The invention concerns a method and an arrangement 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 (2) on which a wheel (4) is supported and provided with an angle sensor (4a) 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 (3) in such a manner that the wheel can be adjusted from positions in contact with and out of contact with a surface (5) of the object whose length is to be measured while the object is fed in its longitudinal direction (5a) 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. Improved measurement precision is achieved according to the method through: - that the suspension system is assigned means (1 ) that allow the measurement wheel (4) to be moved between a withdrawn resting position and an extended measurement position relative to the frame (3)； that the suspension system is assigned means (1 ) that allow the measurement wheel (4) to move in a manner that provides shock absorption relative to the frame (3) during the measurement process； that a measurement process for the measurement of length of a surface (5) is initiated with the measurement wheel (4) being moved forwards from the resting position to make contact with the surface； that the measurement wheel (4) is placed in contact with the surface (5) under force during the complete measurement process and that this takes place through the influence of the operating agent； that the measurement wheel (4) is moved back to its withdrawn resting position after completion of the measurement process.
