Motion4: point A on the component moves in an arc a distance R from pivot point B on the parent model I can see one problem that we would have to deal with - in most cases there would be two possible solutions (above and below the pivot, or behind and in front of it), so we would need some kind of syntax to tell Rail3D which to choose, to avoid getting our levers crossed. Maybe something like "+H/-H" would work. Apart from this particular problem, Motion4 should also be useful for things like eccentric rods.