John, Thanks for the nice explanation and drawing. You are correct about the = lengthening effect. http://jjlahr.com/science/psn/pivots To further the discussion, would not the center of rotation move from = the contact point at no displacement to a point behind the plane and in = the direction of travel as the boom is displaced rather than be = considered a lengthening of the boom on the boom side? The radius to = any point from the center of rotation must be perpendicular to the = velocity of the point. Thus the center of the pivot ball to the contact = point will define one line and a line perpendicular to the now flatter = motion of the mass on the end define a second line. When the system is = rotated to the horizontal, the movement of the center away from the mass = to a point behind the support will also tilt the axis of rotation = between the pivots to a more stable direction. =20 As you say, the upper pivot would best be designed to accomodate axis = change. In the arrangements I have seen in PSN, the upper ball is on = the opposite side of a support so as to be in compression. Therefore = the rolling would now move the center of rotation in the upper pivot = toward the mass with an end result of the new axis tilting the boom down = to a shorter period. A ball style support will require a good bit more = engineering than just using a conveniently sized ball. Do the = proponents have anything to offer regarding design? RandyJohn,Thanks for the nice explanation and = drawing. =20 You are correct about the lengthening effect.To further the discussion, would not the center of rotation move = from the=20 contact point at no displacement to a point behind the plane and in the=20 direction of travel as the boom is displaced rather than be considered a = lengthening of the boom on the boom side? The radius to any point = from the=20 center of rotation must be perpendicular to the velocity of the=20 point. Thus the center of the pivot ball to the contact point will = define=20 one line and a line perpendicular to the now flatter motion of the mass = on the=20 end define a second line. When the system is rotated to the = horizontal,=20 the movement of the center away from the mass to a point behind the = support will=20 also tilt the axis of rotation between the pivots to a more stable=20 direction.As you say, the upper pivot would best be designed = to=20 accomodate axis change. In the arrangements I have seen in = PSN, the=20 upper ball is on the opposite side of a support so as to be in=20 compression. Therefore the rolling would now move the center of=20 rotation in the upper pivot toward the mass with an end result of = the new=20 axis tilting the boom down to a shorter period. A ball style = support will=20 require a good bit more engineering than just using a conveniently sized = ball. Do the proponents have anything to offer regarding = design?Randy