A combine harvester has a rear axle that can be extended on-the-go to enable its ground wheels to be turned more sharply during steering operations without striking nearby portions of the chassis. Extension may be carried out manually by the driver or automatically in response to a predetermined input, such as the commencement of a turn by the harvester. In one embodiment the rear ground wheels are powered, such as by a single hydraulic or electric motor, while in another embodiment they are non-powered.
