outer radius of the drum 16 for the last few centimeters of travel the drum 16 as more clearly shown in fig5 and 6 has a normal circular first outer surface section 92 normally referred to as a drum helix with a first radius indicated at 94 a second outer surface helix section 96 has an elliptical contour that is tangent to the first outer surface section 92 at point 99 in proximity to the major axis 98 of the contour the minor axis 100 of the elliptical contour intersects the axis of rotation 26 the axis of rotation 26 is interposed between the defined center 102 of the elliptical contour and the elliptical contour surface 96 the elliptical contour is positioned such that the effective radius continually decreases from the tangent point 99 to the minor axis 100 to it 39 s minimum radius indicated at 104 it can be readily seen that the motor thus can provide for more torque to overcome the resisting forces of seals and latches by placing the cable along a smaller radius 104 in this fashion when the door is closed and the most tension is placed on the cable the highest bending stresses occur near the tangent point 99 near the major axis 98 and the highest tensile forces are in proximity of the minor.