interchangeable fig1 shows a self propelled windrower 10 incorporating the apparatus and method of the invention however it will be appreciated that the principles of the present invention are not limited to a self propelled windrower or to any specific type of harvesting machine in the illustrated embodiment the self propelled windrower 10 comprises a tractor 12 and a header 14 the header 14 being attached to the front end of the frame 16 or chassis of the tractor 12 the header may be of generally any suitable construction and design and may include not only crop harvesting mechanisms but also crop conditioners such as elongate rolls 15 such attachment of the header 14 to the frame 16 is achieved through a pair of lower arms 18 only the left one being shown the right being generally the same pivoted at one end to the frame 16 and at the other end to the header 14 as well as through a central upper link 20 one or more cylinders such as individual lift and flotation cylinders or a single lift flotation cylinder interconnects the lower arm 18 and the frame 16 on each side of the header typical features and operation of a system for controlling the lift and flotation functions for a header such as header 14 depicted herein are disclosed in u s pat no 6 901 729 incorporated herein by reference referring also to fig2 4 and 5 windrower 10 includes a propulsion driveline 22 controllably operable using operator controls for rotatably driving a left wheel 24 and a right wheel 26 for propelling windrower 10 over a ground or other surface hydraulic motors not shown in connection with each wheel.