Shackles and springs

Driving Pete around the neighborhood Saturday I realized the Black Diamond springs are not very flexy. This should get better w/ time and miles but it got me to thinking about the whole flexing process. There should be an optimum length for the shackles to allow the springs to approach complete straightening of the leaves. I think you wouldn't want to completely flatten out or the springs could eventually have some counter arc in them. So to my questions. Do longer shackles allow for more flex? If so, how much longer can they be before you get into castor/steering issues?:v6:

 

Yeah, Black Diamond 3" lift. Probably harebrained idea, but I thought, as with most production vehicles, jeep may have cut them short of optimum and some increase in length would give better flex. No extra long bumpstops w/ the kit. At this point I don't see it stuffing a tire enough to matter.:v6:

 

First ck to be sure the shackle bolts are not to tight. Then check to see what is limiting the travel, most times it is shock length that restricts flex.