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Chasing Driveline Vibes

dz1087

NAXJA Forum User
Location
Florida
I lifted the Jeep about a year and a half ago with the RE 3.5 superflex, full leaf packs. Did the IRO H&T SYE and am running a rebuilt and shortened front DS and 6* shims at the axle.

Anyway, I've always have driveline vibes since the lift, even after the SYE. My shaft is at about 17.5* and the Chry 8.25 is at about 15.5* Seems that should have fixed all the vibes, but I still get some good ones at around 30mph.

So, what else could be causing these vibes? I think that the springs may have netted over 5" of lift in the rear as I had to add 2" coil spacers to get the jeep near-level. Could it be just too much lift for a simple H&T?

Also, where is the best spot to measure the yoke angle? I just measured it off the rear of the diff at the bolt heads, assuming it was 90* opposed to the yoke output.
 
I did the following:
  1. Pulled the front driveshaft.
  2. Shimmed the rear... Still had vibes.
  3. Rotated my tires. Vibes gone.
  4. Reinstalled front driveshaft, got new (but familiar) vibes.
Correcting my front pinion angle will be a pain, as it became an issue whin I fixed my caster angle. In any case, these steps spared me some agony - should be worth a shot.

And with regard to measuring the rear pinion angle - that's how I did it. Just make sure you take all the measurements at the same time to make sure the plane upon which the Jeep is resting is the same for all measurements.
 
Driving around with my rear pulled, I have no vibes. I think that pretty much narrows it down to the rear shaft or the H&T, right?
 
Driving around with my rear pulled, I have no vibes. I think that pretty much narrows it down to the rear shaft or the H&T, right?

It can also be wheel bearings or the pinion bearing, though those are usually less pronounced than the driveshaft or SYE/transfer case (the output bearing there is a possibility, too).

One really dumb thing that's easy to overlook: lug nuts. Checked mine a couple of weeks ago and found that nearly half of them weren't completely tight. Getting them all the way back down resulted in about 60% of the current vibes (wheel bearings, in my case) disappearing.
 
If any of the bearings are going out, would they be more pronounced under power? Before the lift there were no vibes, then vibes right after lift. The SYE and shaft just made them less noticeable. With the rear shaft pulled there are no vibes.

I had the shaft shortened and rebalanced last April, and don't do any wheeling that would put me in contact with rocks to bend the shaft, but could there be a problem with another part of the shaft? I'll probably pull the front and stick it on the rear sometime this week, but if I still have vines then, what else could it be? Do I just need to try 8* shims and take a .5* positive angle at the rear yoke?
 
What year/what t-case are we talking about,it might be a problem with the H-n-T.
 
U-Joints and centering ball are all brand new NEAPCO's. I had the shaft rebuilt/balanced/ and shortened by a driveline shop here in FL. I've got the AW4/NP-231 paring under the Jeep right now.

I just remeasured the angles at the U-joint and got 15* yoke and 17.5* shaft. Should I try throwing an 8* shim under there? Does anyone make a 7* shim? Should I just pull a leaf or two and see if that works?
 
From the numbers you've posted, it appears your rear pinion angle points down 2.5* relative to your rear shaft. That's too much, if you are using a double-cardan rear shaft. With a leaf-sprung axle, you should probably try to get the pinion-to-shaft angle such that the pinion points approximately 1* below the shaft, to account for spring wrap. I have found it's trial and error to getting the right shims. You might try 5 to 7* to see where that gets you. You could always shim the transfer case down to fine-tune. Also, playing with the length of your rear shackles can be used to fine tune angles.
With regards to the front pinion angle, it's always a compromise between caster angle and pinion angle. Caster around 4-6* is OK. I assume your XJ is later model (Chrysler 8.25), so probably doesn't have a disconnect front axle; that makes it difficult, on large lifts (>4") to "compromise". Of course, the "correct" solution for either front or rear axles is to relocate the spring perches (rear), or inner "C's" (front); I assume you already did this for the rear when you installed the SYE.
 
From the numbers you've posted, it appears your rear pinion angle points down 2.5* relative to your rear shaft. That's too much, if you are using a double-cardan rear shaft. With a leaf-sprung axle, you should probably try to get the pinion-to-shaft angle such that the pinion points approximately 1* below the shaft, to account for spring wrap. I have found it's trial and error to getting the right shims. You might try 5 to 7* to see where that gets you. You could always shim the transfer case down to fine-tune. Also, playing with the length of your rear shackles can be used to fine tune angles.
With regards to the front pinion angle, it's always a compromise between caster angle and pinion angle. Caster around 4-6* is OK. I assume your XJ is later model (Chrysler 8.25), so probably doesn't have a disconnect front axle; that makes it difficult, on large lifts (>4") to "compromise". Of course, the "correct" solution for either front or rear axles is to relocate the spring perches (rear), or inner "C's" (front); I assume you already did this for the rear when you installed the SYE.


No, I did not relocate the perches. Should I do that instead of shimming the rear to 8*?

OK - so, I'm thinking correctly here....

Could I use transfer case dropping to figure out when the driveline angle is correct, basically by dropping it - what, maybe .25" at a time until it doesn't vibrate?

Maybe an easier way at this point would be just to weigh down the rear until I have no vibes, then measure the angle. Would the weight itself have any effect on vibes aside from adjustment of the driveline angle due to sag?

As long as either method would accurately represent the adjustment of the axle angle in order to find the pint of least vibes, it would be easier to do than dropping the axle and playing shim hunt.
 
The correct way is to cut the rear spring perches free, then, with the full weight of the Jeep setting on the axle, place a jack under the pinion and adjust the drive line for correct angle; re-weld, and your perfect--until you add more lift, or add significant weight to the Jeep. When reading the drive- line angles, it would be best to have the Jeep loaded with "normal" cargo; otherwise, the angles will change when you load-up.
A major side-effect of setting up everything to facilitate a 3-1/2 lift, is you'll probably want more lift in the future; obviously, this will result in required drive-line re-adjustments. So maybe using shims and other tricks to get acceptable angles now would be OK, for the short term--until you find your "ultimate" lift numbers. At that time, do it right, and rotate the spring perches.
I don't like the use of transfer case drops to get the correct angles; it causes misalignment with the front shaft, puts unwanted stress on the engine mounts, and raises the fan blad relative to the fan shround. Sometimes it's the only way, if all you need is a +- 1* change. Then, as you suggest, 1/4 inch, or less?? You can also adjust the rear angle by lengthening/shortening the rear shackles--again just for minor adjustments.
Adding weight to the Jeep for the purpose of determining the correct drive-line angle (the angle that reduces vibration to a minimum) works for isolating vibration to drive line angles, however, relating that angle to the number of degrees for a shim is still an iterative process.
 
I'm just making sure that if I add weight to the point I don't get an vibes, I can use that angle separation to determine the proper degree of change I need to see in the axle. Basically, the weight wouldn't have any other effects except to lower the chassis to axle height and lessen the angle between shaft and yoke. At that point I can then determine if I need to re-weld perches or if I can live with the shims.

I want to do this because I have a suspicion that I have something else causing the vibes aside from driveline angle and want to rule out the angle difference.
 
OK, so I just got done with adding about #800 of concrete bags to the rear. Got the angle down to about 1.2* of separation. Still have some vibes at 30. The vibes are mostly in the steering wheel now, I don't feel them hardly at all any more in the seat with this degree of separation.


Anymore tricks I should be trying? I'll rotate my tires tomorrow and see how it drives then, see if the vibes are a little less at 30.
 
Add another 100lbs? 1.2* is still a little greater than it should be. For diagnostic purposes, try getting the angle less than 1*. Problem is, deciding whether your "polishing a turd", or not. From what I've read, with the double cardan "shaft",theoretically you should have 0-degrees at the differential; however, two issues indicate this not practical: joint self-lubrication, and axle wrap. Supposedly you need at least a 1/2-degree error to insure the u-joint will lubricate itself. Secondly, it's hard to determine just how much allowance you need to make for the spring wrap your particular set-up gives you. If you truly raised your suspension 5 inches, as you suspect, I'm not sure you can expect a "stock height" smoothness in the drive train.
As you increase the operating angle of the front u-joint, you quickly exceed the designed angle of operation. Aside from shortened joint life, vibration and other "ugly" consequences can surface.
 
I see what you're sayin' there. Everything I've read points to 2* separation between the angles. I'm also going to pull the front shaft tomorrow an just make sure I've not got something else going on up there.

After I rotate the tires tomorrow, I think I'll just 'self-shim' the axle a bit to get it to .5* without the concrete in there.
 
Yeah, even that page says 1 to 2* of separation. I'm going to throw my 2* shims under the 6* shims and see what happens. If it smooths it out I'll be calling around to get some prices on some welding to move the perches.
 
Sounds like a plan. By the way, here's a quote from the above referenced article: "So, why must you run the u-joint at 0 degrees with a CV joint on the other end. See the discussion above and realize that the only time a single u-joint can operate smoothly is at a 0 degree operating angle. At any non-zero angle, the u-joint will induce a rotational vibration in whatever it is hooked to. This is not desirable, so the u-joint MUST be at 0 degrees operating angle in a CV- or double-cardan type shaft"
 
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