I lowered the rear 2 inches using shackles and I need to install shims to raise the pinion. My question is, does the fat part of the shim face the front or rear? I'm getting conflicting answers. The place I'm buying the shims from says fat part towards the front.

My intuition says towards the front, but with suspension, intuition is sometimes wrong. Thanks

 

Fat end goes in the rear. See below. This is a page from the Ford Performance lowering shackle install instructions.

Side note... which shackles did you use and how much actual drop did you get at the fender well?

 

Thanks for the Ford Performance illustration. That pretty much settles it, even though I cant explain why towards the rear makes sense!

I used MaxTrac shackles. I forgot to measure the height before starting, but based on appearance alone, it appears to be a solid 2 inches.

 

I’m not suspension savvy at all so entirely a guess here...

I know the shackles lower the truck but being longer, seems like they would put more force on the rear mounting points of the leaf springs, which would tend to push the rear of the springs down and tilt the front of the springs up. Basically, if you’re looking straight at the driver rear wheel, this would slightly twist the entire rear end housing clockwise. Shimming the springs with the fat end toward the rear would have the opposite effect. It would twist the housing back counterclockwise. Again, maybe totally off base, just my best guess.

 

Installing a drop shackle tips the pinion angle down. Fat end in the rear creates the opposite effect by tipping it back up.

 

Measure your angle at the pinion yoke and trans yoke angle and install the shim as necessary. The vehicle does not have to be perfectly level because you are interested the difference between the two.