Pinion angle measurements for shims. Sanity check please!

[staying_tuned]

Hey folks, I installed Ford's drop shackle kit on my CCSB and love the reduced rake. I'm noticing some intermittent vibration at very low speeds. I originally planned to grab a set of 2.5° shims to dial it back in but after taking measurements (and compensating for loaded scenarios) I think I might need more like a 3.5° or even 4° shim. Here are the angles.

Tranny: 4.7°
Driveshaft (center): -5.6°
Pinion: -1.7°
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Operating Angle: 3.9° per spicer's angle calculator

If we are supposed to shoot to zero out the difference while also ensuring a -0.5° to -1° resting state to account for load twist, that would mean I'd need more like a 3.5° or 4° shim. I'm hesitant to order that size simply because so many folks report vibrations resolved (using identical drop shackles) with a 2.5° shim.

It seems like either I should run at least a 3.5° shim to get it closer to the published ideal OR something is off in my math or the way I'm using the calculator OR folks are just running a 2.5° because it has worked for others without actually measuring. My vibration is so faint that a 2.5° would likely make it imperceivable but I want it right, not just undetectable when I'm driving it.

Thoughts? Thanks a TON in advance!

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[SALEEN961]

The way I'm picturing your numbers, you have a 0.9° operating angle at the front u-joint and 3.9° at the rear u-joint because the pinion is aimed down too far. If you install 2.5° shims, your angles should be roughly 0.9° and 1.4° which should work well. If you install 4.0° degree shims, that would put you closer to 0.9° and 0.1° with the pinion aimed too high.

 

[staying_tuned]

The way I'm picturing your numbers, you have a 0.9° operating angle at the front u-joint and 3.9° at the rear u-joint because the pinion is aimed down too far. If you install 2.5° shims, your angles should be roughly 0.9° and 1.4° which should work well. If you install 4.0° degree shims, that would put you closer to 0.9° and 0.1° with the pinion aimed too high.

I guess what is confusing is that I’ve seen multiple instances where folks mention targeting -1 to account for twist on load. Is that nonsense and maybe I’m just over complicating it or legit?

 

[SALEEN961]

I guess what is confusing is that I’ve seen multiple instances where folks mention targeting -1 to account for twist on load. Is that nonsense and maybe I’m just over complicating it or legit?

I normally aim to have my pinion nose down 1° from the point where my u-joint operating angles would be perfectly even at rest. With the composite leaf springs used on our trucks, the pinion doesn't seem to climb very much so I would feel comfortable getting the angles a bit closer at rest.

To give you a point of reference, my truck has no vibrations from 0mph all the way up to the speed limiter. My u-joint angle at the slip yoke is roughly 0.2° and the u-joint angle at my rear diff is roughly 1.4° with my truck sitting in park in my driveway. This difference in angle is a little higher than I'd like, but I never installed shims because I never had any vibrations.

 

[Buddy726]

I have a 2023 F150 4 x 4 that I wanna lower level I measured my diff at the yolk and it’s a 2° with the blocks so if I do this, what else do I have to measure and how I’m thinking of removing the blocks and doing shackles in the back. I tried the ride tech kit, but it didn’t work out because it lowered it too much that I wanted it. I just want a simple 2 to 3 inch drop in the back.

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