Lift wheel structure for an implement includes four bar linkage having a curved lower link connected to the frame closely adjacent a first tool-carrying rank. A caster wheel is supported at the forward end of the linkage for castering in an accommodation area defined by the curved lower link. Plates are bolted to the sides of the lower link and extend along sides of an upper link to a connection with a lift cylinder directly above the upper link. The link and the cylinder closely overlie the frame to limit interference with other components mounted above the frame and to facilitate compact folding of implement sections. Loads, including cylinder lift loads, are transmitted primarily through the lower link. A protected transducer assembly is connected between the lower link and the frame to maximize and linearize transducer response in the depth control range. The lift wheel structure can by mounted at different locations on the frame to accommodate different tool configurations and provide the desired automatic depth control and leveling functions. In one embodiment a tension link connects a forward four bar linkage wheel structure with a rear single pivot lift wheel structure such that level lift is provided.