Abstract:
The invention relates to a vehicle ( 1 ) provided with a frame ( 2 ) having a front frame part ( 3 ) and a rear frame part ( 4 ) which can tilt with respect to one another. The vehicle ( 1 ) comprises three or more wheels ( 7, 7′, 13 ), it being possible to rotate the front wheel ( 13 ) with respect to the steering column ( 18 ). In this case, a sensor ( 24 ) determines the angle of rotation between the front wheel ( 13 ) and the steering column ( 18 ) and, as a function of this angle of rotation, actuates the tilting means ( 9, 9′ ) of the vehicle. As a result of the consequent tilting of the front frame part ( 3 ) and as a consequence of the speed at which the vehicle ( 1 ) is travelling, the front wheel ( 13 ) and the tilting angle will automatically adopt the correct level for allowing the vehicle ( 1 ) to travel through the bend in a stable manner at the given speed. Controlling the tilt via the difference in angular rotation between the steering column ( 18 ), which is flexibly connected to the front wheel ( 13 ), and the front wheel ( 13 ), it is possible to control the tilt in a simple and robust manner.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a vehicle provided with 
     at least three wheels, 
     a frame comprising a first frame part and a second frame part, it being possible for the frame parts to tilt with respect to one another about a tilting axis which lies in the longitudinal direction, 
     at least one front wheel which is connected to the first frame part and can tilt about a front-wheel steering axle which lies essentially parallel to the plane of the front wheel, 
     a steering wheel which is rotatably connected to the first frame part, 
     tilting means which are connected to the first and second frame parts for executing a tilting movement between the first and second frame parts, and 
     a sensor which is coupled to the front wheel and to the tilting means for forming a control signal for driving the tilting means. 
     A vehicle of this kind is known from WO95/34459 in the name of the applicant. This document describes a self-balancing, preferably three-wheeled vehicle, in which the sensor, which in this case is formed by a hydraulic rotary valve, measures the force or the moment on the front wheel. In response to the signal from the sensor, the front frame part, which comprises the driver&#39;s cab and the steering wheel, is tilted until the moment on the front wheel is virtually equal to zero. In this way, a self-stabilizing tilting action is obtained, which causes the vehicle to corner in a stable manner at all speeds. Since narrow vehicles as described in WO95/34459 generally have insufficient lateral stability and manoeuvrability to be able to participate in normal (automotive) traffic, a tilting system of this nature increases the stability, in such a manner that a narrow vehicle of this nature can become a fully functioning means of transport. The tilting system described is fully automatic, with the result that the driver does not require any special skills to control the vehicle. The known system provides a vehicle which reacts safely and predictably in all conceivable driving conditions. 
     Although the known tilting vehicle is very effective, the force sensor for determining the force or the moment on the front wheel is relatively complex. Furthermore, the feel of the steering of the known device can be improved further. 
     SUMMARY OF THE INVENTION 
     Therefore, the object of the present invention is to provide a tilting vehicle which comprises a simple and robust control of the tilting action, combined with an optimum feel of the steering for the driver. 
     To this end, the vehicle according to the present invention is characterized in that the steering wheel is connected, via a steering axle, to the first frame part, which steering axle can rotate about its axis with respect to the front-wheel steering axle, the sensor determining the angle of rotation between the front-wheel steering axle and the steering axle. 
     The present invention is based on the insight that a vehicle, which may be a tilting vehicle, can be steered in two fundamentally different ways: 
     Firstly, the driver may have direct control over the position of the front wheel or the front wheels. The driver, by imparting a specific angular displacement to the front wheel, provides the vehicle with a turning radius. 
     Secondly, the driver may have direct or indirect control over the tilting position of the vehicle. In this case, the combination of the vehicle speed and the tilting position provides a specific turning radius. The front wheel assumes an appropriate position which is governed by the speed and the tilting position, which front-wheel position is not used to determine the direction of travel. A motorcycle works on this principle. By means of complex steering manoeuvres, the driver continuously adjusts the tilting position, resulting in the desired direction of travel. In practice, this has been found to provide a vehicle which is easy to control. In the case of the vehicle according to the invention, the driver can fix the tilting position directly by means of the tilting means, in contrast to the indirect fixing of the tilting position via the handlebars, as takes place in the case of a motorcycle. According to the invention, the optimum instrument allowing the driver to control and actuate the tilting system comprising the tilting means, and therefore to adjust the tilting position, is the steering wheel. According to the invention, the standard rigid connection between the steering wheel and the front wheel is replaced by a connection in which the steering axle can rotate about its axis with respect to the front-wheel steering axle. This flexible connection, in which the rotation-angle sensor is accommodated, provides a signal for the desired tilting position of the vehicle. The tilting of the vehicle can then be actuated The optimum position of the front wheel, which is appropriate for the combination of tilting angle and speed of travel, is automatically adopted in a manner which is imperceptible to the driver. 
     The tilting system according to the present invention allows the sensor to be of very simple design and also allows the tilting means to be actuated in a robust and operationally reliable manner. 
     The sensor, the control arrangement and the tilting means themselves may each separately use various technological principles, for example they may be of mechanical, hydraulic, pneumatic or electrical design. The present invention will provide a description of a (largely) hydraulic design of the components. In this case, the rotation-angle sensor may comprise a cylinder which is connected at a first end to the front wheel and at the second end to the steering column. The tilting means may comprise hydraulic or pneumatic cylinders which, via a valve, are connected to a pressure source. The cylinder of the angle-position sensor actuates the valve of the tilting means. This valve may be a simple on/off slide valve. 
     The vehicle according to the present invention may comprise a rear frame part provided with two wheels and a front frame part which has one wheel and can tilt with respect to the rear frame part. It is also possible to use a rear tiltable frame part provided with one wheel and an untiltable front frame part with two wheels. Finally, it is also possible to employ a tiltable frame structure as described in Dutch patent application number 1005894 in the name of the applicant. 
     EP-A-0,592,377 has disclosed a tilting vehicle in which the tilting of the front frame part with respect to the rear frame part, which supports the drive, is obtained by the angular displacement of the front wheel about the front-wheel steering axle with respect to the frame, the steering wheel and the front wheel being rigidly connected. A system of this nature has the drawback that the tilting is not dependent on the vehicle speed and is therefore not accurate. A stable ride is therefore not obtained. 
     EP-A-0,020,835 has disclosed a tilting vehicle in which the tilting position can be obtained by actuating foot pedals or by moving the steering column, transversely to its axis of rotation. The steering axle is again rigidly connected to the front wheel. In this known device, the tilting position is not dependent on the vehicle speed, so that a stable ride is not obtained. 
     The way in which the vehicle according to the present invention operates differs fundamentally from the way in which the vehicles which are known from the prior art and have been described above operate. When the driver of the vehicle according to the invention is driving straight on a flat road surface and wants to turn a corner, he rotates the steering wheel. Since the geometry and gyroscopic stability of the front wheel mean that the latter tends to remain directed straight on, the steering angle of the steering axle as dictated by the driver will lead to an angular displacement a between the front-wheel steering axle and the steering axle. On the basis of this angular displacement α, the rotation-angle sensor forms a control signal which leads to the tilting means being driven and to a predetermined tilting angle β of the first frame part. As the tilting of the vehicle increases, the front wheel will steer in slightly at an angle δ, which is dependent on the speed of travel. If the steering wheel is then held at a fixed angle φ, the angular displacement of the steering wheel is partially converted into a tilting angle and partially into an angular displacement of the front wheel about the front-wheel steering axle. This tilting angle and the angle of the front wheel automatically assume the ideal ratio which is suitable for any speed of travel. 
     By adjusting the ratio between the tilting angle β of the vehicle and the angle α detected by the sensor, it is possible to influence the ride. By way of example, it would be possible to provide a 1:1 coupling, in which case x° rotation of the steering wheel with respect to the front wheel leads to x° of tilting. 
     In a further embodiment, a steering sensation is obtained by connecting a force element to the steering column, which force element, as the angle position of the steering column increases, exerts an increasing restoring force on the steering column. This force feedback, under which a moment builds up on the steering wheel as the steering wheel is rotated further, produces a steering sensation and, when the steering wheel is released, returns the vehicle to the upright position. Since the tilting angle of the vehicle is a measure of the speed at which the vehicle drives through a bend, and owing to the fact that this tilting angle is related to the angle determined by the sensor between the steering axle and the front-wheel steering axle, this angle can therefore be used as a measure for the level of counter-moment exerted on the driver. It is therefore possible to achieve successful feedback from the “sharpness of the corner” to “the amount of force which is exerted on the steering wheel”. As the force element, it is possible, by way of example, to place a torsion spring between the front-wheel steering axle and the steering axle (force as a function of α). This spring may also be accommodated between the steering wheel and the front frame part (force as a function of steering wheel angle φ). 
     In a further embodiment, the vehicle comprises a speed sensor which, at a predetermined limit speed, limits the angular displacement between the front-wheel steering axle and the steering axle. At low speeds, when the vehicle is at a standstill or when it is reversing, it is important that the tilting of the vehicle is switched off. This is achieved by limiting the angular displacement between steering axle and front-wheel steering axle. In one embodiment, this is realized by switching on a power-steering cylinder, which is accommodated between the front wheel and the frame, at low speeds. If the power-steering moment is controlled as a function of the speed, it is possible to achieve an excellent, smooth transition from “power steering without tilting” when at a standstill and reversing to “no power steering, with complete tilting” when driving normally. In the transition range, the situation is that of “slight power steering with slight tilting”. 
     It is also possible, for a vehicle in which power steering is not required or desired, to block the tilting at low speeds by locking the angular displacement between front-wheel steering axle and steering axle in the centre position. In the design being examined here, there is a fixed/hydraulic coupling between the tilting angle β and the angular displacement ac between front-wheel steering axle and steering axle. In an embodiment of this nature, it is possible to select the option of blocking the tilting angle, with the result that the vehicle stands reliably upright and the angular displacement between front-wheel steering axle and steering axle is blocked. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One embodiment of a tilting vehicle according to the present invention will be explained in more detail with reference to the appended drawing, in which: 
     FIG. 1 shows a diagrammatic, perspective view of a vehicle according to the present invention in the straight-on driving position, with a rotation-angle sensor which is of hydraulic design, 
     FIG. 2 shows a perspective illustration of the vehicle in accordance with FIG. 1, in the tilted position, 
     FIG. 3 shows a hydraulic circuit diagram of the vehicle in accordance with FIG.  1  and FIG. 2, 
     FIG. 4 shows a perspective view of a vehicle in which the rotation-angle sensor is formed by a set of bars, 
     FIGS. 5 to  7  show a perspective view of the vehicle in accordance with FIG. 4 in various tilted positions, and 
     FIG. 8 shows a hydraulic circuit diagram of the vehicle in accordance with FIGS. 4 to  7 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a vehicle  1  with a frame  2 . The frame  2  comprises a front frame part  3  and a rear frame pan  4 . The frame parts  3  and  4  are attached at a rotation point  5  so that they can rotate with respect to one another. The rear frame part  4  is provided with a rear axle  6  with two rear wheels  7 ,  7 ′. Via a piston rod, tilting cylinders  9 ,  9 ′ are connected to an attachment plate  11  on the front frame part  3 . At a second end, the cylinders  9 ,  9 ′ are connected to the rear frame part  4 . A slide valve  10 , which is driven by a cylinder  12 , is attached, parallel to the tilting cylinder  9 , between the rear frame part  4  and the front frame part  3 . The rear frame part  4  bears further drive means, such as an internal combustion engine or electric motor, for propelling the vehicle. For the sake of clarity, this drive is not shown in the figures. 
     The front frame part  3  bears the front wheel  13  which, via the front fork  14  and the front-wheel steering axle  15 , is rotatably mounted in a front support  16 . A steering wheel  17  is attached, via the steering axle  18 , to a second support  19  of the front frame part  3 . Via the steering wheel  17 , the steering axle  18  can be rotated in the bearing provided by the second support  19 , independently of the front wheel  13 . A force-exerting device, such as a torsion spring  16 ′, is connected, on the one hand, to the steering axle  18  and, on the other hand, to the front frame part  3 , in order to exert a restoring force on the steering wheel  17 , which force increases as the angular displacement of the steering wheel  17  increases. 
     Respective transverse arms  20  and  21  are attached to the end of the front-wheel steering axle  15  and of the steering axle  18 , the free ends of which arms are connected to a respective part of the rotation-angle sensor  24 , which in the embodiment shown is formed by a hydraulic cylinder. In FIG. 1, the rotation angles of the front wheel  13  and of the steering wheel  17  are shown by diagrammatic rotation-angle indicators  22  and  23  which are purely for the purpose of illustration and will not be present in the final design of a vehicle according to the invention. A power-steering valve  25 , the respective ends of which are attached to the transverse arms  20  and  21 , is accommodated parallel to the rotation-angle sensor  24 . 
     Finally, the front fork  14  is attached, via a transverse arm, to a power-steering cylinder  26 , the other end of which is attached to the front frame part  3 . 
     The front frame part  3  also supports a driver&#39;s seat and driver&#39;s cab which, for the sake of clarity, are omitted from the figure shown. 
     The rear frame part  4  furthermore comprises an oil pump  28 , an accumulator  29  and a reservoir  30  for the hydraulic pressure medium. The tilting cylinders  9  and  9 ′ and the power-steering cylinder  26  are driven by the oil pump  28 . Finally, a speed sensor  27  is connected to the rear axle  6  for switching the power-steering cylinder  26  on and off as a function of the speed. 
     In the straight-on position shown in accordance with FIG. 1, the angle α between the steering axle  18  and the front-wheel steering axle  15  is 0°, so that the rotation-angle indicators  22  and  23  are parallel to one another. The rotation-angle sensor  24  and the cylinder  12  of the slide valve  10  are hydraulically coupled together. Movement of sensor  24  ensures movement of slave cylinder  12 , with the result that slide valve  10  moves. As a result, the slide valve  10  opens, a pressure difference is formed across the tilting cylinders  9 ,  9 ′ and the tilting frame  3  begins to move. As a result, slide valve  10  returns to its centre position, and when this position is reached the pressure difference between  9 ,  9 ′ disappears, so that the tilting movement stops. Each position of the rotation-angle sensor  24  leads to a specific position of cylinder  12  and therefore of the tilting frame  3 . The slight rotation of the front wheel will scarcely be noticed by the driver and will be corrected unnoticed by a small correction to the steering wheel. 
     When the steering wheel  17  is rotated out of the straight-on position until the rotation-angle indicator  23  is at an angle to the rotation-angle indicator  22 , the inertia and other dynamic properties of the front wheel  13  cause the rotation-angle indicator  22  initially to remain in the straight-on position. Tilting cylinders  9 ,  9 ′ are actuated by means of the rotation-angle sensor  24 , so that the front frame part  3  tilts to a predetermined extent with respect to the rear frame part  4 . As the front frame part  3  tilts, the front wheel  13  will steer in slightly, depending on the vehicle speed, until an angle α is established between the front wheel  13  and the steering wheel  17 , as indicated in FIG.  2 . The tilting angle between the front frame part  3  and the rear frame part  4  will ultimately amount to β°. In FIG. 2, α is defined by the diagrammatic rotation-angle indicators  22  and  23 . Thus the angular displacement of the steering wheel  17  is converted partly into the tilting angle β and partly into a front-wheel angle, which both adopt the ideal ratio during any speed of travel. 
     The value α measured by the rotation-angle sensor  24  is equal to the steering angle of the steering axle  18  minus the angular displacement of the front-wheel steering axle  15 . The angle α is used to control the tilting angle β of the vehicle in accordance with the relationship β=f(α). The tilting angle β is entirely defined by the steering wheel measurement α. Both the travel of the steering wheel  17  with respect to the front wheel  13  (the extent in degrees of the steering wheel measurement α) and the conversion of the steering wheel measurement α into the tilting angle β can be selected in such a manner that an optimum ride is obtained. By way of example, it is possible to select β=c.α where c is a constant. 
     FIG. 3 diagrammatically shows the hydraulic system of the vehicle  1  according to the present invention. In FIG. 3, the front frame part  3  and the rear frame part  4  are indicated by the rectangles denoted by a dashed line. Furthermore, in FIG. 3 the same reference numerals denote the same components as in FIGS. 1 and 2. As can be seen from FIG. 3, the oil pump  28  is driven by the engine  31  of the vehicle  1 . In this case, the engine  31  may be an electric motor or an internal combustion engine. However, it is also possible to drive the oil pump  28  by means of a separate motor which is accommodated in the front frame part  3 . 
     The accumulator  29  is situated on the delivery side of the oil pump  28 . Via the  4 / 3  slide valve  10 , the tilting cylinders  9 ,  9 ′ can be connected, by way of their respective lines  32 ,  33 , to the high-pressure line  34  which is in communication with the accumulator  29  or to the return line  35  which opens out into the reservoir  30 . The slide valve  10  is actuated by the cylinder  12  which, via the lines  36  and  37 , is coupled to the rotation-angle sensor  24 . The cylinder  24  is connected, on the one hand, to the transverse arm  20  of the front-wheel steering axle  15 , while the piston rod of the cylinder  24  is connected to the transverse arm  21  of the steering axle  18 . The piston of the cylinder  24  is moved as a function of the relative angular displacement a between the steering axle  18  and the front-wheel steering axle  15 . This travel is followed by the cylinder  12 . In the straight-on position shown, in which the angle α is 0°, both tilting cylinders  9 ,  9 ′ are connected to the high-pressure line  34 , so that the front frame part  3  is upright. In the event of the steering wheel being moved anticlockwise (as seen from the driver&#39;s position), the piston will move to the left inside the cylinder  24 . As a result, the piston in the cylinder  12  is pushed towards the valve  10  and the right-hand tilting cylinder  9 ′ is connected to the high-pressure line  34 . The left-hand tilting cylinder  9  is connected to the return line  35 . The valve  10  and the cylinder  12  are connected, on the one hand, to the rear frame part  4 , as is diagrammatically illustrated by the dashed line  38 , and, on the other hand, are connected to the front frame part  3 , diagrammatically indicated by the dashed line  39 . As a result, when tilting to the left, the cylinder  12  is moved away from the valve  10 , until the valve  10  resumes the centre position, with the result that the movement of the coupled-together piston rods of the tilting cylinders  9 ,  9 ′ stops. 
     FIG. 3 also shows the speed-dependent power steering by means of the power-steering cylinder  26  which, via the  4 / 3  power-steering valve  25 , is connected to a switching valve  40 . The switching valve  40  is actuated by a speed sensor  27 , for example in the form of a geared pump. In the situation shown, the speed of the vehicle  1  is insufficient for the pump  27  to move the valve  40  counter to the spring force. At the low speed, the valve  40  is switched in such a position that the line  41  of the valve  25  is connected to high-pressure line  34 . The line  42  is always connected to the return line  35 . In the event of the steering wheel being rotated, the fact that the valve  25  opens means that the pressure difference will be built up across the piston of the cylinder  26 , so that the front wheel  13  is rotated. In the event of the vehicle speed increasing, the pressure in the switching line  43  will increase to a sufficient extent to switch the position of the valve  40 , so that both lines  41  and  42  are connected to the return line  35 . The power steering can be switched off smoothly by not switching line  41  over abruptly from high pressure to return line, but rather by allowing the pressure to drop gradually (for example by means of a pressure-control valve which is actuated by the speed sensor). In this way, the power-steering cylinder  26  is deactivated. By switching on the power steering at low speeds, in the event of the steering wheel  17  being rotated the front wheel  13  will follow the steering-wheel movements, so that the angle α remains virtually equal to 0°. This prevents the vehicle from tilting. A certain free travel of α, for example ±1°, remains possible. 
     Although in the embodiments described above the sensor  24  is of hydraulic design, the invention is not limited to this arrangement, and it is also possible to use optical, electrical or mechanical sensors for actuating the tilting cylinders. In principle, it is also possible to replace the hydraulics of the tilting cylinders with any other system, such as for example an electrical system. Furthermore, the present invention is not limited to vehicles which have a front tilting part with one wheel and a rear stationary part with two wheels, but rather it is possible for the front frame part to comprise two wheels and to be of non-tilting design, while the rear frame part is able to tilt and comprises one wheel. In this case, the terms “front wheel” and “front-wheel steering axle” also encompass the designs in which these components, if desired, are located in the rear part, with regard to the direction of travel, of the vehicle. “Front wheel” therefore means the steerable wheel of the vehicle and “front-wheel steering axle” means the axle about which the steerable wheel can tilt. Furthermore, it is possible to use a four-wheeled frame, as described in Dutch patent application number 1005894 in the name of the applicant. 
     In alternative embodiments, it is possible for the angle of rotation α between the steering axle  18  and the front-wheel steering axle  15  to be transmitted to the slide valve by two traction cables or by a push/pull cable, instead of via a hydraulic rotation-angle sensor  24 , lines  36  and  37  and the slave cylinder  12  of the slide valve  10 . The angular displacement between the steering axle  18  and the front-wheel steering axle  15  can also be measured by means of a planetary gear system, in which case, by way of example, the sun gear is connected to the steering axle  18  and the planetary gear is connected to the front-wheel steering axle  15 , the rotation of the planet gear providing the tilting angle required. The coupling of the planet-gear rotation to the tilting cylinders can be brought about via various technological principles. 
     In the embodiment which is illustrated in detail in FIG. 4, the difference in angle between the rotation of the front wheel  89  about front-wheel steering axle  81  and the rotation of the steering wheel  72  about the steering axle  71  is determined via a set of bars, which set of bars is mechanically coupled to the slide valve  62  via a rotary shaft  63  and a flange  64 . The tilting vehicle  50  in accordance with FIG. 4 comprises a front frame part  51  which is tiltably connected to a rear frame part  52 . The rear frame part  52  is provided with two rear wheels  53  and  54  and comprises two tilting cylinders  55 ,  56 . The tilting cylinders  55 ,  56 , on the one hand, are connected by means of their cylinder housing to the frame part  52  and, by means of their piston rods  59 ,  60 , are connected to an attachment plate  61  on the front frame part  51 . The rotary shaft  63  extends rotatably as far as the region of the rear frame part  52  and, at the location of this rear frame part  52 , is connected to the slide valve  62  via a flange  64 . At the other end, the slide valve  62  is connected to the rear frame part  52 . Furthermore, the rear frame part  52  comprises an engine, not shown, as well as an emergency accumulator  65 , an accumulator charging pump  66 , a volume sensor  66 ′ (not shown), an oil pump with integrated volume control  67  and a reservoir  68  of a continuous circulation system. The cylinders  55  and  56  are each provided with a respective shock absorber/restrictor valve  57 ,  58 . 
     At the front end of the front frame part  51 , the front fork  69  is suspended so that it can rotate about the front-wheel steering axle  81 , via a transmission  80 . 
     Via the steering axle  71 , the steering wheel  72  is rotatably mounted in the top part of the front support  70 . The bottom part of the support  70  comprises a bearing in which the front end  74  of the rotary shaft  63  is accommodated. Via the swivel bracket  87 , the steering axle  71  is connected to a horizontal arm  76  which engages pivotably on the transverse arm  77  of the front wheel  89 . The swivel bracket  87  is connected, via a ball joint, to a vertical arm  73 , which, via a ball joint, is connected to the front end  74  of the rotary shaft  63 . With the aid of the swivel bracket  87 , the horizontal arm  76  and the vertical arm  73 , the difference in rotation angle between steering axle  71  and of the front wheel  89  about the front-wheel steering axle  81  is converted into a rotation of the rotary shaft  63  about its longitudinal direction. Via the flange  64 , this rotation is transmitted to the slide valve  62  which actuates the tilting cylinders  55 ,  56  in order to tilt the front frame part  51  as a function of the rotation-angle difference detected by the bars  87 ,  73  and  76 . 
     Furthermore, the vehicle  51  comprises a power-steering cylinder  82 , which, on the one hand, is connected to the front support  70  and, on the other hand, engages on a tilting plate  84 . Via the power-steering on  75 , which runs parallel to the horizontal arm  76  of the rotation-angle sensor, when the power-steering cylinder is actuated the transverse arm  77  is moved from the front wheel  89 . The power-steering cylinder  82  is actuated by a power-steering valve  85  which is connected, on the one hand, to the front frame part  51  and, on the other hand, to the rotary shaft  63 . For the sake of clarity, the system of hydraulic lines between the power-steering valve  85  and the power-steering cylinder  82  is omitted in FIG.  4 . Furthermore, an overflow valve  86  is provided, which is actuated by a speed sensor of the vehicle, so that at low speeds the power-steering valve  85  actuates the power-steering cylinder  82 , and at high speeds it is not possible for pressure to build up across the power-steering cylinder via the valve  85 . Finally, a torsion bar  88  is provided which, as a function of the tilting position, exerts a force on the rotary shaft  63 , ensuring that as the position of the vehicle becomes more inclined it is necessary to exert more steering force on the steering wheel  72 . When the steering wheel  72  is released, the torsion bar  88  ensures that the front frame part  51  moves back into the upright position. 
     FIG. 5 shows the tilting vehicle  51  in accordance with FIG. 1 in the upright position, and in this figure the position of the vertical arm  73 , the horizontal arm  76  and of the swivel bracket  86 , which is connected to the steering axle  71 , can be seen more clearly. As is clearly apparent, the vertical arm  73  is connected, via ball joints  92  and  93 , on the one hand to the swivel bracket  87  and on the other hand to the rotary shaft  63 . Via a ball joint  91 , the horizontal arm  76  is connected to the swivel bracket  87  and, by means of a ball joint  90 , is connected to the transverse arm  77 . As is clear from FIG. 6, when the steering wheel is rotated to the left, either the horizontal arm  76  can be moved forwards or the vertical arm  73  can be moved upwards, or a combination of these two movements can take place. As a result of the horizontal arm  76  moving forwards, the front wheel will turn to the left. Moving vertical arm  73  upwards imparts a rotation to the rotary shaft  63  via the ball joint  93 , so that the rotary shaft actuates the slide valve  62  via the flange  64 . As a result, the piston rod  60  is withdrawn into the housing of the tilting cylinder  56  and piston rod  59  is pushed out of the tilting cylinder  55 , so that the front frame part  51  is tilted, via the attachment plate  61 , to the position which is illustrated in FIG.  7 . The rotary shaft  63  can be seen as an extension, towards the front of the tilting vehicle, of the slide valve  62 . Since this slide valve  62  has a very short working travel, the rotary shaft  63  can also be regarded as an extension of the rear frame part  52 . The rotation of the rotary shaft  63  with respect to the front frame part  51  is consequently equal to the tilting angle β between the front frame part  51  and the rear frame part  52 . Via the link formed by the vertical arm  73  and the swivel bracket  87 , the rotation of the steering axle  71 , horizontal arm  76  is converted either into a rotation of the front-wheel steering axle  81 , via the horizontal arm  76 , or into a rotation of the rotary shaft  63 , via the vertical arm  73 , or a combination of these two rotations. The combination of the rotation of the front wheel  89  about the front-wheel steering axle  81  and of the tilting of the front frame part  51  will be adjusted by the vehicle itself as a function of the speed of travel and turning radius. 
     FIG. 8 shows the hydraulic circuit diagram for controlling the tilting of the tilting vehicle  50  in accordance with FIGS. 4 to  7 . The system in accordance with FIG. 8 is a constant circulation volume system, in contrast to the constant pressure system which is shown in FIG.  3 . The oil pump  67  is equipped with an integrated constant-volume regulator and, at high speeds, pumps a constant volume through the circuit  130  which includes the slide valve  62 , the overflow valve  86  and the reservoir  68 , and at low speeds pumps a constant volume through the circuit formed by the slide valve  62 , the power-steering valve  85  and the reservoir  68 . 
     Via a high-pressure line  100 , the oil pump  67  is connected to a supply inlet  101  of the slide valve  62 . A cylinder outlet  102  of the slide valve  62  is connected, via a shock absorber  57  and outgoing line  103 , to the tilting cylinder  55 . A return inlet  109  of the valve  62  is connected, via a shock absorber  58  and a return line  108 , to the tilting cylinder  56 . A return outlet  110  of the valve  62  is connected, via the line  111 , both to the inlet of the overflow valve  86  and to the supply inlet  125  of the power-steering valve  85 . The discharge line from the overflow valve  86  and the return outlet  128  of the power-steering valve  85  are connected, via line  113  and volume sensor  66 ′, to the reservoir  68  which lies on the low-pressure side of the pump  67 . 
     A speed sensor  112 , which if appropriate may be designed as an oil pump, generates an electrical control signal which is transmitted to the overflow valve  86 , in such a manner that, at high vehicle speeds, this overflow valve is switched over and connects the line  111  to the line  113 , so that there can be no pressure buildup across the power-steering valve  85  and the power steering is deactivated. 
     The mechanical connections between the slide valve  62  and the tilting cylinders  55 ,  56  and the mechanical connection between the pistons  59 ,  60  and the rear frame part are indicated by dashed lines. The same applies to the mechanical connection between the power-steering cylinder  82  and the power-steering valve  85  and the front frame part. 
     If the valve  62  is situated in the centre position, the hydraulic fluid flows directly from the supply inlet  101  to the return inlet  110 . If, at high vehicle speeds, under which line  111  and line  113  are connected, the slide valve  62  is actuated, via the rotary shaft  63 , by the rotation of the steering wheel  72 , in the event of the steering wheel being rotated to the left, as seen from the driver&#39;s position, the outgoing line  103  of the tilting cylinder  55  will be increasingly connected to the low-pressure line  111 , while the return line  108  of the tilting cylinder  56  will be increasingly connected to the high-pressure line  100 . As a result, the piston rod  59  will be pushed out of the cylinder housing of the tilting cylinder  55 , while the piston rod  60  will be retracted into the cylinder housing of the tilting cylinder  56 . As a result, the front frame part  51  of the tilting vehicle tilts to the left. 
     Via feedback lines  114  and  115 , the outgoing line  103  and the return line  108  are connected to respective sides of the slide valve  62 . As a result, a force which acts in the opposite direction to the steering force and is coupled back to the latter is generated on surfaces in the valve. Thus, if it is necessary to generate more pressure, more force has to be applied to the slide valve  62 . As a result of the counter-moment applied to the rotary shaft  63  in this way, a force which is a measure of the tilting acceleration which is perceived by the driver is imparted to the person controlling the steering wheel  72 . The result is a favourable steering sensation. By providing the restrictor valves  57 ,  58 , the actuation of the tilting cylinders  55 ,  56  is damped, so that the feedback of the supply outlet  102  and return inlet  109 , via feedback lines  114 ,  115 , generates a counter-force on rotary shaft  63  which is a measure of the tilting speed which is perceived by the driver on the steering wheel  72 . The slide valve  62  could be replaced by a rotary valve, in which case the feedback is effected by means of a hydraulic motor. 
     If the vehicle speed decreases, the overflow valve  86 , via the sensor  112  is moved into the position as shown in FIG. 8, so that a pressure difference is built up across the overflow valve  86 . As a result, increasing amounts of oil have to flow through the power-steering valve  85 , with the result that pressure can be built up in the power-steering cylinder  82  if the valve  85  is activated. In the same way as slide valve  62 , the power-steering valve  85  is provided with a supply inlet  125 , a supply outlet  126 , a return inlet  127  and a return outlet  128 , as well as with a feedback line  116 . Since the tilting of the front frame part  51  is adjusted, via the rotary shaft  63  and the slide valve  62 , on the basis of the difference between the rotation of the steering axle  71  and the rotation of the front wheel  89  about the front-wheel steering axle  81 , when the power-steering cylinder  82  is fully active the tilting of the vehicle will be switched off. As a result of the power-steering cylinder  82  being activated, the front wheel  89  is rotated about the front-wheel steering axle  81  (for example to an extent equivalent to the angular displacement of the steering axle  71 ) and the steering input will not cause any rotation of the rotary shaft  63 . As a result, the vehicle remains in the upright position at low speeds. 
     Another option for blocking the tilting of the vehicle at low speeds is, by means of the feedback line  116 , to place the power-steering valve  85  very much in the centre position, so that the valve cannot be moved by rotating the steering wheel, so that all that happens is that the front wheel  89  can be rotated about the front-wheel steering axle  81  without the vehicle being tilted. As a result of this feedback in the power-steering valve  85 , the advantage is obtained that the driver feels some of the forces which are acting on the front-wheel steering axle  81 . In FIG. 8, there is a certain degree of feedback across the power-steering valve  85 , so that as the vehicle speed decreases the rotation of the rotary shaft  63  by means of the power-steering valve  85  is increasingly blocked and the stiffness of the upright position increases. 
     As shown in FIG. 8, each cylinder  55 ,  56  comprises, in addition to pistons  131 ,  132 , an emergency piston  120 ,  121  which can move separately from the pistons  131 ,  132 . In the event of a failure in the hydraulic system, for example in the event of the oil pump  67  breaking down, a volume sensor  66 ′ detects a fall in volume, and the emergency pistons  120 ,  121  are connected, via an emergency line  124 , to the emergency pressure accumulator  65  which is held under pressure via a pump valve  66 . As a result, the emergency pistons  120 ,  121  are pushed against respective internal stops  122 ,  123  into the position which is shown in FIG. 8, so that the front tilting frame  51  is placed in the upright position. 
     Other possibilities for an emergency system for placing the vehicle in the upright position in the event of the hydraulic system failing are, for example, the arrangement of a spring element parallel to the tilting cylinders  55 ,  56  and allowing the tilting to take place counter to the spring force. If the hydraulic tilting force disappears, the spring force will move the vehicle into the upright position. 
     For a hydraulic system which acts at constant pressure as shown in FIG. 3, the pressure can be measured using a sensor and, in the event of a certain pressure drop, emergency pistons of similar type to the emergency pistons shown in FIG. 8 can be activated. 
     In order to improve the steering sensation, it has already been described above that feeding back the pressure across the slide valve  62 , via feedback lines  114 ,  115 , results in the person controlling the steering feeling a force on the steering wheel  72  which is a measure of the tilting acceleration. By applying a traditional damping action via the restrictor valves  57 ,  58 , the force felt on the steering wheel  72  is converted into a measure of the tilting speed of the front frame part  51 . 
     As a result of the action of the torsion bar  88  as shown in FIG. 4, as the tilting increases, the force which has to be exerted on the steering wheel  72  to tilt the vehicle increases and, when the steering wheel  72  is released, the vehicle returns to the upright position. 
     Acceleration or deceleration transmissions  78 ,  80 ,  79  can be arranged in the steering axle  71 , in the front-wheel steering axle  81  and in the bearing of the rotary shaft  63 , in order to change the steering sensation. Together with the selection of the lengths of arms  73 ,  76  and of the swivel bracket  87 , the relationship between the steering angle, the rotation of the front wheel  89  about the front-wheel steering axle  81  and the tilting angle can be adjusted. 
     If the suspension of the front wheel  89  is fitted with a large fork head angle (the tilting axle  81  forming an angle to the vertical), geometric properties mean that at low speeds the tilting angle of the front frame part  51  is not optimum. This behaviour can be eliminated by imparting an opposing turning moment when the front wheel is rotated about the front-wheel steering axle  81 , by means of a force-exerting element which, by way of example, may be designed as a tension/compression spring  83  which is accommodated between the front frame part  51  and the front-wheel steering axle  81 .