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how will i know how long to build the shackles to match whatever front lift the hanger adds?


As mentioned above, you basically make the shackles that many inches longer than stock. In my case, I did 2.5" hanger drop and shackles 3.5" longer than stock (I added 1" for a little more caster angle), they ended up 7" center-center length. Or, the AllPro hanger is 1" drop, their shackles are 1.5" longer than stock. You do want the shackles *at least* as much longe as the hanger drop, a little more is OK, a little less is bad news.