A known practice is to use support wheels on heavier powered vehicles such as trucks and buses to distribute the weight of any load over more wheel axles. It is also known that when such a vehicle has to start moving or travel up steep hills on running surfaces with low friction its powered wheels may slip on the running surface, thereby limiting the possibility of driving the vehicle. This problem may be solved by relieving the load on support wheels, thereby increasing the pressure which applies the powered wheels to the running surface and hence providing more friction force between the powered wheels and the running surface. This is traditionally achieved on trucks by support wheels being raised by a lifting device intended for the purpose. This may, for example, involve using the arrangement described in W099/55576. In the case of buses, the pressure applying support wheels to the running surface may be reduced by decreasing the pressure in the air suspension of their axle. In certain cases, support wheel load relief may only be permissible at relatively low travel speeds depending on vehicle design and any relevant laws and regulations in the country where the vehicle is at the time.

Support wheel load relief was previously effected manually by the driver pressing buttons in the driving cab. This may cause the driver inconvenience in situations where he needs to concentrate on surrounding traffic.

Another patent concerning automatic support wheel load relief is US 4 577 557 referring to a vehicle with continuous support wheel load relief depending on the vehicle's load

and braking capacity. Moreover, another patent, US 3 877 715, refers to automatic raising of an extra support wheel axle during braking. The amount of rise depends on the vehicle's load.

SUMMARY OF THE INVENTION The object of the present invention is to provide automatic support wheel load relief which takes into account the grip of the vehicle's wheels on the ground surface. It aims in particular at automatic support wheel load relief when the rotation speed of at least one wheel (other than support wheels) differs from the vehicle's travel speed. Said wheel may be a powered wheel, a steered wheel or any other wheel.

This object is achieved with the arrangement indicated in the introduction which is characterised by incorporating a control unit designed to automatically control said actuating device with reference to said parameter. The pressure acting on the powered axle can thus be influenced automatically without the vehicle driver having to pay attention to support wheel axle load relief. A further advantage may also be achieved with respect to time spent. Said automatic load relief arrangement and load relief method may involve a shorter time from detection of a deviation of said parameter to the desired load relief result than corresponding manual methods. Said load relief may nevertheless still be achieved by button pressing by the driver. Main wheel here means a wheel other than a support wheel. Unlike the previously mentioned patents US 4 677 557 and US 3 877 715 in which load relief depends on the vehicle's load and the braking procedure, load relief according to the present invention takes place upon detection of slipping of any of the wheels, thereby initiating load relief for the support wheels or the support wheel axle, and the vehicle's wheels gain a better grip.

According to a preferred embodiment of the invention, the control unit is designed to control the actuating device in such a way that the pressure acting upon the support wheels decreases when the value of said parameter indicates that this is required, e. g. when the value of said parameter exceeds a certain level. The pressure acting upon the

main wheels may thus be increased so as to improve their grip on the running surface.

The parameter value may thus be restored. The support wheels may also be reinstated after certain conditions, e. g. advantageously (but not necessarily) a certain time after load relief commenced.

According to a further preferred embodiment of the invention, said main wheels include at least one powered wheel and said parameter depends on the rotation speed of said powered wheel (s) relative to the vehicle's travel speed. This means that if one of the vehicle's powered wheels slips on the running surface, automatic load relief can be initiated to increase the pressure acting upon the powered wheel. This has the advantage of making it easier for the vehicle to start moving or travel up steep hills on running surfaces which present low friction.

According to a further preferred embodiment of the invention, the sensor arrangement incorporates at least one primary sensor designed to detect the rotation speed of the powered wheel, thereby creating the possibility of direct and hence reliable measurement of powered wheel rotation speed which may improve the accuracy of the sensor arrangement.

According to a further preferred embodiment of the invention, the sensor arrangement incorporates at least one secondary sensor designed to ascertain the vehicle's travel speed by detecting the rotation speed of an unpowered main wheel. This is an example of creating the possibility of accurate measurement of the travel speed.

In an advantageous application of the invention, the load relief arrangement is fitted to a heavy vehicle in cases where heavier vehicles mostly have more need of support wheels than lighter vehicles. The heavier vehicle may for example be a bus.

According to a further advantageous embodiment of the invention, the actuating device incorporates a pneumatically or hydraulically powered lifting means. Said lifting means

may incorporate an air bellows, which may also constitute the support wheel suspension device.

The object may also be achieved with the vehicle indicated in the introduction which is characterised by incorporating a control unit designed to automatically control said actuating device in response to said parameter.

The object may further also be achieved with the method indicated in the introduction which is characterised by receiving and processing said parameter signal and automatic control of the actuating device by a control signal provided by the control unit. The sensor arrangement thus delivers a parameter signal which depends on said parameter and is received by a control unit which processes said parameter signal and delivers a control signal to the actuating device. The actuating device thereafter relieves the support wheel load in response to said control signal.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be explained in more detail on the basis of various embodiments described by way of examples with reference to the attached drawings.

Fig. 1 depicts schematically a sideview of a heavy vehicle, Fig. 2 depicts schematically a first embodiment of an arrangement designed to automatically relieve the load on the support wheels of the vehicle in Fig.

1.

Fig. 3 depicts schematically a second embodiment of an arrangement intended to automatically relieve the load on the support wheels.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION Figs. 1 and 2 depict a heavy powered vehicle 1 on a ground surface 2. The vehicle 1 incorporates a support wheel arrangement 3 which comprises a support wheel axle 4 with

two support wheels 5, a first main wheel arrangement 6 incorporating a powered wheel axle 7 with powered wheels 8, and a second main wheel arrangement 9 incorporating a wheel axle 10 with two unpowered main wheels 11, e. g. steered wheels. In the present example, the vehicle 1 thus incorporates a total of four main wheels 8,11. The vehicle 1 is not limited to incorporating two wheels of each wheel type but may for example incorporate one or several wheels of the same type. The vehicle 1 may also incorporate more than the three depicted wheel axles 4,7,10. The invention is suitable for all types of heavier vehicles with a support wheel axle or other extra wheel axle which is capable of being raised, e. g. trucks and buses. The invention would also be applicable to tandem- drive vehicles, e. g. vehicles with two powered axles where one of them is capable of being raised or being relieved of load.

When the vehicle 1 is travelling on a ground surface 2 with low friction, the grip of the powered wheels 8 on the ground surface 2 may be improved by relieving the load on the support wheels 5, which may be achieved by raising the support wheels 5 or by reducing the pressure which applies the support wheels 5 to the ground surface 2, thereby increasing the pressure which applies the main wheels 8,11 to the ground surface 2 and their grip on said surface. The vehicle 1 may thus be driven in situations where, for example, the powered wheels 8 would otherwise slip, as in the case of starting to move or on uphill gradients where the ground surface 2 is slippery.

The vehicle 1 further incorporates an arrangement for automatic support wheel load relief which includes a sensor arrangement with a primary sensor 12 for each wheel 8 and a secondary sensor 13 for each wheel 11. The sensors 12,13 are designed to continuously detect slipping between, for example, the powered wheels 8 and the ground surface 2.

The arrangement for relieving the load on the support wheels or the support wheel axle also includes a control unit to which the sensor arrangement is connected, and an actuating device 15 which is itself connected to the control unit 14. The vehicle 1 further incorporates an engine 16 and a universal joint 17.

The control unit 14 receives from the sensors 12,13 a parameter signal which it processes and passes on to the actuating device 15 in the form of a control signal. The actuating device 15 is designed to alter, in response to the control signal which is delivered by the control unit 14 and which is itself determined by the value of the parameter signal, the pressure which applies the support wheels 5 to the ground surface 2. The control unit 14 and the actuating device 15 are with advantage designed to continuously adapt the pressure acting upon the support wheels 5 to any slipping detected, so that the grip of, and the pressure acting upon, the powered wheels 8 can be continuously adapted to the ground surface 2. The actuating device 15 may also be designed to raise the support wheels 5 completely in response to said control signal. According to one embodiment, the actuating device 15 consists of the lifting arrangement described in W099/55576 and depicted in Fig. 2. According to a further embodiment, the actuating device 15 consists of air bellows incorporated in the compressed air suspension 18 (depicted in Fig. 3) of the vehicle 1.

The sensors 12,13 incorporated in the sensor arrangement are designed to detect the value of a parameter which depends on the rotation speed of at least one main wheel 8,11 relative to the travel speed of the vehicle 1. In an advantageous version, the primary sensors 12 measure the rotation speeds of the powered wheels 8 and the secondary sensors 13 the rotation speeds of the unpowered main wheels 11. The parameter relationship may be obtained in various ways and the following are a number of non- limitative examples: - Parameter depending on the difference in rotation speeds between two powered wheels 8 with the travel speed of the vehicle 1 as reference.

- Parameter depending on the difference between the rotation speed of at least one powered wheel 8 and the travel speed of the vehicle 1 with said travel speed as reference.

The parameter may also be set to zero if the travel speed of the vehicle 1 exceeds a certain value.

In an advantageous version of the invention, said parameter depends on the difference between the rotation speeds of the two powered wheels 8. In this case the travel speed of

the vehicle 1 is used as a reference speed whereby the travel speed can be estimated from the rotation speed of the unpowered main wheels 11. The reference speed is used for deciding whether the discrepancy between the rotation speeds of the powered wheels 8 is due to their slipping or is caused, for example, by the vehicle 1 turning. In the first case the discrepancy is due to a powered wheel 8 slipping on the ground surface 2 while the other powered wheel 8 continues to grip. In the second case the rotation speeds of the powered wheels 8 will be different because of the difference between the distances travelled by the right and left sides of the vehicle. On the basis of said reference it is possible to decide which case prevails at the time. As the design of the vehicle 1 may, as previously mentioned, impose limitations on the travel speeds at which load relief is permissible, it is also advantageous if the parameter is set to zero if the travel speed exceeds a certain specified level.

In one version of the invention, the control unit 14 may be integrated with the actuating device 15. In another version of the invention, the control unit 14 is not integrated with the actuating device 15 and the control unit 14 sends a control signal to the actuating device 15. In a further version the control unit 14 is not integrated with the actuating device 15 and does not send the control signal direct to the actuating device but to some other device such as a compressed air device which itself powers the actuating device 15.

The invention is not limited to the embodiments depicted but may be varied and modified within the scopes of the ensuing patent claims.

When a powered wheel 8 slips, its grip on the ground surface 2 may be restored if its rotational moment is reduced. This may be achieved by reducing the power extracted from the engine 16 and hence the power transmitted to the powered wheel 8. Known prior technology allows such engine deceleration on the vehicle 1 when slipping of any of the powered wheels 8 is detected. There is also a later known system for more efficient braking of a vehicle on slippery running surfaces, referred to as an ABS brake system.

The technology of engine deceleration and ABS braking may with advantage be integrated with the present invention. The control unit 14 may also be caused to continuously adjust the pressure which applies the support wheels 5 to the ground surface and said parameter in order to achieve optimum performance. The sensor arrangement may thus also be conceived as incorporating sensors designed to detect the nature of the ground surface.

The travel speed may also be measured in other ways within the scope of the invention, e. g. by GPS.