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Build: "full body" tons and 40s

1) The current center to center is 48" unsprung. If it changes under the weight, im not sure how to calculate that.

2) If I put a level between the centers as it sits, with a digital angle finder.. its 5 degrees exactly.

On another note, it does look like the center pin needs to go back 5". I do have kind of perches that can go back another 1" as well iirc.

Thanks for the help!
Sorry, I need to ask more question:

- So c-to-c of the leaf spring eyes unsprung is 48.0" and not 48.75"? If yes, then what is 48.75" c-to-c? Is it the straight line distance between the leaf spring front mount and the shackle mount?

- Is your XJ currently sitting approximately level or close to the rake you intend when its a ride height?

- Since you have the level sitting on the leaf spring, I'm wondering if you misunderstood my question about angle between the leaf spring front mounting point and the shackle mounting point. Just to be clear, if you set your angle finder on a straight edge sitting on top of the bolt heads at the leaf spring front mount and on the bolt head at the shackle mount (at the frame), you get 5 degrees?
48" c to c is the leaf spring only.
48.75" c to c is the mounts on the body.

This was the video I watched to calculate the mount holes at 48.75"

I think it is close to ride height. Im not sure exactly how to do it, but I figured id get the rear in and then get it "level" with what coil springs I need up front.

Sorry for the confusion, thats exactly what I thought after posting that pic. I did infact put a straight edge c to c of front mount and rear shackle where spring mounts. When I put that angle finder on the straight edge it was 5 degrees exactly.
According to my shackle angle calculator, your shackles be at 51 degrees relative to vertical when your leaves flex to straight. If I assume that the c-to-c distance between your leaf eyes is 51" at ride height, the shackle angle relative to vertical will be 30 degrees.

This is pretty close to my setup. This range of shackle angles nets you an addition 0.55" of rear wheel travel for your 5.25" c-to-c shackles, and this reduces the effective wheel spring rate by ~10%.

If you move the shackle mounting point forward by 1/2", then your shackle angle relative to vertical with a straight leaf pack will be 59 deg, and I'm guessing the angle at ride height will be ~33-37 deg. Since you don't know the c-to-c leaf eye spacing a ride height, I can only guess the angle at ride height.

You might want to consider building shackle mounts that have 2-3 holes to allow you to experiment. I've seen one kit that uses a diamond pattern to allow spacing the holes more tightly while also allowing the shackle mounting point to be raised and lowered to change ride height slightly.