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Old 04-23-2013, 06:29 AM   #1
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Need help with rear drive line vibes.

For those not familiar with my recent build my 98 had a 4" frankenlift with the stock skid and 1.25" drop. I replaced all arms with Savvy adjustables. I also installed: JBC SS SYE, UCF ultra high skid with lowpro mount, and a double cardan drive shaft. The tcase slope is 4° and ds is 19.75" long. I initially set ds and pinion angle at 22.5°.
The problem.
1st and 2nd are smooth, around 25mph I get a moderate vibe that slackeneds about 35. At 50 change in the console starts to rattle then stops at 70. This happens at acceleration, steady speed, and deceleration. If I shift into neutral at speed the vibes stop while I coast.
What I have tried.
I reduced the pa so the ds was 1° higher, 22° and 23° respectively. No change. I went to the drive line shop that built my ds and took a tech for a ride. He recommended dropping pa to 2° differential. Tried it and no change. I reset ds/pa to 23°/22° and removed the rear ds, it drives smooth as silk. I am having the ds rechecked today for any problems.
Thoughts or advice are welcome. Thanks

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Old 04-23-2013, 07:09 AM   #2
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The first thing I would do is what you're doing today. I've seen more than one newly rebuilt/ built drive shaft have issues, including from famous name drive shaft shops.

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Old 04-23-2013, 12:31 PM   #3
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I talked to the drive shaft tech and he said the ds is good to go. He also said the dc joint is rated at 26° max operating angle. With my tcase slope of 4° and ds angle at 23° my joint angle is 19°. Any thoughts or ideas?.
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Old 04-23-2013, 01:10 PM   #4
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It seems odd to me that you have an off/on vibration problem at different speeds and are looking at the drive shaft. Do you know for a fact your wheels/tires are balanced, no play when mounted and your wheels aren't bent?
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Old 04-23-2013, 01:17 PM   #5
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Thanks for the reply. I thought about that to which is why I removed the rear ds and and drove with the front. Drives smooth without rear ds.
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Old 04-23-2013, 01:21 PM   #6
Knows a couple things...

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I would stop worrying about all those precise angle measurements your making, the only thing to worry about is getting the pinion shaft angle at the same angle as the driveshaft. So the pinion shaft points directly at the CV joint. Nothing else matters, not even the t-case output shaft angle.

The pinion shaft can point perhaps 1-2 degrees under that of the driveshaft but it can't point even 1 degree above the driveshaft.
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Old 04-23-2013, 02:32 PM   #7
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So your telling me not to over-think it Jerry? Not many people can mountain climb a mole hill better than me lol. I'll set the ds and pa the same and test drive. Thanks
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Old 04-30-2013, 10:18 AM   #8
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Update: I had the drive line shop recheck the DS, they said it was all good. I reinstalled the DS and set the PA at .5° down. The change doesn't rattle anymore but I still get some vibe at 25-30 mph accel and deccel. None on coast. It is felt through the seat and steering wheel. I don't think it's tires cause It doesn't vibe while coasting in neutral. I removed front DS and it is still there. Removed rear and vibes are gone. Could this have anything to do with the lowpro mount? I get a little more idle vibes with the LPM. Even though I'm confident it's not tires, I will have them rebalanced today. Thoughts or advise?

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