About the Shackles I believe if the length of your springs flatened = the distance between the center of the eye in the front spring mount to the center of the shackles eye before it contacts the frame you will get the most flex. If the springs flatened length is for example longer than the distance between the front spring mount center and the rear spring mount on the shackle fully stretched out backwards the spring can't flex it will just stay arched and not bend
Does this make any sense
