Figuring out my Driveline Vibrations after 4" Lift...

zachpeakee

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Bear with me, this is going to be a bit longer than I hoped...

We've been discussing this issue in my build thread for a few days now, but I decided it was time to make a thread entirely for the issue.
( there are plenty of threads like it, however I can't figure out some of the answers I need )
Not to mention driveline angle lingo gets confusing depending on the context...

I've got a 2006 LJ, with 4" Suspension lift.
Factory/OEM Driveshafts.
Full Adjustable Control Arms.
4.88 Revolution Gears.
(adding this because i'm un-familiar with gearing, or if there are any gearing-related vibrations)

I've got mystery drivetrain vibrations
( slight vibration around 30mph but goes away around 35mph, with a more audible noticeable vibration starting at 65mph and getting louder with speed)

It is text-book example of driveshaft vibrations , as the typical VROOOM - VROOOOM - VROOOOOM "drone-y" sound.
Similar to a sine wave.
It's not an aggressive vibration, as it's rather mild, but still I know that ANY vibrations can wreak havoc on joints/bearings/seals.

I took the incentive this morning to pull my front driveshaft out, so that the issue could be isolated front/rear.

The vibrations were still there, so it can be ruled that the issue is not in relation to the front driveshaft/axle.
So it is contained entirely to either my Transmission/Transfer Case, Rear Driveshaft, or Rear Axle/Pinion.


I used my Klein Digital Angle finder on the Rear Pinion, as well as the rear Transfer Case Output Shaft.

Rear Transfer Case was read to be 8.4 Degrees - as measured from the top of the harmonic balancer, as well as the driveshaft knuckle.
Rear Pinion is set to a matching 8.4 Degrees - measured from the flat/machined spots on the rear of the housing.

1708120517314.png


1708120530644.png


I have since tried to adjust the pinion angle to 7.9 Degrees with no discernable change to vibrations (0.5 degree difference to Output Shaft to account for axle rotation).

Here's where things get interesting.
At 70mph, during the vibrations, if I push the clutch pedal to the floor the vibrations stop entirely.
The same happens if I let OFF the gas pedal, and coast downhill.

The vibrations seem to be following/related to drivetrain load.
(which theoretically COULD still be a pinion angle/axle rotation based issue)


Now the fun part.
There's a ton of 'fixes' for vibrations, all of which I am willing to try.

MML seems to be a hot topic in the vibration threads, as well as a slight transfer case drop being another common one.
SYE/CV is top priority for vibrations (however, I currently see no need in wildly throwing $600 at it in hopes of fixing the issue)

I'm willing to pick up a few washers, and drop the Transfer Case temporarily to see how/if that changes things.
But I do have some concerns with that method.

It's been pretty heavily regarded that a transfer case drop is needed with the factory YOKE style driveshaft, EVEN IF you have adjustable control arms.
I've read that on about 10 different threads in the last 20 minutes, and it just makes absolutely zero sense to me as to why.

The transfer case drop essentially just changes the angle of the output shaft/driveline, to better align to your rear pinion angle.
It's a 'band-aid' fix at best, but that is all I need at the moment until I can figure out exactly what is causing the vibrations.

However, seeing that I already have fully adjustable arms, I can ALREADY match the pinion angle to whatever the transfer case angle is.
Does that, or does that not, render playing with Transfer Case angles useless in my scenario?

(it is also worth keeping in mind the length of the LJ driveshaft already. It sits a LOT flatter than the TJ rear shaft)

1708121496117.png


At this point, I am willing to try anything/everything to at least verify what is causing the issues/vibrations.

There are a lot of people in this forum smarter than myself, hence why I felt the need to add all the information, as well as reach out to you guys about solutions.
If there's any information I left out that could be useful, let me know. I'm willing to measure/photo anything that is needed.
 
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With a two joint shaft the pinion angle needs to be 1-2 degrees lower than the tcase angle. Since the conditions you are describing when it goes away that corresponds with having an "acceleration" vibration and you need to try and lower the pinion two degrees and see if/how it changes.

Using washers is a good tool that can help confirm a diagnosis of whether you could need a SYE but LJs for the most part just need to be dialed in the majority of the time.
 
Just by eyeballing it that pinion angle looks too high, as in higher than the tcase angle by at least a few degrees. As blackjack said, 1-2° lower than t-case for that type of shaft. You look to have small enough angles to keep that shaft happily once the angles are right.
 
I've successfully run an LJ vibe free with an OE shaft, no transfer case drop, and a 3" lift, but never tried a 4".

I know I posted this on your build thread but it works here too, even if the angles match there's still an angular velocity phenomenon that takes place when you have operating angle and the more angle you run the lower driveshaft rpm it takes to feel the resulting vibration.

Screenshot_20240216-160114.png


You're at a hair over 5 degrees and your driveshaft rpm at 65 is about 3400, so your vibes are kicking in right about where Spicer says is the limit.

Dropping your transfer case reduces your operating angle in two ways:

1. pitches the engine/trans/TC rearward, thereby increasing your output shaft angle
2. Lowers the output yoke, thereby decreasing the angle of your driveshaft

Some napkin math guessing you have about 22" between the u joints, a 1/2" t case drop would take 1.3deg out of your driveshaft angle and add about half that to your output angle, getting you down to about 3 degree op angle and buying you enough rpm to go 95mph. It would be a quick way to at least determine whether it's angle related or not.

A motor mount lift accomplishes #1 as well, and a little bit #2 but not nearly as much, so a 1" MML (didnt catch if you already have one) could get you a little more but might still come in before 80.

An SYE adds length to the shaft, which decreases it's angle, and also splits that angle in half between the two U joints in the double cardan joint. I think you'd end up around 11-12 driveshaft meaning your operating angles would be about 2 or less.
 
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With a two joint shaft the pinion angle needs to be 1-2 degrees lower than the tcase angle. Since the conditions you are describing when it goes away that corresponds with having an "acceleration" vibration and you need to try and lower the pinion two degrees and see if/how it changes.

Using washers is a good tool that can help confirm a diagnosis of whether you could need a SYE but LJs for the most part just need to be dialed in the majority of the time.

Okay, so at least my assumptions of TCase drops are somewhat reasonable...
At least in my situation.

I've dialed in the Pinion now to 7.5 degrees (so 1 degree lower than TCase).
Tried to take it for a "test drive" but it's 20 degrees and icy here now.

Heart rate is through the roof at the moment. I sent the LJ sideways on the highway just now at 60mph, and decided the 'testing' was over, and wasn't worth the risk right now. So for the rest of the evening, i'm just pondering my options/ideas with you guys.
( side note: Cooper STT Pro's are the worst tires i've EVER experienced in icy conditions )

However, I will say before the whole sideways on the highway ordeal, my vibrations DID seem to be about 1/2 of what they were.
So I would assume another degree down should get me within a reasonable ballpark.

Which could potentially lead me into another issue entirely, which i'm replying to directly underneath this...

I know I posted this on your build thread but it works here too, even if the angles match there's still an angular velocity phenomenon that takes place when you have operating angle and the more angle you run the lower driveshaft rpm it takes to feel the resulting vibration.

View attachment 501319

You're at a hair over 5 degrees and your driveshaft rpm at 65 is about 3400, so your vibes are kicking in right about where Spicer says is the limit.

Dropping your transfer case reduces your operating angle in two ways:

1. pitches the engine/trans/TC rearward, thereby increasing your output shaft angle
2. Lowers the output yoke, thereby decreasing the angle of your driveshaft

Some napkin math guessing you have about 22" between the u joints, a 1/2" t case drop would take 1.3deg out of your driveshaft angle and add about half that to your output angle, getting you down to about 3 degree op angle and buying you enough rpm to go 95mph. It would be a quick way to at least determine whether it's angle related or not.

A motor mount lift accomplishes #1 as well, and a little bit #2 but not nearly as much, so a 1" MML (didnt catch if you already have one) could get you a little more but might still come in before 80.

An SYE adds length to the shaft, which decreases it's angle, and also splits that angle in half between the two U joints in the double cardan joint. I think you'd end up around 11-12 driveshaft meaning your operating angles would be about 2 or less.

I had to draw this out on paper to understand, and NOW I understand.

However, in realistic "fix the problem" sense, if I'm thinking about this clearly...
It would probably be worthwhile to stack a few washers, at least see if the joints are causing partial issue, and then go for a more worthwhile solution
(such as MML for future use of Tummy Tuck...)

From the looks of that chart, that may very well be at least part of the issue, because as you said, the vibrations are kicking in almost EXACTLY at the limit according to that chart. And I'm fairly positive that chart was probably based on an "ideal" scenario, versus my unknown age joints.


-
-

All of this is exactly the information I was not only hoping to see, but also trying to understand.
Thank you guys for the wisdoms. 🤘
 
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Just by eyeballing it that pinion angle looks too high, as in higher than the tcase angle by at least a few degrees. As blackjack said, 1-2° lower than t-case for that type of shaft. You look to have small enough angles to keep that shaft happily once the angles are right.

Angles in the photo's were set identical to each other. 8.4 Degrees for BOTH.
(which still, as you said, was a bit too high on the pinion)

Admittedly, the photo isn't the best, but it's the best I had without taking wheels/tires off.