For lifted kjs - custom driveline required?

[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!

 

[24turbo]

Im not talking from direct KJ experience here so take it for what it is worth.

There is adjustable hardware for the rear differential and specifically the upper control arm. I believe it is from a YJ and there is a vendor that makes a bracket to adapt to a KJ differential upper mount (WWdiesel?) I would start there and change your pinion angle to something a bit closer to 3 degrees. You may also need to change the lower control arms to something a bit more rigid, as the factory ones have soft rubber bushings which probably are not helping. You could also go the route of just doing adjustable lower control arms and adjust pinion angle that way.

Also worth noting is you want the driveshaft angles at your 2 locations (transfer case and differential) at as close to the same angle as possible, whatever that happens to be. U-joints operating at an angle actually change the rotational speed of the driveshaft (Im sure there is a u-tube video explaining this) through its rotation and if the 2 are matched they cancel themselves out.

 

[derekj]

Look into the Iron Rock rear upper control arm assembly - direct replacement for the stock arm and gives you adjustability to correct driveline angles.

 

[mercdudecbr600]

Appreciate the feedback but I already have adjustable rear lower control arms, so changing the pinion angle is not the issue. Right now my tc and pinion measure at 5.7 to 5.8deg, so their parallelism is fine. The issue is that the operating u joint angle exceeds 3deg - I’m sitting at double that, around 6deg.

Even with fresh oem mounts, a balanced driveshaft with the oem harmonic balancer and the harmonic weight at the rear diff, im still getting vibs at final drive above 24-2500 rpm.

To note, I have the 45rfe trans which runs higher rpm’s than the later trans (so this magnifies the issue) but I believe the operating angles are an issue for every lifted kj.

That said, I’m sure someone has had this before and ended up with a double cardan (cv) driveshaft.

 

[mercdudecbr600]

Im not talking from direct KJ experience here so take it for what it is worth.

There is adjustable hardware for the rear differential and specifically the upper control arm. I believe it is from a YJ and there is a vendor that makes a bracket to adapt to a KJ differential upper mount (WWdiesel?) I would start there and change your pinion angle to something a bit closer to 3 degrees. You may also need to change the lower control arms to something a bit more rigid, as the factory ones have soft rubber bushings which probably are not helping. You could also go the route of just doing adjustable lower control arms and adjust pinion angle that way.

Also worth noting is you want the driveshaft angles at your 2 locations (transfer case and differential) at as close to the same angle as possible, whatever that happens to be. U-joints operating at an angle actually change the rotational speed of the driveshaft (Im sure there is a u-tube video explaining this) through its rotation and if the 2 are matched they cancel themselves out.

You can’t just adjust the pinion to reduce the u joint angles because it will throw off the parallelism and make things worse.

Theoretically you could drop the trans cross member and then tilt the pinion up to match the tc angle and see if that would get you under 3 degrees but using my math skills the best I could get reasonably is 4deg operating angle.

 

[mercdudecbr600]

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[24turbo]

Thanks for the lesson on driveshaft angles!

I guess you didnt bother to read my entire post:

"Also worth noting is you want the driveshaft angles at your 2 locations (transfer case and differential) at as close to the same angle as possible, whatever that happens to be. U-joints operating at an angle actually change the rotational speed of the driveshaft (Im sure there is a u-tube video explaining this) through its rotation and if the 2 are matched they cancel themselves out."


In case you didnt get what I was saying, its the same as "parallelism" (fancy term!). When you raise the rear suspension it changes pinion angle and you need to adjust it so its the same at the transfer case (which if not done will make it worse). All I was trying to do was offer some help. Maybe you should remove this from your first post:

"Help and/or thoughts are welcome!"

Since you are so convinced your driveshaft angle is too much (which it is) and cant be changed- easy button: BUY A NEW DRIVESHAFT

 

[mercdudecbr600]

Sorry about that, not intended to be critical. Yes I understood what you meant about pinion angles.

The thing is: with coil spring 4-link rear suspensions found in jeep, raising or lowering the rear ride height doesn’t affect pinion angles. The lengthening or shortening or suspension does though. raising and lowering does affect u joint operational angles because the driveshaft is at a higher angle to connect the tc to rear diff.

My pinion and transfer case angles measure as matched and cancel out. But, the high u joint operating angles create (basically) binding and oscillation at high speed, aka vibration. And that situation is not unique to my rig, same for every vehicle that uses a single u joint driveshaft.

What is unique to my rig is that I have the older driveshaft with the harmonic balancer on it which adds considerable rotational mass and that (I’m guessing) amplifies the effect. U joints are new and so is tube. So really no difference if I get another driveshaft or not.