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Unread 04-02-2005, 07:00 PM   #1
wherndon
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Acceleration Vibration

My new Jeep has a slight vibration when accelerating.
Doesn't happen when coasting or decelerating.
RE Superflex, with 3.5" springs.

Two questions...
Should I shorten, or lengthen the upper control arms?
Can I do this without removing the wheels and supporting the axle?

thanks,
Bil

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Unread 04-02-2005, 08:42 PM   #2
Jerry Bransford
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Acceleration vibrations usually mean the rear pinion angle is too high. You could either lengthen the upper arms or shorten the lower arms to reduce the pinion angle slightly.

Whether your wheelbase should be slightly lengthened or shortened (likely lengthened) will dictate if you adjust the uppers or lowers. I'd lean towards lengthening the upper arms which will slightly lengthen your wheelbase that is naturally slightly shortened by the lift... which would essentially move the axle back a little towards the factory wheelbase length.
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Unread 04-02-2005, 08:49 PM   #3
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Thanks Jerry.
I don't have adjustable lowers, so lengthening the uppers it is...

Can I disconnect the upper just by jacking up the body, or do I need to support the axle too?

thanks again,
Bil

BTW, you can see my new Jeep here.
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Unread 04-02-2005, 10:23 PM   #4
Jerry Bransford
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You can disconnect the control arms without jacking up the body or the Jeep itself. Just place your factory jack under the snout of the axle pumpkin where the pinion shaft exits to support it. Then I would unfasten both control arms and drop the pinion slightly down, maybe two degrees, then set the control arm lengths so they match the mounting holes again and bolt them back into place.

By the way... NO GUARANTEES your pinion angle is too high which is a common cause of acceleration vibrations, but there's a good chance lowering your pinion angle will take care of it.
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Unread 04-03-2005, 11:37 AM   #5
wherndon
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When you say, drop the pinion slightly down, wouldn't that in effect be shortening the upper control arms, rather than lengthening them?
I'm kinda confused here.
thx,
Bil
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Unread 04-04-2005, 02:44 PM   #6
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What type of driveshaft are you using? Is it the stocker or a double cardan shaft. If you are running the later, then it is likely you need to rotate the pinion down a little by shortening the uppers. The pinion will rise a little under hard acceleration.
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Unread 04-04-2005, 05:38 PM   #7
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I'm running a Fabtech double cardian, and an RE Superflex with 3.5" springs.
I haven't really tried any hard acceleration yet, want to get a few miles on the engine first. Only 220 miles on the odometer.
Mainly my vibes are at moderate acceleration, or just cruising to maintain a steady speed.
As soon as I let off the gas the vibes go away.
Regards,
Bil
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Unread 04-04-2005, 06:32 PM   #8
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I couldn't find info on the Fabtech double cardan. Does that kit get rid of the factory vibration dampener or keep it? Auto or standard tranny? Is the vibration a pulse or constant?

Sorry for all the questions. If I were you I would get an angle finder and find out what kind of angle the pinion is at right now. A good starting place is down 1-2 degrees on the pinion (in relation to the driveshaft), and fine tune it from there. If you can't dial it out with pinion angle changes, then there is a whole new list of things to try. Or maybe it is just a jeep thing.



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Unread 04-04-2005, 07:21 PM   #9
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thanks Fritz.
the DS did away with the vibration dampener.
Automatic trans.
the vibibration is sort of a pulse, Rrrrrrr.....Rrrrrrr....Rrrrrrr. ha ha, need audio on the forum...
Bil
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Unread 04-04-2005, 08:03 PM   #10
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Quote:
Originally Posted by wherndon
the vibibration is sort of a pulse, Rrrrrrr.....Rrrrrrr....Rrrrrrr. ha ha, need audio on the forum...
Bil
I think I have the same noise in my XJ, smaller lift though, just a BB,

I plan on putting in some 2* shims to correct this, but this thread has me confused, am I to roll the axle towards the rear of the jeep 2* or roll the axle forward 2*? I figured rolling the axle towards the rear 2* would work?

really what I want to know is, should i put the fat end of the shim to the front or the back of my leaf pack?
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Unread 04-05-2005, 09:07 AM   #11
Jerry Bransford
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Quote:
Originally Posted by wherndon
When you say, drop the pinion slightly down, wouldn't that in effect be shortening the upper control arms, rather than lengthening them?
I'm kinda confused here.
thx,
Bil
WHOOPS you're right, I meant to say that you need to shorten the upper arms to lower the pinion angle. You woulda figured it out and seen what I meant anyway.
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Unread 04-05-2005, 10:48 AM   #12
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cool, thanks.
I'll try and get to it this weekend, then post the results.
Bil
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Unread 04-05-2005, 11:52 AM   #13
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creature, it seems you would need to roll the axle fwd.
and I think this would mean installing the fat part of the shim toward the rear.
Bil
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Unread 04-05-2005, 12:00 PM   #14
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Quote:
Originally Posted by wherndon
creature, it seems you would need to roll the axle fwd.
and I think this would mean installing the fat part of the shim toward the rear.
Bil
Cool, I shall give it a try.
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Unread 04-05-2005, 12:21 PM   #15
Jerry Bransford
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Quote:
Originally Posted by wherndon
creature, it seems you would need to roll the axle fwd.
and I think this would mean installing the fat part of the shim toward the rear.
Bil
Shim? You're thinking about a leaf-spring vehicle like an XJ or CJ, TJs don't use axle shims to set pinion angles.
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