A novel dual mast storage and retrieval vehicle comprising structure which maintains each mast essentially vertical as variably weighted heavy loads are placed on a carriage disposed between the masts. Each mast is disposed near the center of a separately supported base such that bending of the base under such loads induces little off-vertical deflection to the mast. The separately supported bases are coupled by a connector which comprises cross-aisle and vertical degrees of freedom but is strictly constrained in the down-aisle direction. In the currently preferred embodiment, two down-aisle drive motors, in master/slave relationship, provide a synchronous drive for the vehicle. Further, interconnecting joints a strut at the top of the masts and of the carriage comprise hinges which resist propagation of mast induced moments caused by beam deflection or vehicle dynamics. The invention comprises a lift cable vertical drive assembly wherein a motor is horizontally disposed down-aisle in line with a lift cable winding and unwinding drum. The vertical drive assembly is modularly assembled on a frame and is attachable and detachable from the vehicle as a unit. The vertical drive assembly comprises, in seriatim, the motor, a belt drive sprocket wheel, a drive belt, a belt driven sprocket wheel, and, juxtaposed and on the same axis as the driven sprocket wheel, an input adapter, a speed reducer, and a two lift cable drum. The motor is attached to the frame by jacks used to raise and lower the motor to adjust tension in the belt.