Method for determining a functional status of a vehicle shock absorber arrangement

1. A method for determining a functional status of a vehicle shock absorber arrangement arranged between a frame and a wheel axle of a vehicle, the vehicle shock absorber arrangement being compressible and expandable during operation for absorbing motions from the wheel axle, the method comprising:

determining a first force value of an external force exposed to the shock absorber arrangement, the first force value being determined when the shock absorber arrangement is arranged at a first position during compression of the shock absorber arrangement;

determining a second force value of the external force exposed to the shock absorber arrangement, the second force value being determined when the shock absorber arrangement is arranged at the first position during expansion of the shock absorber arrangement;

determining a difference between the first and second force values;

comparing the difference between the first and second force values with a predetermined threshold; and

determining that the shock absorber arrangement is degraded if the difference between the first and second force values is below the predetermined threshold.

2. The method of claim 1 , wherein the shock absorber arrangement comprises a piston movable within a shock absorber housing of the shock absorber arrangement, wherein the first position of the shock absorber arrangement is a first piston position of the piston within the shock absorber housing.

3. The method of claim 2 , wherein the piston is movable within the shock absorber housing between respective piston end positions within the shock absorber housing, wherein the first piston position is a non-end position within the shock-absorber housing.

4. The method of claim 1 , further comprising:

determining a position of the shock absorber arrangement when the vehicle is arranged at stand still; and

setting the first position to the position of the shock absorber arrangement at vehicle stand still.

5. The method of claim 1 , wherein the shock absorber arrangement is connected to a vehicle spring arrangement, wherein the method further comprises:

determining a characteristic of the vehicle spring arrangement;

determining the predetermined threshold based on the characteristic of the vehicle spring arrangement; and

comparing the difference between the first and second force values with the predetermined threshold specific for the characteristic of the vehicle spring arrangement.

6. The method of claim 5 , wherein the spring arrangement comprises a flexible bellows and the characteristic of the spring arrangement is a measured gas pressure level within the flexible bellows.

7. The method of claim 5 , wherein the spring arrangement comprises a leaf spring arrangement and the characteristic of the spring arrangement is a vertical displacement position of the leaf spring arrangement.

8. The method of claim 1 , wherein the external force is an oscillating force acting on the shock absorber arrangement, wherein the method further comprises the steps of:

determining a frequency level of the oscillating force acting on the shock absorber arrangement;

determining the predetermined threshold based on the frequency level of the oscillating force; and

comparing the difference between the first and second force values with the predetermined threshold specific for the frequency level of the oscillating force.

9. The method of claim 1 , wherein a plurality of first force values are determined for a plurality of positions of the shock absorber arrangement during compression of the shock absorber arrangement, and a plurality of second force values are determined for the same plurality of positions of the shock absorber arrangement during expansion of the shock absorber arrangement.

10. A vehicle suspension system of a vehicle, the suspension system comprising a vehicle shock absorber arrangement connectable between a frame and a wheel axle of a vehicle, the vehicle shock absorber arrangement being compressible and expandable during operation for absorbing motions from the wheel axle; and a control unit connected to the vehicle shock absorber arrangement, wherein the control unit is configured to:

determine a first force value of an external force exposed to the shock absorber arrangement, the first force value being determined when the shock absorber arrangement is arranged at a first position during compression of the shock absorber arrangement;

determine a second force value of the external force exposed to the shock absorber arrangement, the second force value being determined when the shock absorber arrangement is arranged at the first position during expansion of the shock absorber arrangement;

determine a difference between the first and second force values;

compare the difference between the first and second force values with a predetermined threshold; and

determine that the shock absorber arrangement is degraded if the difference between the first and second force values is below the predetermined threshold.

11. The vehicle suspension system of claim 10 , further comprising a spring arrangement connectable between the vehicle shock absorber arrangement and the frame of the vehicle, and a spring sensor connected to the control unit and arranged to detect a characteristic of the spring arrangement, wherein the control unit is further configured to:

receive a signal from the spring sensor indicative of the characteristic of the spring arrangement; and

determine the predetermined threshold based on the characteristic of the spring arrangement.

12. The vehicle suspension system of claim 11 , wherein the spring arrangement comprises a flexible bellows and the spring sensor is a gas pressure sensor arranged within the flexible bellows for detecting a characteristic in the form of a gas pressure level of the flexible bellows.

13. The vehicle suspension arrangement of claim 10 , wherein the vehicle suspension system is arranged between the frame and the wheel axle of the vehicle.