SYE or custom driveshaft?

[mercdudecbr600]

Hi all. For the last 5 years or so I’ve had a lifted kj. Nothing crazy, just the ome lift with no spacers.

After regearing to 4.10, I’ve got vibrations through floorboard at 70mph+. Had both driveshafts rebuilt or replaced but I continue to get the same at high speed 70+. I Have gone through a litany of new parts, wheel bearings, intermediate shaft, cv axles, oem mounts which did help significantly but it’s still there.

Having pestered the driveline shop enough they finally started asking hard questions like, what is your rear pinion angle relative to your tc output shaft, and your u joint operating angle?

Long and short of it, I’ve got an excessive u joint operating angle at something like 5-6deg. Supposed to be 1-3deg.

And since I’m not the most extreme of lifted kjs figure others here MUST have figured out a solution.

Mind you, this only came about because I’m now spinning 24-2500 rpm at 70mph. I’m guessing I always had this issue after lifting and didn’t realize it because with 3.73 and 31” tires, I never got to the propshaft speed necessary to vibrate.

Help and/or thoughts are welcome!

 

[mercdudecbr600]

Found this old thread... glad to know I'm not the only one dealing with vibs and pinion angle issues.

Slip Yoke

This is exactly what I was saying in my post, and your results supported that. Careful man, you might be accused of "not knowing suspensions" too (people can ROFLMAO all they want, I could give a flying crap)!! Mainly what I don't like about what the lift does, is how goofy the Liberty looks...

www.jeepkj.com

 

[u2slow]

Tried shimming down the trans crossmember?
Its easy to loosen the bolts at the frame and slide 1/4" something in there for a test.

 

[mercdudecbr600]

I tried and only measured a 0.1-0.2deg improvement. Not significant enough for it to matter. Operating angles are still high.

I’ve now tried removing the harmonic balancer, lengthening the ds, rebalancing, and playing with pinion angle. Can’t get it smooth at high speed.

But I did notice my rear upper control arm has at least some tearing in the bushings. Will replace and then most likely buy a cv driveshaft from Tom woods.

 

[u2slow]

Best I can measure I have 11.5° on the shaft and 3-4° at either end. Surprised they are that close a match - it looks like more at the tcase end. The pinion is tilted up slightly because of how the JBA tri-link bracket is built.

I'm using the driveshaft with the damper weight ring.

 

[mercdudecbr600]

I’ve got something like 5.2* at tc and rear yoke with 11.5* slope.

So far the best ive been able to do is get the vibration down to low harmonics but as you get above 75 goes to a noticeably felt vibration.

I’ve got the iron rock rear uca to install and then I’ll report

 

[u2slow]

I should add, I have the engine cradle dropped 3/4", and the trans crossmember dropped 1/2".

 

[mercdudecbr600]

Did you consider a double cardan style ds? I’m thinking I will still ultimately have to do this because of the u joint oscillation - even with a 0.2-0.3* variance between tc and rear yoke (which should be fine) I get harmonics. This even with new oem mounts across the board. Just isn’t right and it’s driving me nuts

 

[u2slow]

Did you consider a double cardan style ds?

No, because I haven't had issues. I spaced the cradle down to help my lift goals. Spacing the crossmember down was a stab-in-the-dark.

Have you considered wear in the tcase output shaft splines, or the sliding yoke? Or the pinion bearings howling/swishing because they're shot?

 

[mercdudecbr600]

New/rebuilt transfer case, new driveline u joints, fully rebuilt rear diff (no noise). Slip yoke feels tight. Drives butter smooth until 65+ mph.

Pretty sure it’s rear ds related because I can get better/worse vibe if I change the pinion angle.

I should mention my 02 kj has the 45rfe trans which may make a difference in both the ds length and sensitivity to vibration (just guessing).

 

[u2slow]

I should mention my 02 kj has the 45rfe trans which may make a difference in both the ds length and sensitivity to vibration (just guessing).

I'm running an '04 auto trans shaft behind the 6spd and np242. Same length.