Abstract: A base frame is provided defining two pairs of opposite marginal portions with the pairs of marginal portions spaced horizontally apart in vertical planes intersecting each other and the marginal portions of each pair spaced on opposite sides of the point of intersection of the planes. A first pair of omnidirectional support structures are mounted from a first pair of the marginal portions and a second pair of reversely and variably drivable support structures are steerably mounted from the second pair of marginal portions for angular displacement about upstanding axes relative to the second pair of marginal portions. Further, steering control structure is operably connected between the frame and the second pair of support structures operable to selectively, variably and equally angularly displace the second pair of support structures relative to the second marginal portions.