Suspension and axle placement Help Needed

[Abusfullofnuns]

Advice needed. Feel free to drop a link if there’s good info somewhere.

Rear suspension was 6” rough country springs with a stock hanger and shackle.

New springs are 64” with 3” lift and 4” shackle flip.

I am trying to mount the front and rear hangers for the rear springs.

I have an 88 GMC 2500 suburban and always thought it had 56” rear springs. The rough country spring I pulled off measures at 54” and is centered. I measured with a wire along the arch and am pretty confident in the length. The lift was installed when I bought the truck. The rear hangers were bolted on, so they could have been moved to use these springs. A friend is telling me that he thinks that 54” probably means I had 52” springs and the extra length is because of the arch of the 6” springs.

When I thought I had 56” springs, my plan was to move the front hanger 4” forward and rear 4” back(or flip the hangers if that worked). I hadn’t considered the potentially offset spring of a 56 and the centered 64. Before I got started, I read that I needed to move them less than the variance between the springs. Say 6” change instead of 8” of variance. Then I realized that I didn’t pull 56’s off.

What is the correct process to make sure that my rear axle is close to where it should be? I know I can ez inch to dial in the final spot.

 

[Abusfullofnuns]

I also found centerline in the wheel well and marked that on the ground using a plumb bob. So I have a measurement marked on the ground 32” from that center point front and rear. I also know that the shackle flip usually moves the wheel placement 1” at Ride height.

Should I be basing my new mounting points on the wheel well and these measurements? Or should I be focusing on the factory mounting points and measuring from there?

 

[Abusfullofnuns]

A friend is telling me that he thinks that 54” probably means I had 52” springs and the extra length is because of the arch of the 6” springs.

@truck-oholic was right. I just went and looked at the truck. The factory rivets are still stuck in the 56” hanger location. Looks like the last owner did move the hanger to use this lift. I guess I get to remove more rivets and should have been more observant.

Solves that part, but I still need clarity on where to base my measurements.

 

[truck-oholic]

The centered spring pin was the give away. 56" springs have 4" pin offset. Basically front half of 52 and 56 are exactly the same, the extra 4" all goes at the rear of the spring. That's why you can replace 52s in rear with 56s and keep axle in same location and just swap the offset shackle flip brackets from driver to passenger.

The shackle flip brackets themselves are 2" offset so by swapping them side to side you're going from 2" forward of center to 2" rearward of center giving you 4" total of rearward movement.

 

[truck-oholic]

For bracket placement going from 56 to 64 you have to think of it in terms of spring pin placement. Stock 52 and 56 are 26 to pin from front eye/hanger. I'm pretty sure 64 is centered so it is 32 to pin from front eye/hanger. That's a difference of 6" on the front so front hanger moves forward 6".

For rear, pin to shackle eye on 56 is 26 (1/2 of 52) + 4 = 30. 32 -30 = 2" difference so shackle hanger would ideally move 2" backwards. That being said this is all based on springs having same arch which they don't. So, instead of moving the rear hanger back 2" it was suggested by others to simply add on 1" to the front 6" making a total of 7" frontward bracket movement and the rear stays where it is and you have a little more shackle angle. Axle is close to center and fine tuned with easy inch block, redrilled or aftermarket perches, or pin adapter plate ORD now sells.