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Unread 02-13-2013, 04:49 PM   #1
habbyguy
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1996 XJ Cherokee 
 
Join Date: Feb 2010
Location: Mesa, AZ
Posts: 574
Driveline vibe - wrong shim?

Hi, all. I've had a fairly benign vibration in my XJ since I did a 3" lift (that included slightly longer rear shackles to get to the right height). I finally decided to install a pair of shims I had laying around (3 degree shims, IIRC), and the vibration is gone, except for under hard(ish) acceleration, when it shakes the whole vehicle, though not severely... just enough to feel like a vibrating massage chair. If I accelerate slowly, or am just driving along, there are no vibes at all.

I measured my pinion and output shaft angles, and the pinion shaft is pointing down 2 degrees more than the tranny output shaft. I figured this would be about right, assuming the pinion shaft raises a bit under hard acceleration, as the leaf springs wind up (presumably bringing the two angles to effective parity). But I never noticed this kind of vibration under acceleration before, when the angles were worse.

I am not running a SYE, just the stock shaft, and have new U-joints installed. There doesn't seem to be any play anywhere in the system.

If I'm calculating correctly, the original (non-shimmed) configuration would have the pinion angle 1 degree too high to start with, rather than 2 degrees too low, like it is now.

So... I'm interested in your best guess for my best option:
1) Find a 1.5 degree shim
2) Do a 1" xfer case drop (hate to do that because of ground clearance reduction... I've already got a bunch of dings on the crossmember)
3) Pull three spark plug wires so I can't accelerate hard any more

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Unread 02-13-2013, 07:17 PM   #2
donthelegend
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1999 XJ Cherokee 
 
Join Date: Apr 2010
Location: STL, MO
Posts: 931
4. Install an SYE.

The reason you're getting vibes (assuming the u-joints aren't shot, which is a possiblity) is because in the stock configuration, the pinion is NOT supposed to be pointing at the t-case or a couple degrees down. Its supposed to be parallel.

See this: http://www.4xshaft.com/driveline101.html
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Unread 02-13-2013, 10:14 PM   #3
habbyguy
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1996 XJ Cherokee 
 
Join Date: Feb 2010
Location: Mesa, AZ
Posts: 574
I did a bad job describing my set-up (and agree that I'd be shaking the paint off if it was set up like you mentioned). Basically, there's a 2 degree difference from parallel between the xfer case output shaft and the pinion shaft, with the xfer case shaft pointing down 8 degrees, and the pinion pointing up 6 degrees (both measurements relative to vertical, netting a 2 degree difference). I figured that the difference would shrink under power, due to the windup on the leaf springs that would raise the pinion shaft (bringing it closer to paralleling that 8 degree xfer case output shaft measurement). But the only time I get the vibe is when I've got my right foot down pretty hard, and it gets worse as the engine makes more power. I should mention that the Jeep is sitting in my driveway so those angles seem awfully small (since the nose of the Jeep is higher than the tail, and I'm measuring the angles from vertical, not the ground under the Jeep).

I think I have a line on a cheap replacement drive shaft, just to make sure I don't have something odd going on there (one of the rear U-joints is really tight when I press it in, and I can barely get the c-clip installed... it's almost like one of the "ears" might have gotten tweaked on a rock or something). But I'm thinking that I might need to change the shim as well - seems like this might be a case where "less is more".
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Unread 02-13-2013, 11:44 PM   #4
sAe23
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1995 XJ Cherokee 
 
Join Date: Apr 2009
Location: Los Angeles / SFV, California
Posts: 2,290
Do the "slightly longer" shackles have more than one set of holes to use?
If not, something longer will change the pinion angle a few degrees.
But as Donthelegend stated, the T-case yoke and the differential pinion need to be parallel. I've never read anything about needing to compensate for axle wrap.
And a SYE is a good thing to have even if you don't have vibes.
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