axle castored most, indicating an 21 increase in effective wheelbase. In turns at higher speeds, castoring movement of the axle was relatively small.

The test indicated that contrary to prior art trailers the ride quality was unusually good. It is most unusual to be able to ride comfortably on an unladen semi-trailer.

Traversal of a severe spoon drain showed that the suspension has the ability to accommodate large variations in terrain without losing contact at any wheel.

Further tests were conducted to determine offtracking in "810 a 90 degree turn, high speed tracking and negotiation of difficult road geometry. The low speed turning test was comparative, wherein a man, auvre was marked out on the test track, and the performance of the trailer described above was compared to that of two current typical tractor semi-trailers with fixed triaxle groups.

The results of this test are reproduced at Table 1. It is known that swept path performance is affected by a factor known as the Equivalent Wheelbase, plus the front overhang of the prime mover beyond the steering axle and the forward projection of the trailer beyond the kingpin. The Equivalent Wheelbase depends on the wheelbases of the prime mover and semi-trailer, plus the kingpin lead on the prime mover (the distance of the kingpin position ahead of the centre of the rear axle group) and is given by the formula: EQ WB [(PM WB) 2

(KP)

2 (S DIM) 2 0 5 where PM WB prime mover wheelbase (m) KP kingpin lead (m) S DIM =trailer s-dimension (in) .0.

do a 23 TABLE 1. CHARACTERISTIC DIMENSION OF TEST TRAILERS CHARACTERISTIC PRESENT

TRAILER

STANDARD

TRAILER #1

STANDARD

TRAILER #2 .9 9

**O

5 6 9* *ft Prime Mover 4.1 3.77 4.4 Wheelbase (m) Prime Mover 1.1 1.3 1.1 Overhang (m) Kingpin Lead .20 .30 Trailer S-Dimension 8.75 8.75 8.65 (m) Trailer Forward .95 .65 1.05 Projection (m) Triaxle Spread 3.4 2.45 3.05 Equivalent 9.67 9.53 9.71 Wheelbase (m) Table 1 shows that, although the test trailers varied 9o 9 somewhat in their dimensional characteristics, their Equivalent Wheelbases are similar, and the prime mover overhangs and trailer forward projections do not vary significantly (although the standard Trailer #1 had a short forward projection of the trailer). It is reasonable to conclude that differences in swept path performance are due primarily to the present inventive system.

Table 2 illustrates the comparison of the swept paths of the test subjects, referenced against the standard performance envelope prescribed for such trailers, expressed as the minimum clearance measured between the swept path of 24 the trailer and the standard envelope of Fig 1.

TABLE 2. TEST RESULTS: CLEARANCE TO STANDARD ENVELOPE TRAILER SWEPT PATH CLEARANCE (m) Present 2.52 2.55 Standard Trailer #1 2.04 S* Standard Trailer #2 1.34 The above results indicate an improvement in compliance with the standar of from 20 to 90%. As Standard Trailer #1 had the advantage of a short forward projection of the 4 trailer and a close-spaced triaxle group, it is concluded that the advantage of the adparatu, in accordance with the present invention over conventional trailers would generally be in the upper part of the above range, that is, 50 to In a high speed tracking test, the trailer apparatus in accordance with the present invention was driven around a driver training track and the performance at road speeds was observed. There was no tendency for the trailer to sway at the higher speeds.

A direct comparison of the trailer in accordance with the present invention and standard apparatus was conducted in respect of negotiation of a difficult combination of horizontal and vertical curvature. The present apparatus accommodated severe geometry without losing contact at any axle. The standard trailer lost traction in negotiating tlis geometric situation, whereas the present apparatus had no such problem.

Road loading tests were also undertaken on a comparative basis. Three aspects of the road loading performance of the apparatus were tested: static load shariny, dynamic load sharing and impact loading, with reference to static load sharing, this refers to the ability of the suspension to maintain equal axle loads in lo -speed situations. The static axle loads were found to be equalised to a level of S* 99.6%. It is known that the large proportion of conventional triaxles equalise to no better than .00. The sensitivity of the load sharing to the attitude of the trailer was also determined. For every 100mm that the fifth wheel is raised, tie present apparatus transfers 1.25% of load from the leading axle to the trailing axle; this may be compared with 37% for conventional suspensions.

Dynamic load sharing relates to the aoility of the suspension to maintain equal average axle loads under actual highway operating conditions. The present apparatus returned a dynamic load sharing performance, expressed as a percentage of equal distribution, of 98%. This may be compared with 79 to 98% for all other suspension types.

The dynamic load coefficient is defined as the amount of dynamic wheel loading related to road roughness and trailer speed. the present inventive apparatus returned a coefficient of 0.121. This may be compared with coefficients ranging from 0.133 to 0.135 for conventional triaxle suspensions using air and steel leaf springs.

The results of the aforedescribed loading tests arc summarized in FIG. 26 Referring to FIG. 5, in the case of low-speed manoeuvers, the maximum axle load under a typical conventional triaxle carrying 20 tonnes on the group is 7.3 tonnes, In the case of the present trailer apparatus, this reduces to 6.3 tonnes. 1. the attitude of the trailer was changed by lifting the front of the trailer by 100mm in each S case, the maximum load under the conventional triaxle was 0* increased to 10 tonnes, while the maximum load on the present apparatus remained 6.3 tonnes.

In the case of highway-speed travel on a reasonably rough road, the highest peak axle load under a typical conventional triaxle is 9.1 tonnes, while that under the present apparatus is 8.1 tonnes. Accordingly, for the same axle group weight, the present apparatus produces 1 tonne less actual road loading than conventional triaxles under both low-speed and high-speed conditions, and is also insensitive to changes in the height of the front of the trailer.

The roll stiffness of the suspension has a major effect on the rollover stability of trailers with high centre-ofgravity loads. The roll stiffness of the present apparatus was found to be 1.88 x 106 Nm/rad, as compared to 0.76 x 106 Nm/rad for the best existing suspension configured for dual tyres.

Apparatus in accordance with the abovedescribed embodiment, in addition to providing advantages in terms of offtracking, stability and road loading, provides advantages to ride comfort and minimization of freight damage, tyre wear reduction along with reduction in wear on wheel bearings and !W 27 suspension mountings.

Additionally, the tare weight of the trailer as described above may be reduced given the advantages described above. Fuel costs are reduced through reduction of tyre scrub, poor trailer stability and poor offtracking. Trip times are also affected by the ability of the trailer to obtain access to delivery sites and the reversing ability of t the present apparatus improves site access.

It will of course be realised that while the above "tO*0 has been given by way of illustrative example of this invention, all such and other modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of this S*a d invention as defined in the claims appended hereto.

CS

4o 9 o*

Claims (3)

1. Steerable triaxle semi trailer apparatus including:- a trailer chassis; a pair of steerable axle assemblies pivotally mounted to respective ends of said chassis, the steerable axle of each said axle assembly being selectively locatable between positions ahead of and behind its respective pivot axis; an intermediate axle assembly disposed between said steerable axle asseublies and movable iore and aft relative to said chassis; link means interconnecting each of said steerable axle .assemblies with said intermediate axle assembly such that said steerable axles are movable in concert with said S intermediate axle assembly, and brake means operable on the wheels of said intermediate axle assembly whereby selective operation of said brake means **e whilst said chassis is moving in forward or reverse effects selection of respective ones of said positions. C C*

2. Steerable triaxle semi trailer apparatus according to Claim 1, wherein said brake means includes control means adapted to operate a brake on the wheels of said intermediate axle assembly and responsive to both a reversing electrical circuit of the apparatus and limit switches located at the limits to fore and aft movement of said intermediate axle assembly and/or said steerable axles relative to said chassis. D TLA

3. Steerable triaxle semi trailer apparatus according to Claim 1, wherein each of said axle assemblies includes suspension means of the type including axle locating means and a spring and damper assembly at the ends of each axle supporting said axle assemblies, said spring and damper assemblies comprising:- a housing having a bore defined therein; a plunger having a chamber defined therein and sealingly slidable within said housing; a port for hydraulic fluid to pass between said bore and said chamber, and a movable member disposed in said housing or said chamber and defining a gas cavity substantially sealed from said port, wherein the bores of the said spring and damper assemblies of each side of the trailer apparatus are hydraulically interconnected. DATED THIS Ninth DAY OF September, 1993. QUEENSLAND TRANSPORT INDUSTRIES PTY. LTD. by PIZZEY AND COMPANY PATENT ATTORNEYS