BACKGROUND AND STATE OF THE ART It is desirable to be able to connect as large towed vehicles as possible to tractor vehicles, with a view to being able to utilise the largest possible cargo spaces.

However, a vehicle combination comprising a large towed vehicle connected to a tractor vehicle may entail problems not only in that the vehicle combination occupies a large amount of space at bends on narrow roads and at roundabouts and street corners but also that small movements occurring in the towed vehicle during operation are transmitted to the tractor vehicle and affect driving.

A type of vehicle combination in which the maximum vehicle length permitted in Sweden is utilised is a truck with cargo carrier of standard size, to which truck a semitrailer of standard size is coupled by means of a so-called dolly. A dolly is a towed vehicle in the form of an intermediate carriage used specifically for converting a semitrailer to a trailer. The state of the art in arrangements of this kind has used various linkage systems to cause the towed vehicle to follow in the wake of the tractor vehicle and therefore occupy less space at bends. The small movements have been eliminated by a so-called roller cage (or turning cage), i. e. a large rolling bearing at the coupling between the dolly and the semitrailer. The actual coupling device between dolly and semitrailer has thereafter been placed on the roller cage. This coupling device is usually called the"fifth-wheel coupling"and takes the form of a spigot coupling with slide surfaces between the dolly and the semitrailer. However, the arrangement with roller cage and fifth-wheel coupling on top of one another occupies a large amount of space in the height direction, leading to expensive cargo space having to be removed in order to comply with the laws and regulations which govern maximum permissible vehicle heights.

Document EP 0 126 387 refers to a control device on a trailer whereby the steering of the trailer can be regulated by means of an additional control device which at low speeds is connected to prevent angular differences between tractor vehicle and trailer becoming so great that the two vehicles collide. Absorbing the trailer's small movements during operation still requires a roller cage.

SUMMARY OF THE INVENTION One object of the present invention is to provide an arrangement whereby a carriage unit accurately follows in the wake of a tractor vehicle. A further object is to provide an arrangement whereby the roller cage can be eliminated without small movements of the carriage unit in operation being transmitted to the tractor vehicle. This is achieved by a steerable carriage unit according to claim 1.

A variable ratio between the angular deflection of the towbar and the angular deflection of the steered wheels enables both good carriage unit tracking in the wake of the tractor vehicle and damping of the small movements of the carriage unit so that they are not transmitted to the tractor vehicle. The roller cage can thus be eliminated, making it possible to increase the cargo space in the height direction.

Further features and advantages of the solution according to the invention are indicated in the description and the other claims.

The invention is described in more detail below with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS -Fig. 1 depicts schematically a vehicle combination comprising truck, dolly and semitrailer; Fig. 2 depicts schematically in perspective a steerable carriage unit according to the invention; and

Fig. 3 depicts schematically an enlarged view of a forward portion of the carriage unit according to Fig. 2.

DESCRIPTION OF A PREFERRED EMBODIMENT It is desirable to be able to couple a semitrailer to a truck in order to be able to utilise the maximum vehicle length permitted on Swedish and Finnish roads, which is 25.25 metres. A vehicle combination of that length is depicted in Fig. 1 and comprises a cargo carrier 13 of standard size on a truck 14, with a semitrailer 15 of standard size coupled to it. Coupling a semitrailer 15 to a truck 14 involves using a so-called dolly 1 which is depicted in more detail in Fig. 2. The dolly 1 is thus a carriage unit for connecting a semitrailer 15 to a truck 14. Semitrailer 15 means the type of trailer which has no front axle of its own and is usually coupled to a tractor vehicle which has a very small or no cargo carrier of its own. The dolly 1 is thus needed to support the front of the semitrailer 15 for coupling to a truck 14 which has its own cargo carrier 13 and therefore takes the semitrailer 15 in tow behind it by means of the dolly 1, thus converting the semitrailer 15 to a trailer 1,15.

The dolly 1 according to Fig. 2 has two axles and comprises a towbar 3 which has its rear end connected to the dolly 1 and its forward end connected to a tractor vehicle such as a truck 14. The towbar 3 transmits tractive and braking forces between the dolly 1 and the truck 14 and is articulatedly connected to a frame 7 of the dolly about a vertical axis A (Fig. 3) of a coupling means 6. The towbar 3 is also connected, via a linkage system 4, to a steering bar system 2 of the dolly for pivoting of the front pair of wheels 8a, 8b of the dolly 1 and hence for pivoting of the dolly 1 and the whole trailer comprising the dolly 1 and a vehicle unit, such as a semitrailer 15, connected to the dolly 1 via a fifth-wheel coupling 11. When the truck 14 and hence the towbar 3 pivot, the linkage system 4 causes a change of ratio between the angular deflection of the towbar 3 and the angular deflection of the front wheels 8a, 8b of the dolly 1.

The steering bar system 2 comprises a number of steering bars 2a, 2b, 2c, and the linkage system 4 comprises a number of linkages 4a, 4b, 4c, 4d, as more clearly

illustrated in Fig. 3. For the sake of simplicity, however, the steering bar system and the linkage system in Fig. 2 are each herein referred to by a single reference notation, refs. 2 and 4 respectively.

As illustrated in Fig. 3, the frame 7 supports a front axle 12 on which two wheels 8a, 8b are arranged. The towbar 3 is connected articulatedly to the frame 7 about a vertical axis A of a coupling device, e. g. in the form of a spigot 6. The towbar 3 takes the form of a two-armed lever with a short first lever arm 3a pointing towards the dolly 1, and a long second lever arm 3b pointing away from the dolly 1 relative to the spigot 6. The towbar's short lever arm 3a is articulatedly connected to one end of a linkage coupling element 4a which has its other end connected articulatedly to a first crosslink 4b which is connected articulatedly to a linkage 4d which is pivotable horizontally relative to one side of the frame 7 and is itself articulatedly connected to a left steering bar 2a and a parallel bar 2b of the steering bar system 2. The steering bar system 2 also comprises a right steering bar 2c so that the two wheels 8a, 8b are each steered by their respective steering bars 2a and 2c which are connected together by the parallel bar 2b. The towbar 3 thus cooperates with the steering bar system 2 to cause the wheels 8a, 8b supported by the front axle to pivot when the towbar 3 pivots horizontally relative to the frame 7.

A sleeve 5 surrounds at least partly the linkage coupling element 4a, is movable together with a second crosslink 4c along the linkage coupling element 4a and is guided in a recess 9. This second crosslink 4c is connected articulatedly to the sleeve 5, extends in an opposite direction as compared with the previously mentioned first crosslink 4b and is articulatedly connected to the frame 7 on an opposite side as compared with said link 4d which is pivotable horizontally relative to the frame 7.

When the sleeve 5 runs along the linkage coupling element 4a, the mutual distance L between the articulation points of the crosslinks 4b, 4c changes, and the geometric relationships in the linkage system 4 are thereby also changed in such a way as to cause variation of the ratio between the angular deflection of the towbar 3 and the angular deflection of the wheels 8a, 8b. The second crosslink 4c is connected to an actuator, such as a spring-loaded compressed air cylinder 10, which is arranged to

pivot the crosslink 4c in order to move the sleeve 5 between forward and rear positions.

The compressed air cylinder 10 is arranged to shift the crosslink 4c between the forward and rear positions depending on the speed of the dolly. It is thus possible to obtain a low speed ratio adapted to the dolly 1 accurately following in the wake of the truck 14, and a high speed ratio adapted to preventing small movements of the dolly 1 in operation from being transmitted to the truck 14.

The low speed ratio applies when the sleeve 5 is in a rear position (not depicted in the drawings) whereby the mutual distance L between the articulation points of the crosslinks of the linkage coupling element 4a is relatively small. This ratio results in good tracking by the dolly 1 in the wake of the truck 14, whereby the vehicle combination of truck 14 and trailer comprising dolly 1 and semitrailer 15 occupies less space at bends on narrow roads and at roundabouts and street corners and can therefore keep within the rules set by authorities for maximum permissible space occupied, i. e. maximum permissible sweep surface. The vehicle combination can also be made longer than previously without thereby occupying more space at bends.

The high speed ratio applies when the sleeve 5 is in the forward position depicted in the drawings whereby the mutual distance L between the articulation points of the crosslinks of the linkage coupling element 4a is relatively large. In this situation, the movements arising from the dolly 1 of the linkage system 4 are damped before they reach the towbar 3, so that small movements of the trailer 1,15 are transmitted to a lesser extent to the truck 14. The effects of small movements arising from the trailer 1, 15 upon the drivability of the truck are thereby also diminished. These movements might otherwise cause safety problems, since the semitrailer 15 may be of considerable size, possibly larger and heavier than the truck 14 itself.

The spring-loaded compressed air cylinder 10 may with advantage be controlled by a conventional control device 13. The control device 13 may for example take the form of a microprocessor which receives information (not depicted) about the vehicle's

current speed from an ABS system, a gearbox or a speedometer and which on the basis of the speed sends signals to the air cylinder 10 for corresponding guiding of the sleeve 5 to the position appropriate to the current speed. In the absence of signals, e. g. in the event of any kind of electrical fault or if the control system for any other reason is not working or not activated, the air cylinder 10 places the sleeve 5 in the high speed position, which thus constitutes a basic setting.

With an arrangement according to the invention, the coupling device between dolly 1 and semitrailer 15, i. e. the so-called fifth-wheel coupling 11, may be situated at a relatively low level, since the so-called roller cage (or turning cage), i. e. a large rolling bearing traditionally used in the coupling between the dolly 1 and the semitrailer 15, can be eliminated. The roller cage was previously needed to eliminate the small movements arising from the semitrailer 15, to prevent their being transmitted to the truck 14. The total height of the vehicle combination is reduced by eliminating the roller cage, which means that a total height conforming to the EU rules about maximum vehicle height of 4 metres is thus possible with a standard ISO semitrailer.

During reversing and marshalling, i. e. fine manoeuvres relative for example to a loading bay, angular movements of the towbar 3 may be blocked either by the driver or automatically. It is often easier for the driver if in such situations the dolly 1 behaves like a dolly with a fixed towbar. The towbar 3 can then be fixed with respect to angular movements by means of, for example, another air cylinder (not depicted).

In the above description, a carriage unit in the form of a dolly 1 is used for exemplifying the invention. Other types of carriage units are of course conceivable, e. g. an ordinary trailer, i. e. a carriage unit with both cargo carrier and front and rear axles. More positions for the linkage system 4 are also conceivable than the low and high speed ratio positions described, for a greater number of different ratios. The sleeve 5 may also be replaced by any other means, e. g. a peglike element, designed to be able to run in a stable manner in the recess 9, or some other means than a recess 9 may be used for guiding the sleeve 5, e. g. a stop lug means. The device for manoeuvring the sleeve 5 or equivalent may also take the form of some other device than the compressed air cylinder exemplified, e. g. a power cylinder situated directly between, and acting directly upon, the articulation points of the crosslinlcs 4b, 4c of the linkage coupling element 4a.