A detachable power drive unit for propelling and steering a manual wheelchair is provided which comprises a frame, two independently controlled drive wheels, a power source, a hitching unit for coupling the device to the wheelchair, a force transfer apparatus, a user interface and a controller. The frame, motors, batteries and wheels have a low profile, which allows them to fit in the space underneath a standard manual wheelchair. The hitching unit couples the device to the wheelchair, yet allowing a fair degree of rotational motion to occur which minimizes the possibility of a wheel losing contact with the ground. The force transfer apparatus is a spring loaded arm that gets compressed by the wheelchair chassis during docking, thus transferring a significant portion of the wheelchair and occupant weights onto the drive wheels and ensuring adequate ground contact force and, as a consequence, drive traction.
