Driveline Vibes With RC Lift Kit

T

TJX02

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New member here... Trying to figure out how to resolve some driveline vibes on my TJ. It came with a lift, which appears to be the 4in RC lift kit. Has the TC drop from the kit, appears to also have the lower control arm from the kit, and also has a body lift. No MML. When I get it up to highway speeds, it's a smooth ride when coasting but the vibrations hit on acceleration, especially in lower gears.

I tried to take some measurements of the drive train angles using my iphone.

Driveshaft: 17*
TC output shaft: 14*
Differential cover: -8*

Don't know how accurate it is to use the rear diff cover, but that's what I've got so far. Attached photo was taken with phone sitting flat on the ground.

I've got u-joints in the mail, along with pinion seal. Planning to do that and refill the fluid.

How do these angles seem? I'm decently mechanically inclined and am trying to get this driving decently. I don't mind going lower with a TC drop temporarily to resolve this issue, but I'm trying to hold off on a SYE/CV. Would a lower drop or a MML help sort this out?

Greatly appreciate any feedback, thanks in advance!

IMG_4474.JPG
 
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TJX02 said:
New member here... Trying to figure out how to resolve some driveline vibes on my TJ. It came with a lift, which appears to be the 4in RC lift kit. Has the TC drop from the kit, appears to also have the lower control arm from the kit, and also has a body lift. No MML. When I get it up to highway speeds, it's a smooth ride when coasting but the vibrations hit on acceleration, especially in lower gears.

I tried to take some measurements of the drive train angles using my iphone.

Driveshaft: 17*
TC output shaft: 14*
Differential cover: -8*

Don't know how accurate it is to use the rear diff cover, but that's what I've got so far. Attached photo was taken with phone sitting flat on the ground.

I've got u-joints in the mail, along with pinion seal. Planning to do that and refill the fluid.

How do these angles seem? I'm decently mechanically inclined and am trying to get this driving decently. I don't mind going lower with a TC drop temporarily to resolve this issue, but I'm trying to hold off on a SYE/CV. Would a lower drop or a MML help sort this out?

Greatly appreciate any feedback, thanks in advance!

View attachment 343096
Click to expand...

Going by the picture your pinion needs to be pointed downward more at the same time really you look like you’re in the territory of just needing a CV drive shaft

It isn’t the brand of lift, it is the amount of lift
 
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AndyG said:
Going by the picture your pinion needs to be pointed downward more at the same time really you look like you’re in the territory of just needing a CV drive shaft

It isn’t the brand of lift, it is the amount of lift
Click to expand...

I wonder if swapping out the RC control arms for stock or adjustable ones would correct my pinion angle without a CV? Really would like to avoid the SYE/CV route at the moment.
 
TJX02 said:
I wonder if swapping out the RC control arms for stock or adjustable ones would correct my pinion angle without a CV? Really would like to avoid the SYE/CV route at the moment.
Click to expand...

I wish I could tell you for sure- Everyone of these ends up a little different when the build happens-

For example I once did a half inch change and it introduced vibes I could not dial out with fully adjustable arms
 
You really should fix it right with the SYE/CV driveshaft. I got vibes with just a 2 inch lift, installed the MML and that to care of it. Then I couldn’t stand how far down the stock TC skid was. Did the tummy tuck and SS SYE. No vibes at any speed
 
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TJX02 said:
I wonder if swapping out the RC control arms for stock or adjustable ones would correct my pinion angle without a CV? Really would like to avoid the SYE/CV route at the moment.
Click to expand...

Without going with the SYE/CV, you have mainly the option of TC drop. HOw much of a TC drop do you have now?

Fixing the Axle Pinion angle to match the TC output seems like would certainly be an adjustable arm add, I don't see that goign to a stock CA would help, I would guess it would be shorter and make it more pointed up (working against your goal).

If you're going to alter your lift, and bring it down, then you have a math problem to solve. I would calculate the cost of going SYE/CV with adjustable CAs to dial it in (and all the other pieces you might need) vs a shorter lift. As important, do you want it lower, or really want it at 4" etc. You don't have an option that is cheap or free unfortunately. TC Drop or more of it would be cheapest review.
 
Apologies, others are right, a MML would alter the angle and bring the TC output possibly closer to the Pinion, so that may be the cheapest, best possible short term solution.
 
You have a rear pinion angle problem, and that someone installed control arms (or adjusted the uppers) to lift the pinion angle as it is didn't know what they were doing. With a stock two u-joint driveshaft the rear pinion angle MUST be parallel to the output shaft from the transfer case. Like this...


2joint_angle-jpg.jpg



The pinion angle is only raised up for a double-cardan (aka constant velocity) driveshaft which has 3 u-joints (2 up front 1 in the rear).

But the main issue is that a 4" suspension lift is WAY too tall for the factory 2 u-joint driveshaft. The angles your driveshaft's u-joints are operating into are excessive and causing the u-joints to vibrate. That the vibration gets worse with acceleration is that the rear pinion angle increases even more during acceleration.

For a 4" suspension lift you really need to install a SYE (slip yoke eliminator) kit into your transfer case and install a double-cardan (aka CV) rear driveshaft. The pinion angle must be like below which requires adjustable length arms so it can be adjusted properly.


cv-driveshaft-angles-gif.gif



I'd normally say to just drop the t-case another inch or two and install a 1" motor mount lift but your rear pinion angle is screwed up so not even that band-aid fix is likely to help. I'd do whatever it takes to fix it right and install the SYE kit and DC (aka CV) driveshaft plus the adjustable length control arms. No it won't be cheap but you absolutely cannot drive a vehicle that has drivetrain vibrations that will not only eventually take out u-joints it can take out seals and bearings too.
 
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Jerry Bransford said:
You have a rear pinion angle problem, and that someone installed control arms (or adjusted the uppers) to lift the pinion angle as it is didn't know what they were doing. With a stock two u-joint driveshaft the rear pinion angle MUST be parallel to the output shaft from the transfer case. Like this...


View attachment 343124


The pinion angle is only raised up for a double-cardan (aka constant velocity) driveshaft which has 3 u-joints (2 up front 1 in the rear).

But the main issue is that a 4" suspension lift is WAY too tall for the factory 2 u-joint driveshaft. The angles your driveshaft's u-joints are operating into are excessive and causing the u-joints to vibrate. That the vibration gets worse with acceleration is that the rear pinion angle increases even more during acceleration.

For a 4" suspension lift you really need to install a SYE (slip yoke eliminator) kit into your transfer case and install a double-cardan (aka CV) rear driveshaft. The pinion angle must be like below which requires adjustable length arms so it can be adjusted properly.


View attachment 343125


I'd normally say to just drop the t-case another inch or two and install a 1" motor mount lift but your rear pinion angle is screwed up so not even that band-aid fix is unlikely to help. I'd do whatever it takes to fix it right and install the SYE kit and DC (aka CV) driveshaft plus the adjustable length control arms. No it won't be cheap but you absolutely cannot drive a vehicle that has drivetrain vibrations that will not only take out u-joints it can take out seals and bearings too.
Click to expand...

When Jerry posts like this, most engineering schools will give you at least several hours of class credit for reading it.

We hang on his every word- Even the prepositions.
 
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Correction: It does not have a body lift.

Alright, doesn't look like lowering the pinion angle is an option, only lowering the TC shaft angle.

Think I'll just go with the SYE/CV and adjustable rear upper CAs.

Gonna try adding a few more washers to the 1" TC drop bar to see if I can improve the angles before I perform the SYE installation.

Great info, thanks all!
 
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Keep in mind; lowering the TC will also affect the shift linkage, and a MML will affect the fan to shroud clearance.
Nothing is free. ;)

Also, some will say it takes both upper and lower adj. CAs to properly adjust the pinion angle.
 
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LONGJP2 said:
It's not just the pinion angle. The u-joints can only handle so much angle.
Also, at some point the length of the shaft comes in to play.
Click to expand...

Good point. I decided to just go the SYE/CV route. After installing new rear driveshaft I plan to replace the front driveshaft u-joints, at least to start fresh with new driveshaft u-joints all around.
 
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Today I refilled the transmission fluid with Red Line MTL (also did two kerosene refills) and refilled the front/rear differential fluid with Red Line 75W90 (used two cans of brake fluid on the gears) and WOW! I'm still in shock at the difference it made. Absolute night and day. Vibrations much more tame now that the shudders and clunks are gone. PO really neglected those areas, but the Jeep is coming along...
 
Update
Replaced transmission mount and removed front drive shaft and nearly all of the vibrations are gone. Inspected rear DS u-joints and they looked surprisingly okay. When I accelerate hard in first the steering wheel and dashboard shake, but can avoid that with slow accelerations.

Observations
I did a drivetrain inspection and found the following components very loose which I could rock considerably with my hand:
  • Driver's side front sway bar end link
  • Rear track bar relocation mount non-angled (extremely loose)
  • Rear sway bar (both end links and axle mounts)
I bought a JKS angled relocation mount/adjustable track bar and sway bar bushings to add to my parts pile. I didn't buy end link bushings because I have aftermarket lengthened rear sway end links and not sure what bushings to get.

Plan
This weekend I am doing the following work:
  • Phase 1
    • Replace u-joints on front drive shaft and reinstall
    • Replace rear sway bar axle bushings
    • Tighten front/rear sway bar end links
    • Install adjustable upper rear control arms
    • Install adjustable rear JKS track bar
If I succeed with those in reasonable time, I will install SYE/DCDS, adjust pinion angle and add JKS angled relocation bracket:
  • Phase 2
    • Install SYE kit
    • Add custom made DCDS
    • Remove 1.5" TC drop
    • Install JKS angled relocation bracket for rear track bar
    • Install rear shock relocation brackets
I can't overnight park where I'm doing the work so I planned the above sequence to avoid downtime. Rebuilt and reinstalled front DS is first to serve as backup if I can't finish rear drivetrain work.

Will do rear UCAs and TB next but leave untorqued and move straight to SYE/DS. If successful will add the angled relocation bracket and finish adjustments. If unsuccessful will torque suspension, remove rear DS and drive home in 4hi on the front DS, then resume another time (following new parts/tools/etc).

Thoughts on my plan or anything I missed/should know?
 
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Well my weekend plans got slightly derailed and I did not accomplish everything, but I did solve my driveline vibrations and greatly improve my driving experience.

Installations & Adjustments
  • JKS Rear Track Bar
  • Rear Sway Bar Moog Bushings
  • Added 3/4" washers to the TC drop for a total of 2 1/4"
  • Replaced u-joints (Spicer 1310X) in front driveshaft (not yet reattached)
Had a friend in the car and suddenly it was driving incredibly well without vibrations. Then they came back when I was alone, so that motivated my to try the increased TC drop, which worked. But the rear track bar replacement and sway bar bushings really brought event thing together. Having so much fun driving it.

I hope to finish installing the SYE/DCDS, UCAs, angled TB bracket and rear shock relocation brackets by end of the month. A bit busy next couple weeks but I'm really looking forward to finishing these upgrades and getting everything dialed in.
 
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After several failed attempts to remove a seized rear upper control arm bolt with a both a breaker bar and high torque impact wrench, a couple blasts of Liquid Wrench sitting for 5 minutes got it off with the low impact setting. Had a torch on hand in case Liquid Wrench was a bust. I'm very impressed, it's way better than PB Blaster.

Installations Update
  • Adjustable Rear UCAs (stock length)
  • Rear sway bay end links
Gonna try to finish the remaining work tomorrow. I hope the pinion adjustment doesn't give me trouble.

Remaining Installations
  1. SYE & DCDS
  2. Angled track bar bracket
  3. Shock relocated brackets
  4. Rear pinion angle adjustment
  5. Rebuilt front DS
 
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