The invention concerns a drive unit for moving a door relative to a door's opening. The drive unit comprises a drive motor and a wheel transmission assembly arranged for transfer of power to a drive wheel configured for engagement with a running surface for moving the door. The wheel transmission comprises a transfer wheel driven by the drive motor. A movable wheel carrier comprises a first running wheel and a second running wheel arranged in in engagement with the transfer wheel. The movable wheel carrier is arranged for displacement between a free wheel position where the first and second running wheel are arranged disengaged from the drive wheel for no power transfer to occur from the drive motor to the drive wheel, at least one running position where either of the first and second running wheel is positioned in engagement with the drive wheel for the transfer of power from the drive motor to the drive wheel.