Abstract: An overhead bi-fold door mounted on a freestanding header has a door lift device with polymer cylindroids and capstans operated with an electric motor driven power transmission to selectively move the bi-fold door from an upright closed position to a folded open position and allow the bi-fold door to move from the folded open position to the upright closed position. Anchors mounted on the bi-fold door adjust the working length of the cylindroids between the capstans and anchors. The capstans have laterally spaced disks engageable with the cylindroids during the helical winding and unwinding of the cylindroid on the capstans as the bi-fold door moves at an increasing rate of speed from the upright closed to folded open positions and a decreasing rate of speed from the folded open to upright closed positions.