A method for a suspension system that may be, but does not necessarily need to be, constituted by a harvesting unit of a forestry harvester and which suspension system is of the type that has an arm on which a wheel is supported and provided with an angle sensor in order to form a measurement wheel for the measurement of length, and which arm is attached in a manner that allows it to pivot to a frame in such a manner that the wheel can be adjusted from positions in contact with and out of contact with a surface of the object whose length is to be measured while the object is fed in its longitudinal direction past the measurement wheel running against the circumference of the measurement wheel, whereby the said object may be, but does not necessarily need to be, constituted by a tree trunk taken up into a harvester unit, characterised in that the method comprises the following steps: - that the suspension system is assigned means comprising a hydrostatic system that allow the measurement wheel to be moved between a withdrawn resting position and an extended measurement position relative to the frame and that is designed to maintain a combination of shock absorption and a constant force of pressure against a surface during the measurement process; - whereby the hydrostatic system uses a primary piston in the system that demonstrates different effective piston areas (9a, 9b) and a channel for the passage of fluid through the piston to maintain the combination of shock absorption and the constant force of pressure; - that the suspension system is assigned means that allow the measurement wheel to move in a manner that provides shock absorption relative to the frame during the measurement process; - that a measurement process for the measurement of length of a surface is initiated with the measurement wheel being moved forwards from the resting position to make contact with the surface; - that the measurement wheel is placed in contact with the surface under force during the complete measurement process and that this takes place through the influence of the operating agent; - that the measurement wheel is moved back to its withdrawn resting position after completion of the measurement process. - that the suspension system is assigned means comprising a hydrostatic system that allow the measurement wheel to be moved between a withdrawn resting position and an extended measurement position relative to the frame and that is designed to maintain a combination of shock absorption and a constant force of pressure against a surface during the measurement process; - whereby the hydrostatic system uses a primary piston in the system that demonstrates different effective piston areas (9a, 9b) and a channel for the passage of fluid through the piston to maintain the combination of shock absorption and the constant force of pressure; - that the suspension system is assigned means that allow the measurement wheel to move in a manner that provides shock absorption relative to the frame during the measurement process; - that a measurement process for the measurement of length of a surface is initiated with the measurement wheel being moved forwards from the resting position to make contact with the surface; - that the measurement wheel is placed in contact with the surface under force during the complete measurement process and that this takes place through the influence of the operating agent; - that the measurement wheel is moved back to its withdrawn resting position after completion of the measurement process.