Driveline angles - sanity check

I have been chasing an annoying vibration since I replaced my rear leaf springs some months ago.

Originally I had an OEM pack with a 1.5" 3-leaf AAL + 1.5" lift shackle. I replaced the OEM pack with a General Springs pack, disassembling the new pack to remove the overload, and inserting the original 3-leaf AAL.

I thought this would result in the same overall lift, it ended up being WAY more, and it resulted in the vibration I have been chasing since.

I figured that I would reduce the unintentional bonus lift by making shorter shackles- which reduced the ground to fender height by .5" as intended. Leaving me with new springs, 1.5" AAL, and 1" Shackle. Still had the vibration.

I followed the thread somewhere on here about measuring driveline angles and followed the advice there- which was more or less, zero your angle finder on the diff, and try to match the driveshaft at 90°. Which I did by inserting a 3° shim.

While this helped, I still had a vibration, so I took out the driveshaft and rebuilt it- new u joints, carrier bearing (OEM for both), and put the driveshaft back in phase (it was out). Stuck it back in, and STILL had the vibration.

I bought a new driveshaft and installed it. MUCH better- *almost* fixed the vibration, but it's still there.

Which has led me to watch every video, and read every article I can on how to properly measure and assess driveline angles. I think I've finally identified that the 3° shim is too much:



It looks like I need to decrease the shim to a 1° shim so that the 3rd and 4th operating angles are matching/closer- or in other terms, so that the angle pinion flange (currently 9.3 down) matches the cardan (7.1 down).

Am I on the right track here?

I'm also going to reduce the shackles again (to stock height) because I still think the back is too high. Obviously, I'll remeasure before I do anything with the shims.

I have a road trip this weekend, so the goal is to rip this stuff apart tonight and get it sorted. I have new u-bolts, and shims from 1° to 6°.

 

Think the differential should point right at the cardan joint.

 

and the front half should point strait to the carrier bearing.

 

I've seen that diagram. It follows the logic that the DS should be 90 degrees to the face of the diff. But I don't think that is a two shaft installation. It applies when there is a double cardan directly after the xfer case.

I've also found additional diagrams here:
https://www.4crawler.com/4x4/CheapTricks/Driveline-101.shtml
which have other advice.

And additional advice that the difference in operation angle should not be more than 2 degrees.

 

I don't really consider this a 2 shaft install, the front shaft is more like a really long stub shaft than a drive shaft, the u joint in it really shouldn't change angle in operation, no angle means no change in velocity, which means no vibes. The tcase stub shaft should have the same angle as the front half, and the cardan joint cancels out any change in velocity at that angle, so any angle at the diff will add velocity that isn't compensated for. so to not add velocity the diff needs to point strait at it.

 

Have you tried to shim down your CB to see if it goes away?

 

This will likely add vibes no take them away, the front shaft needs to be at the same angle as the tcase stub shaft.

 

You would think so but I've read a thread here from someone chasing vibes too and shimming the CB dowm with washers ended up fixing it.

 

Yes- it did not help.

I had 3/4" of washers in there. No difference.

More to decreasing the slope on the intermediate shaft- that's not possible. The carrier bearing is as high as it will go. And other sites for sure mention having an angle between the two shafts, so I don't think the concept of the intermediate shaft being inline with the xfer case is correct either. This is a two shaft driveline, with angles on both shafts- which is why I think the parallel (canceling) angles is key, and why I should remove 2 degrees from the back shim.

Specifically:
from https://www.4crawler.com/4x4/CheapTricks/Driveline-101.shtml

 

From what I've read, it's a fine tune once you have the majors ironed out.

 

Ok, a universal joint doesn't have constant velocity, this is why they can get vibrations because as the joint flexes it changes the rotational speed. So the angles of the joint need to cancel each other out so the out shaft maintains a constant speed. The confusion here is because a cardan joint IS a constant velocity joint, it does not add or reduce velocity, this means its angle doesn't matter. So whatever angle the intermediate shaft joint is at and the angle that the differential joint is at are the ones that are adding or reducing velocity, those are the ones that need to cancel out.

 

Ok so three leading options:

1) Point the diff in line with the prop shaft



which would suggest to increase the shim another 1 degree to match the diff (currently at 9.3) to 10.3.

2) Align the diff with the intermediate shaft

which would suggest inverting the shim to align the (currently 9.3) diff to the 4.6 degree intermediate shaft (this seems unreasonable).

3) Matching the cardan and pinion operating angles (1° shim)


Given that the truck has spent the majority of it's life with 3" of lift and no shim, this seems like the "closest to how it used to be, before accidental mega lift" configuration.

 

Or go buy a 1 piece ds?

 

For real though, I got my icon rxt pack going in soon and I've been doing my own research into this and a think option 1 may be right. My intermediate shaft does seem to go strait to the carrier bearing though. That cardan joint really screws with the napkin math on this one.

 

Well... went with option 3. Drove around for a little bit- no discernible difference from before. Changing the shackles did fuck all of an 1/8"- So these General Springs must have a non-advertised higher spring rate than OEM- I even have a HD custom rear wrap around bumper with a swing out (and tire) on it...

I'm going to swap the 1 degree for the 4 degree in the morning before taking the dog to beach.

 

its not something dumb like a bad carrier bearing or ujoint that was exacerbated by the accidental mega lift is it?

 

I appreciate your work in documenting and explaining what you have tried and are working towards. I have an almost identical situation and wanted to finally alleviate a vibration that I have at 62mph. I’m looking forward to a solution.

 

I rebuilt the OEM shaft after the mega lift- new OEM carrier bearing and ujoints. No change. Then I bought a whole new non-oem driveshaft. So- pretty much ruled out anything to do with the DS. Also, the vibration is not present when I remove the drive shaft.

 

I don't know anything about DS specs but this:

would make me think the DS is the problem.

 

There is/are carrier bearings with built in drop......

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