This invention in general concerns cargo vehicles and in particular lorries or trucks having a semi-trailer or trailer, i.e. vehicles of the type that comprises a pulling vehicle having a trailer mounted, or a so-called tractor and a semi-trailer attached thereto. Although the invention is defined to concern trucks or lorries it will be obvious to persons skilled in the art that also vehicles for transporting persons, for example busses, may be de- signed in accordance with the teaching of the present invention so the in¬ vention is also to comprise the use of the invention within such closely re¬ lated and technically equivalent areas. More particularly, the invention concerns a technique whereby the maneuvering abilities of such vehicles is improved without traffic safety being deteriorated.

The invention particularly concerns a system for guiding at least one set of rear wheels on a semi-trailer or trailer of a cargo vehicle, which comprises said semi-trailer or trailer and a truck trailer or tractor, and comprising

- bearing means in which said rear wheels are journalled rotatably with respect to the chassis of the semi-trailer or trailer so that said rear wheels can turn from neutral position, in which the rear wheels gener¬ ate no steering effect, to one side in order to generate steering of the vehicle in one direction with respect to said neutral position, or to the other side side in order to generate steering of the vehicle in the oppo¬ site direction with respect to said neutral position,

- power generating means adapted to affect said rear wheels and, when activated, to generate said turning of said rear wheels to one or the other side,

- activation means, mounted in the cabin of the vehicle and adapted to be operated by the driver of the vehicle for activating one of the power generating means and for generating steering of said at least one set of rear wheels independently of the steering of the truck trailer or tractor, and

- locking means adapted to be operated from the driver's cabin of the vehicle for generating, in a first position locking of said rear wheels in said neutral position, and being switchable from said first position to a second position, in which the rear wheels are released and may be brought to turn by activation of the activation means.

Systems of this kind are known within the art, more particularly from the following patents, US 5,213,353, US 5,035,439 US 2,930,631 and FR 2.645.105. These publications disclose systems of the kind stated above as well as components for such systems. As described in for example US 5,035,439 these systems are operated by the driver of the vehicle, in which the system is mounted, activating a switch, which is connected to the locking means, and which by activation switches the locking means from the first position to the second position, upon which it is possible for the driver to generate the turning of the guided steering rear wheels by activating the activation means. By generating the guided steering of the rear wheels a substantial reduction of the total turning diameter is gener¬ ated, as realized in the said prior art, which, during forwards or back¬ wards driving, enables the vehicle to turn round even very sharp corners and thus maneuver at a very limited area, including maneuvering in an area considerably smaller than what is required for maneuvering a similar vehicle having no guided steering system, for instance to turn round a sharp corner, back up to a loading ramp, turn in a limited area etc. It will be evident to persons skilled in the art, however, that a reduction in the turning diameter to such a substantial extent as achieved by a guided steer¬ ing system of the this art necessarily entails instability in driving at normal speed on a road and thereby entails a safety risk not only to the driver of the vehicle but also for other persons in the traffic in case the driver acti¬ vates the guided steering system when driving on a normal road and brings the vehicle to turn very sharply and thereby provokes a considerable risk of said vehicle turning over or loosing its steering capability because of skidding.

It should be noted that in connection with a different technique, i.e. co- steering on trucks or busses, safety systems are known, but said co-steer¬ ing systems, which are described in for example EP 0 710 601 and DK 94 00327 U3 differ from the guided steering system in that the rear wheels of

the vehicle perform co-steering corresponding to the steering deflection brought about by means of the steering wheel of the vehicle and thus cor¬ responding to the steering front wheel of the pulling vehicle or the truck. Consequently, co-steering systems of said type have no activation means which are operated from the driver's cabin of the vehicle and may gener¬ ate steering, i.e. a guided steering of the rear wheels. Co-steering systems of said type provide interconnection between the co-steering rear wheels and the steering front wheels and comprise locking means corresponding to the locking means of the guided steering system described in the pre- amble of this description and they may furthermore comprise a speed rela¬ ted locking and corresponding return facility of the co-steering rear wheels when the vehicle moves at a speed exceeding a preset limited speed level, typically a speed of the order of 50 km/h as described in EP 0 710 601.

The basis of the present invention, consequently, lies in realizing that known guided steering systems in spite of their excellent qualities in im¬ proved maneuvering may entail a safety risk to the driver of the truck in question as well as to other persons in the surrounding traffic in case the driver intentionally or unintentionally activates the system for guided steer- ing of the rear wheels of the truck when driving on an ordinary road and at normal speed.

The object of the present invention, therefore, is to eliminate the above described safety risk in known systems of guided steering of rear wheels of trucks or other similar vehicles in order to improve the maneuverability of said truck or similar vehicle and thus eliminate the safety risk inherent in systems of this art.

The object stated above is achieved by a system of the art stated above, said system in accordance with the teaching of the invention is character¬ ized in that the system further comprises blocking means comprising speed detecting means or being adapted to cooperate with a speed detector of the vehicle for detecting the speed of the vehicle and for determining whether the speed of the vehicle is higher or lower than a preset limited speed level and cooperating with said locking means and being adapted to generate blocking of the locking means in the first position when the speed detected by the blocking means is higher than the speed limit, and to release die

locking means from said blocking when the speed detected by die blocking means is lower than the speed limited.

By the guided steering system according to me present invention the afore - said safety risk is readily eliminated, as the blocking means characterizing the invention and comprising speed detecting means or cooperating with a speed detector on the vehicle block the locking means of the system and d us prevent the driver of the truck or similar vehicle from intentionally or unintentionally making a guided steering of the rear wheels of the vehicle at a speed so high that activation of the guided steering system and a guided steering of the rear wheels of the vehicle entail a risk that the vehi¬ cle either turns over or skids, upon which the driver loses control of the vehicle. It should be remembered in this connection that vehicles of the type in question are even very large and heavy vehicles, which for exam- pie in the case of semi- trailer vehicles may be up to 18 meters long or in certain countries even longer and weigh up to 52 tons or more, so it will be readily understandable why it would be even very dangerous, not only to the driver but also other persons in the surrounding traffic, if the driver of such a vehicle should lose control of the vehicle. The system according to the present invention consequently only allows guided steering of the rear wheels of the vehicle when the speed detected by the blocking means is lower than the preset speed limit, and when the guided steering of the rear wheels of d e vehicle may take place without safety risk.

For further improvement of the safety in the guided steering system ac¬ cording to the present invention it is preferred that the blocking means are adapted to lock me vehicle to drive at a speed below the said speed limit, preferably by locking the motor of the vehicle to run idle when the locking means are in said second position. It is thereby ensured that, after said locking means are brought to said second position and the said at least one set of rear wheels of the semi-trailer or the trailer are turned by activation of said activation means, the driver of the vehicle cannot, intentionally or unintentionally, accelerate the vehicle more than to said preset speed limit and thereby expose him/herself or other persons in the surrounding traffic to a considerable safety risk.

It will be obvious to persons skilled in the art that the system according to

the present invention having the means known per se for generating said guided steering of the at least one set of rear wheels of the truck may be implemented in a variety of ways and comprise a single set of rear wheels, since the other sets of rear wheels of the semi-trailer or trailer may be either attached to the chassis of the semi-trailer or trailer and therefore cannot turn with respect to it or, as an alternative, may be embodied so mat die guided steering of d e said one set of rear wheels at the same time cause an elevation of the other sets of rear wheels as described in US 5,035,439. In accordance with the teaching of the invention it is preferred that the system comprises all sets of rear wheels of the semi-trailer or trail¬ er and that, consequently, the system comprises bearing means for rotat- able journalling of all sets of rear wheels of the semi-trailer or trailer, since it is hereby ensured that the rear wheels of the semi-trailer or trailer will turn in a predictable way and, particularly in consequence of die im- proved safety characterizing the system according to the present invention, may generate a maximum guided steering effect on the rear wheels of the semi-trailer or trailer. By simultaneously turning all rear wheels of the semi-trailer or trailer it is further achieved that tyre wear is reduced in comparison with a construction in which only one set of rear wheels gen- erate guided steering while me remaining rear wheels carry out no guided steering function.

In accordance widi the above described preferred embodiment of the sys¬ tem according to die present invention, in which all rear wheel sets of the semi-trailer or trailer are brought to turn by activation of the guided steer¬ ing system, it is preferred that the power generating means are designed so as to generate a guiding in parallel of all rear wheels of the semi-trailer or trailer when the rear wheel is turned to one or the other side in order to achieve on d e one hand a maximum steering function and on the other hand minimizing me tyre wear.

In the present application the expression 'guiding in parallel' of tine rear wheels of the semi-trailer or trailer is used as an expression describing die fact that sets of rear wheels are turned simultaneously and not independ- ently of one anodier even if the rear wheels of the semi-trailer or trailer when turning, like the front wheels of a vehicle, do not perform a perfect¬ ly parallel movement but a movement which, with a limited deviation from

die geometrically ideal parallel guiding, corresponds to me curve tracks which the wheels in question are to follow during turning of the vehicle.

The system according to die present invention, as mentioned above may be embodied in a variety of ways in accordance with technical principles known per se, for instance comprising the technical solutions described in the publications mentioned above. According to me presently preferred embodiment of the system according to the present invention, particularly die power generating means may be adapted to generate turning of me rear wheels from the said neutral position to eidier a maximum turned position to die one side or a corresponding maximum turned position to d e other side. Consequently, the system according to mis preferred embodiment of the system according to the present invention is adapted so that the guided steering rear wheels are either in said neutral position or in said maximum turned position to the one side or the od er side for providing maximum maneuverability. In this embodiment the vehicle thus performs in a com¬ pletely predictable way, since die rear wheels are positioned in one of diree possible positions and in die steering position to either one or the other side provide a unique and completely predictable steering effect.

In accordance with an alternative embodiment of me system according to diis invention the power generating means are adapted to generate a con¬ tinuous turning of the rear wheels from said neutral position to either one or the other side, whereby it is possible for the driver of the vehicle to provide more or less steering effect from the rear wheels and thus, by acti¬ vating ie activation means provide more or less steering effect corre¬ sponding to me generating of more or less turning of the steering wheel of die vehicle to provide more or less turning by die steering front wheels of the vehicle.

Depending on the vehicle construction in question and in consideration of feasible and reliant technical solutions, die system for guided steering of the truck or similar vehicle may be embodied as an electrically, a pneuma¬ tically or preferably a hydraulically generated system or a combination of such systems, since the individual parts of the system, for example the power generating means may be electrically, pneumatically or preferably hydraulically activated, whereas the other elements or parts of the system

may be implemented as electrically, pneumatically or hydraulically acti¬ vated or controlled components or parts, or a combination thereof. In me preferred embodiment of me system according to the present invention d e system is a combined hydraulically activated and electrically controlled system but omer system constructions are possible within the scope of the protection witiiout thereby deviating from die scope of the invention.

In die blocking means characteristic to the invention, the detection of die speed of die vehicle as mentioned above may be performed by means of a separate speed detector or a separate speed detecting means mounted on d e vehicle and connected to and cooperating wi the blocking means ac¬ cording to d e teaching of die present invention. Alternatively and prefer¬ ably, the system according to die present invention utilizes a speed detec¬ tor which is mounted on die vehicle, and which may be connected to eidier the speedometer of the vehicle or in modern vehicles be inherent in the

ABS brake system of the vehicle, and the system is consequently designed as an integrated safety part of the general safety equipment of the vehicle.

For improvement of me safety of the system it is preferred diat the block- ing means are adapted to provoke the said blocking of the locking means in case no speed is detected, and to only generate said release of die lock¬ ing means from said blocking provided die speed is positively detected as a speed which is lower than said speed limit. It is thereby ensured that, in case of malfunction, e.g. in the speed detector or in other parts of the mechanic, hydraulic, pneumatic or electric system, no overruling of die overall safety consideration takes place, in odier words diat the system ac¬ cording to the present invention can only be activated to provide guided steering of die rear wheels of the vehicle when the vehicle is moving un¬ ambiguously at a sufficiently low speed in consideration of safety. De- pending on die size and stability of the vehicle in question said speed limit may be chosen relatively high or low, but it is presumed diat in no system for any use at all may d e limit be set higher than 30 km/h and preferably lower than 20 km/h. In die presently preferred embodiment of me system according to me present invention said limit has been set at 10 km/h in order to ensure diat the increased maneuverability, but simultaneously re¬ duced stability of the vehicle, which is achieved when guided steering af¬ fects die rear wheels of the vehicle, does not occur at irresponsibly high

speed.

The system according to the present invention may be elaborated in a vari¬ ety of ways and as stated above may be combined wid d e od er safety equipment of me vehicle. In this connection it may also be particularly adapted so that the blocking means are adapted to cooperate with die trans¬ mission of the vehicle, and be adapted to only allow release of die locking means from the said blocking when the transmission is set in a lowest gear for driving forwards or backwards. A double security is diereby achieved in mat die activation of die system is prevented for example in case of a fatal malfunction if die vehicle is set in any other ian the lowest gear, and dius drives at a speed higher than the speed determined by the lowest gear of d e vehicle. The system may alternatively be designed to only allow guided steering of the rear wheels if the vehicle is in reverse gear; this however, will unambiguously reduce the maneuverability of the vehicle in comparison with a vehicle being steerable by guided steering of die rear wheels when driving forwards as well as backwards.

The invention is explained in die following wid reference to the drawing in which

Fig. 1 is a schematical and partly cross-sectional view of a semi-trailer vehicle seen from above and having guided steering means for the rear wheels of the vehicle,

Fig. 2 is a schematical and partly cross-sectional view of a rear wheel axle arrangement for generating guided steering of rear wheels on a semi-trailer or a trailer,

Fig. 3 is an electric and hydraulic circuit diagram of the safety blocking part of the system, and

Fig. 4 is an overall diagramatic view of the hydraulic system part of the system.

Fig. 1 schematically shows a truck 10, comprising a pulling vehicle or tractor 12 and a trailer or semi-trailer 14. The pulling vehicle or tractor 12

has a driver's cabin 16 mounted on d e chassis of die tractor, said chassis rotatably supporting two front wheels, a left front wheel 18 and a right front wheel 20, respectively, as well as two rear wheels, a left rear wheel 22 and a right rear wheel 24, respectively. The longitudinal axis of the tractor is marked by the reference numeral 26 and the center axis of the rear wheels 22 and 24 is marked by die reference numeral 28. Reference numeral 30 is die rotation axis of the semi-trailer with respect to die chas¬ sis of the tractor 12.

The semi-trailer 14 comprises a chassis or a semi-trailer box 32, which is rotatably journalled with respect to the tractor about the above described rotation axis 30. The semi-trailer 14 has two sets of rear wheels, a fore¬ most set of rear wheels comprising a left rear wheel 34 and a right rear wheel 36, and a hindmost set of rear wheels comprising a left rear wheel 38 and a right rear wheel 40. In fig. 1 me rear wheels 34, 36, 38 and 40 are shown in full lines in a position turned so diat the maneuverability of the total vehicle is improved, as die steering rear wheels 34, 36, 38 and 40 increase the turning ability of the vehicle 10. In fig. 1 the front wheels 18 and 20 of the tractor 10 are turned to die left so that the total vehicle turns sharply to the left, and at the same time the rear wheels 34, 36, 38 and 40 shown in full lines are turned so diat die total vehicle turns sharply to the left, thereby turning more sharply to the left than a vehicle having no guidedly steered rear wheels on the trailer or semi-trailer. Here it should be noted diat the rear wheels 34, 36, 38 and 40 of die semi-trailer 14 natu- rally can turn not only from the positions shown in dotted lines in fig. 1 to the positions shown in full lines in fig. 1, but also may turn to die other side, diereby generating turning of die vehicle to me right. In fig. 1 die rear wheels 34, 36, 38 and 40 are shown in dotted lines in a center posi¬ tion, in which the rear wheels 34, 36, 38 and 40 are not steering, and in which center position or neutral position the rear wheels are locked when die vehicle 10 moves at a speed exceeding a maximum speed preset for safety reasons, above which speed die rear wheels 34, 36, 38 and 40 of die semi-trailer must not be activated to generate guided steering of the vehicle 10. It should be realized diat the vehicle shown in fig. 1 generates an even very sharp turning of the rear wheels 34, 36, 38 and 40 in d e position shown in full lines, and, provided me vehicle moves at a speed of 60 km h or 70 km/h, a turning or swaying so sharp may entail that die vehicle turns

over or that die vehicle is brought to skid, upon which die driver of d e vehicle involuntarily will lose control of the vehicle.

The reference numeral 42 in fig. 1 designates a foremost rear wheel axle arrangement for the foremost rear wheels 34 and 36 of me semi-trailer, said wheels being rotatably journalled with respect to die chassis of d e semi-trailer 14 and dius with respect to the rear wheel arrangement 42 from the positions shown in dotted lines in fig. 1 to the position about rotation axes 44 and 46, respectively, shown in full lines in fig. 1. The turning of the rear wheels 34 and 36 is generated by means of a connection rod, shown in fig. 2 and designated die reference numeral 43, as well as by means of a a main cylinder 48 and a subcylinder 50, respectively, which are interlocked by means of a hydraulic circuit, which will be de¬ scribed below widi reference to fig. 4, to generate a parallel guiding of die two rear wheels 34 and 36 pertaining to the foremost set of rear wheels of the semi-trailer 14. Likewise, the two hindmost rear wheels 38 and 40 of die semi-trailer 14 are mounted on a rear wheel axle arrangement 52 which, corresponding to the rotation axes 44 and 46 has rotation axes 54 and 56, about which die rear wheels 38 and 40 may rotate with respect to the rear wheel axle arrangement 52, me same way as the rear wheels 34 and 36 may turn about the axes 44 and 46 widi respect to the rear wheel arrangement 42. The hindmost rear wheel arrangement 52 like the fore¬ most rear wheel arrangement 42 has a connection rod, which is shown in fig. 2 and designated the reference numeral 53, as well as two hydraulic cylinders in the form of a main cylinder 58 and a subcylinder 60, which like the main cylinder 48 and die subcylinder 50 in the foremost rear wheel arrangement 42 are interlocked in a hydraulic circuit for generating a parallel guiding of die two rear wheels 38 and 40. The rear wheel ar¬ rangement 52, however, differs from the foremost rear wheel arrangement 42 in that d e main cylinder 58 acts on die right rear wheel 40 whereas the subcylinder 60 acts on die left rear wheel 38, the main cylinder 48 and die subcylinder 50 likewise in me foremost rear wheel arrangement 42 acting on die left rear wheel 34 and the right rear wheel 36, respectively. This difference between the two rear wheel arrangements 42 and 52 solely serves to provide a symmetric rear wheel construction and improve the inherent stability of the entire rear wheel construction.

Fig. 2 shows, in greater details, d e rear wheel arrangement 42 of me two foremost rear wheels 34 and 36 of die semi-trailer 14. As in fig. 1 the rear wheel arrangement 42 is seen in planar view. Apart from mat fig. 2 shows die same components pertaining to the rear wheel arrangement 42 as shown in fig. 1 and described above, and further shows die detailed loca¬ tion of the main cylinder 48 and d e subcylinder 50.

Shown in fig. 3 is an electric diagram of a safety system in d e system ac¬ cording to the present invention. The safety system serves to ensure that the guided steering of die rear wheels of die vehicle on me semi-trailer or trailer is only generatable when the vehicle moves at a speed which is lower that the above mentioned maximum speed of preferably 10 km/h. The overall safety system is designated d e reference numeral 70 and comprises a switch 72, which is activated by a speed relay of the ABS brake system of the vehicle, said relay being activated when die speed of die vehicle exceeds the said maximum speed. In fig. 3 a situation is shown where d e vehicle eidier stands still or moves at a speed which is lower than said maximum speed or speed limit. The switch 72, consequently, supplies power to an indicator lamp 74 on an operation panel 76 in me driver's cabin of the vehicle. The lamp 74 thereby informs the driver of the vehicle that it is possible to perform guided steering of ie rear wheels of the semi-trailer. The operation panel 76 further contains a switch 78, which die driver of die vehicle may operate in order to release the rear wheels of d e semi-trailer from the center positions shown in dotted lines in fig. 1 of d e drawing, and tiiereby bring the rear wheels to turn to either the left or the right. The safety system further includes a relay 80 having a coil 82, and over ie coil a mm off diode connected in parallel as well as a contact 86, which in die rest position of die relay is open. The relay 80 is activated simultaneously wi i a power being supplied to die control lamp 74 via the switch 72, whereby the contact 86 closes. The contact 78 of die operation panel 76 is connected in a series configuration widi die contact 86 and, when die relay 80 is activated and d e contact 86 closed, may sup¬ ply power to a magnetic valve 88. The magnetic valve 88 provides normal pressure via die pneumatic circuit of the vehicle to two spring-loaded pneumatic locking devices 90 and 92, which are normally, i.e. without air pressure, locked, and which are mounted on die rear wheel arrangement 42 and me rear wheel arrangement 52, respectively. By activation of the

magnetic valve 88 air pressure is supplied to die spring-loaded pneumatic locking membranes 90 and 92, as will be seen from the upper part of fig. 4, upon which it is possible, as will be explained below widi reference to fig. 4, to perform a guided steering of the rear wheels 34 and 36 as well as the rear wheels 38 and 40 pertaining to the rear wheel arrangements 42 and 52, respectively. The safety system shown in fig. 3 further comprises a number of switches 94 and 96, which are preferably embodied as non- contact switches pertaining to the locking membrane devices 90 and 92 and mounted on one of d e two rear wheel arrangements. Likewise, corre- sponding switches connected in a series configuration are provided for me second rear wheel arrangement, said switches being embodied as non-con¬ tact switches. The switches 94 and 96 are turned off when the correspond¬ ing rear wheels are in the central or neutral position, but close when the rear wheels are turned from said positions shown in dotted lines in fig. 1. Power is diereby supplied through the switches 94 and 96 to a relay 98 which, like the relay 80 described above has a coil 100, a turn-off diode 102 connected in parallel with the coil and a switch 104. When the coil 100 is activated, the switch 104 is turned off, which again disconnects die supply of power to a control lamp 106 included in the operation panel 76, said control lamp informing the driver that the rear wheels of the vehicle are no longer in d e locked central or neutral position.

The safety system further includes a relay 112 which, like the relays 80 and 88 described above, includes a coil 114, a turn-off diode 118 con- nected in series with the coil and a contact 112. Power is supplied to die relay eidier via a diode 108 from the switch 72 or via a diode 110 from the contacts 94 and 96. The contact 118 of the relay 112 is connected to die ETC steering of the vehicle's motor and keeps die motor running idle when power is supplied to die relay 112 through eidier the diode 108 or the diode 110, corresponding to respective detection of whether or not the speed of d e vehicle is slower than 10 km/h, and whether the axle locks pertaining to die locking membrane devices 90 and 92 are released, which is detected by die switches 94 and 96.

Fig. 4 shows in details die hydraulic circuit of the system according to the present invention. The upper part of fig. 4, as mentioned above, shows d e locking membrane devices 90 and 92, die protruding locking taps of

which, 120 and 122 respectively, are adapted to interlock with die pertain¬ ing connection rods 43 and 53, respectively, of the rear wheel arrange¬ ments 42 and 52, respectively. The lower half of fig. 4 shows the rear wheels 34, 36, 38 and 40 or the semi-trailer, the location of d e rear wheels in the diagram of fig. 4 not corresponding to die physical location of die rear wheel, but rati er determined by die consideration of the clarity of me diagram. Similarly the corresponding cylinders, ie. the main cylin¬ ders 48 and 58 pertaining to the wheels 34 and 40, respectively, and the subcylinders 50 and 60 pertaining the die wheels 36 and 38, respectively, are placed juxtaposed die respective wheels. The hydraulic circuit includes a hydraulic engine, which supplies hydraulic pressure liquid via a pump to a feeding line 124 dirough a one-way valve 126. The pump 122 sucks hydraulic liquid from a hydraulic liquid container 128, which is further¬ more in connection widi die hydraulic circuit via a return line 130. A pres- sure relief valve 132 is inserted between die feeding line 124 and the re¬ turn line 130. The feeding line and d e return line, 124 and 130 respective¬ ly, are in connection widi an electric/hydraulic three-position valve 134. In the neutral position shown in fig. 4 the connection from the feeding line 124 and die return line 130 to the main cylinders 48 and 58 and further- more to the subcylinders 50 and 60 is shut off, but when die electric/hy¬ draulic diree-position valve 134 is activated to eidier side, a connection is established between the feeding line 124 and the return line 130 to die hy¬ draulic circuit comprising the main cylinders 48 and 58 as well as die sub¬ cylinders 50 and 60. The main cylinders 48 and 58 are coupled in a sym- metric cross coupling, the front compartment of die main cylinder 48, i.e. die compartment in front of the plunger of die cylinder, being coupled to the same hydraulic line 136 as die rear compartment of the main cylinder 58. Similarly, the rear compartment of die hydraulic cylinder 48 is coupled to die same hydraulic line 138 as the front compartment of die hydraulic cylinder 58. Pressure relief valves, 140 and 142 respectively, are inserted between die hydraulic lines 136 and 138 and die return line 130. The hydraulic circuit further comprises a branch from the hydraulic line 138, said branch leading through two adjustable valves 144 and 146 and two respective check valves 148 and 158 to die front compartments of die subcylinders 50 and 60 and die rear compartments of die subcylinders, respectively. Safety valves 152 and 154 are furthermore inserted in the respective branches of the subcylinder 50. The branches to the subcylin-

ders 50 and 60 serve the purpose of providing a hydraulic guided leading of die set of rear wheels comprising the rear wheels 34 and 36 in parallel widi die set of rear wheels comprising e rear wheels 38 and 40, tiius pre¬ venting one set of rear wheels from turning more or less than the otiier set of rear wheels. The subcylinders 50 and 60 are coupled in a cross coupling to die main cylinders 48 and 58 and are furthermore interconnected in said cross coupling. It is tiiereby achieved diat provided one set of rear wheels performs a non-parallel track with respect to the otiier set of rear wheels, a pressure difference is accumulated over one of the subcylinders, causing diat by release through the cross coupling of die hydraulic circuit said pressure difference performs guided steering of the rear wheels so that not only die rear wheels pertaining to a set of rear wheels in consequence of die mechanical coupling via the pertaining connecting rod are led in paral¬ lel with each other, but so that the two sets of rear wheels are continuously led or steered in parallel with each other.

The electric control of die electric/hydraulic tiiree-way valve 134 is achieved by means of a three-position switch 156 which is likewise in¬ cluded in d e control panel 76, and which receives power through the switch 72. By shifting the switch 156 from a central position, in which the connection through the switch 156 is disconnected, to one of die two outer positions, one of which is shown in fig. 4, power is supplied to die two coils of the electric/hydraulic diree-position valve 134. The electric/hy¬ draulic three-position valve 134 is thereby activated for establishing a con- nection from the hydraulic feeding line 124 to one of die two hydraulic lines 136 and 138, and from the hydraulic return line 130 to the otiier of d e two hydraulic lines 136 and 138. By movement of die rear wheels 34, 36, 38 and 40 from the central positions shown in dotted lines in fig. 1, a switching of sets of contacts mounted on die respective axles is provided, said sets of contacts being shown in fig. 4 and designated the reference nu¬ merals 160, 162, 164 and 166. The switch sets 160 and 162 are mounted so as to detect a turning of the wheels to the left, whereas the switches 164 and 166 are mounted so as to detect a turning of the wheels to the right into the position shown in fig. 4. A connection is thereby established dirough the switches 164 and 166 from the feeding line, which also sup¬ plies die three-position switch 156, dirough a corresponding lamp 172 which indicates to die driver diat the rear wheels have been turned to die

right. By turning in the opposite direction, i.e. turning to die left, the switches 160 and 162 are closed, thereby turning on a corresponding lamp 170 to indicate to die driver that the wheels 34, 36, 38 and 40 have been turned to die left. When the wheels 34, 36, 38 and 40 are in the central position shown in dotted lines in fig. 1, all the contacts 160, 162, 164 and

166 are turned off, entailing mat also die lamps 170 and 172 are turned off.

Although the invention has been described above widi reference to a spe- cific example of an embodiment it will be obvious to persons skilled in the art diat witiiin the scope of die invention a variety of changes and modifi¬ cations may be made according to die present invention witiiout thereby deviating from the spirit and die aim of the invention as it is defined in the patent claims below. Thus, in alternative embodiments of the system ac- cording to die present invention it is possible to implement the system as a pneumatic system and further to use separate detectors, possibly widi a corresponding microprocessor control for activating the respective pressur¬ ized-air cylinders monitored by a speed detector, separately mounted on the vehicle, as well as to either detect the movement of die vehicle with respect to the earth or in a satellite navigation system to receive informa¬ tion from a communication satellite. Apart from such microprocessor con¬ trolled pressurized-air implementing of the system according to die present invention, it is possible to replace parts of the system described above by equivalent electric or pneumatic elements and replace parts of the hydrau- lie components described above by corresponding mechanic, pneumatic or electric components.