In a crop harvesting machine there is provided a pair of hydraulic float cylinders or springs for a header such as a rotary mower relative to a self-propelled vehicle, where a float spring force is controlled to provide a predetermined lifting force is provided to the header. The spring force is varied in response to detection of ground speed to decrease the lifting force at higher ground speeds. A position sensor is used to generate an indication of movement and/or acceleration. The electronic control is arranged, in response to changes in the sensor signal, to temporarily change the control signal to vary the lifting force and thus change the dynamic response of the hydraulic float cylinder. In order to reduce static friction so that the system can react quickly, an arrangement is provided for causing relative reciprocating movement in an alternating wave pattern between the piston and cylinder.