Abstract: A vehicle undercarriage includes a pair of track laying assemblies each provided with at least two pairs of wheels having an endless belt tread wrapped thereabout. Variation of the vehicle wheelbase is achieved upon actuation of an extend/retract device serving to displace in a fore and aft direction at least one wheel of each track laying assembly. Further actuation of the same extend/retract device or of a separate variable pitch assembly produces an elevation or depression with respect to a horizontal plane, of at least one distal portion of each track laying assembly to accommodate varying terrain while also altering the size of the tread footprint. During all of the displacements of the tread, a take-up assembly automatically absorbs and lets out excessive portions of the endless belt tread. An improved tread construction provides a medial rib offering an axially-extending ground engaging contact surface.