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One other thing I'll mention...as for parallel drag link and all:
The further up we go into our imaginary arc the more effect axle movement has on the movement of the third arm. If the drag link is in the center of that circle (a parallel arm), then up/down movement will cause the least amount of pull/push/pull action. If we move the radius upward or downward (but upward always in the case of worn springs), the movement of the axle will then cause an increased pull/push/pull action due to the fact the drag link end is now moving through a more severe arc at a different angle. Man, I hope that makes sense. Reading it again I'm not so certain, but moving the drag link upward will create a more dramatic movement to the point where any given upward drag link movement may cause a greater measurement of third arm swing.
Jon