There is previously known a device of this kind wherein a braking means on the recording wheel is actuated and wherein the braking force to prevent the wheel from rotating is a function of the friction between the wheel and the underlying surface.

Since measurements are made continuously whilst the vehicle is in motion, the method is unduly unreliable, in particular on an underlying surface where the friction keeps varying the direction of travel. Moreover, it is unfortunate that the wheel must be constantly subjected to a braking action, which is apt to cause the recording wheel to run hot.

It is the object of the present invention to provide a simple solution to the problems associated with this prior art solution, and according to the invention the device is characterised in that the recording wheel is free-running with no braking means, that the recording wheel is mounted on the vehicle in such a manner that with the aid of an actuator it is capable of turning to an angle relative to the direction of travel of the vehicle, and that a means is provided to register the friction between the wearing surface of the recording wheel and the underlying surface as a function of either a) the force acting on the actuator when the angle is maintained virtually constant, or b) the angular deviation when the force acting on the actuator is maintained virtually constant.

Further embodiments of the device are described in the attached patent claims and in the description that now follows.

Advantageously, the device is used to measure roadway or runway friction and at the same time, as a function thereof, to actuate a roadway gritting apparatus mounted on the vehicle.

The device will now be described in more detail with reference to the attached drawings: Fig. 1 shows a vehicle with the device mounted thereon.

Fig. 2 is a schematic illustration of the lateral movement of the device in relation to the vehicle.

Fig. 3 is a perspective view of the device mounted on a gritting truck.

Fig. 4 is a simplified coupling diagram for the device.

Figs. Sa, 5b and 5c show the device mounted on a vehicle, in respectively an inactive transport position, an active position with high coefficient of friction and an active position with low coefficient of friction.

Fig. 6 shows an enlarged section of the illustration shown in Fig. 5.

Advantageously, the recording wheel 1 of the device is mounted in connection with the chassis 3 of the vehicle 2, e. g., on the left-hand side of the vehicle between the front wheels 4 of the vehicle and its rear wheels 5. The recording wheel 1 runs freely without any braking action and measures the forces necessary to turn the wheel 1 at different friction ratios. When the coefficient of friction is high, the wheel 1 will run almost straight ahead (as indicated in Fig. Sb), whereas when the coefficient of friction is low, it will run turned substantially inwards beneath the vehicle (as indicated in Fig. Sc).

The recording wheel is vertically adjustable between an inactive transport position, as shown in Fig. 5a, and a friction-measuring position, as shown in Fig. 5b and Fig. 5c, with the aid of a first actuator 6, e. g., a pneumatic or hydraulic jack. Furthermore, the device has a second actuator 7 which can cause the wheel 1 to be capable of turning to an angle relative to the direction of travel 8 of the vehicle. In order to record the angular deviation as a function of the friction between the tread of the recording wheel and the underlying surface, there is provided a recording means, such as the recording means 9 or 10. as indicated in Fig. 4. When the actuator 7 is a hydraulic or pneumatic jack, the said means 9 will expediently be a pressure meter arranged in connection with the pressure hose 11 of the jack 7. If the actuator 7 is an electric jack, the said means 9

could be an ammeter arranged in connection with the jack power supply 11', as indicated in Fig. 4.

In an alternative embodiment, the said means as indicated by means of reference numeral 10 may be an angle gauge that is known per se which is, e. g., arranged in connection with the vertical pivotable 12 of the recording wheel. When using an angle gauge of this kind, it will be necessary to maintain the power from the actuator 7 virtually constant at all times. The pressure gauge 9 will via a recording unit 17 be able to cause the pressure to be kept almost constant.

As shown in Fig. 4, the wheel has a wheel suspension 13 that is conventional per se. If the actuator 6 and/or 7 is a hydraulic or pneumatic jack, it will receive its fluid supply from a fluid reservoir 14 and/or 15, as indicated in Fig. 4 and Fig. 6. However, if one or both of the actuators are in the form of an electric jack, it/they will receive power supply via supply line 16 and then via line 11'. The measuring device comprises a recording unit 17 which relays power supply to the actuator 6 and 7, whilst receiving signals either from the pressure gauge 9 or the gauge 10 via respective lines 18 or 19. This signal is relayed to a calculating means 20 which calculates the coefficient of friction that applies to the underlying surface. Advantageously, the value of friction measured is also related to the speed of the vehicle 2 in that a speed signal from a speedometer 21 is fed into the means 20. The coefficients of friction can be fed to a display unit and/or a printer 22. The means 20 may, if desired, also be capable of automatically or semi- automatically controlling a gritting apparatus 23 via a control line 24, although the gritting apparatus may, of course, also be controlled manually via a control line 25.

When readings are to be taken, the recording wheel is lowered to the underlying surface (roadway/runway) and begins to measure the friction as the vehicle moves forwards.

Correct coefficient of friction will normally only be obtainable once the vehicle has moved a minimum distance, e. g., 50 m, and at a specific driving speed, e. g., 50 km/hour, straight ahead. The recording wheel should not be engaged at excessively high speeds, e. g., speeds of more than 40 km/hour.

The air pressure in the recording wheel should be virtually constant. If the underlying surface presents relatively high coefficients of friction, e. g., more than 0.5, it will be appreciated that when the vehicle is driven continuously the tread of the wheel 1 will be worn down relatively severely. In such cases it is not necessary to measure across the whole measuring distance, but just at certain test points.

As previously indicated, the friction measuring device will be capable of making continuous measurements of roadway or runway friction and, as a function thereof, operating a gritting apparatus 23 that is mounted on the vehicle.

The word"vehicle"in this description should be understood to mean either a self- propelling vehicle or a non-self propelling vehicle which can be pulled or pushed by a self-propelling vehicle. Thus, e. g., a gritting truck (not shown) can be towed behind the vehicle shown in Fig. 1 and Figs. 5a)-c).