vibes issue

A couple of years ago, I installed a RE 4.5" lift with JKS adjustable lowers. I had to set the caster pretty steep to get rid of vibes, like 15" on the lowers with stock 1/4" shims in the LCA buckets. Steering was awkward but it did get rid of the vibes, highway driving was very smooth.

I got some 32" tires on the jeep this winter. The tires rubbed the inner fender on sharp turns, so I bought some adjustable uppers and set all four links to 16" (LCA buckets still had the 1/4" shim), so basically it moved the axle forward by 1" (maybe some slight changes to geometry). The tires no longer rubbed but I picked up some vibes in the 55-65 MPH.

While I was chasing some noises, I stuck a video camera under the jeep and noticed that the pinion was above the driveshaft most of the time. I thought, well, I changed angles a bit, let me set the pinion angle again, so I measured a bit and extended the JKS LCAs by another 1/4". That puts the uppers at 16" and the lowers at 16.5" (including the shim). The driveshaft is 9-degree at rest, and the spreader mounts on the D30 are also at 9-degree angle, so it should be dead on. But I am still getting vibes 55-65 MPH and its different from what before, its a rapid vibration, a chatter, and it makes the whole jeep buzz and rattle, whereas the other vibe was more mellow.

I am thinking that I should go back to the steeper angle and get the tires rebalanced. Alternatively, I could try extending the LCAs a little more, since I suspect the driveshaft angle becomes shallower when I sit in the seat. At this point I am looking for some input.

Can you post some pictures? You were very thorough, but I'm a visual guy so it would help me understand it a little more angles wise.

At any point, have you removed the front drive shaft to confirm that is not causing any problems? Axle shaft joints good?

It might be the driveshaft. I was looking through some videos and noticed it has an apparent wobble. Look at the first and last few seconds of this https://youtu.be/6YGFTK7JP4s

I haven't removed it yet. Without a third shaft to replace with, all it would tell me is if the vibes are from the tires or the shaft, which would be useful, but it wouldn't tell me anything about the shaft or the pinion. I need to come up with a way to measure if the shaft is bent or out of round or out of balance

There is a chart that shows control arm length for any given lift. I don't have a link handy. I recall you had to get a pretty big lift before the uppers needed to be changed.

I measured against my stock 01 XJ and my lifted 97 XJ (2.5") which has adjustable control arms 5/16" longer per the chart, and both have the pinion at 2 degrees below driveshaft angle at rest.

The smoothest ride I have gotten out of my 97 XJ has been by throwing the factory original front driveshaft from my 01 XJ in it. That driveshaft has 332,000, miles give or take, and is factory original. The original front driveshaft in my 97 XJ runs smooth as a rear driveshaft in the 97 XJ. With more effort and rotating the yokes, I may be able to get one of the other driveshafts to run as smooth, but I haven't had the time to fuss with it.

I doubt you'll get it to "stand still" end to end as there is some leeway, but any wobbling should be minimal.

You may first try removing the front driveshaft and pull the slip end out pretty far and grease the heck out of it. The vacuum of greasing seems to pull the slip part back in. Purge all the old grease and see how it does. Hang an angle meter on it and see if the driveshaft is about 2 degrees lower than the pinion.

I am still wondering about this, because my front transfer output shaft has some slop in it. Even after changing the bearing - the slop is because the bearing does not fit perfectly tight in the front case half. But something about that factory original front shaft makes it ride like glass.

ehall said:

It might be the driveshaft. I was looking through some videos and noticed it has an apparent wobble. Look at the first and last few seconds of this https://youtu.be/6YGFTK7JP4s

I haven't removed it yet. Without a third shaft to replace with, all it would tell me is if the vibes are from the tires or the shaft, which would be useful, but it wouldn't tell me anything about the shaft or the pinion. I need to come up with a way to measure if the shaft is bent or out of round or out of balance

Click to expand...

That video tells most of the story- that ds is definitely wobbling and is almost certainly the source of your vibes.

I've had a mystery vibe in the front that turned out to be a worn slip-spline on the front drive shaft. Drove me nuts for weeks! Vibes on the freeway were so bad I couldn't even use my rear-view mirror. It would vib in 2WD on acceleration and deceleration BUT in 4wd-hi it would only vib on deceleration and was 100% GONE on acceleration.

I finally found it by crawling under and pushing and pulling up and down on the front drive shaft. There was visible play in the slip spline.

The upside (if there is an upside) is that in the process of hunting this down I replaced all the ujoints in the front drive train, new wheel bearing units, rebuilt control arm joints, re-balanced tires, etc. So once the drive shaft was finally rebuilt she rode smooth as glass. :doh:

I had some few minutes after dinner so I pulled the front driveshaft. Damn jeep drives like my Cadillac without it, smooth and quiet.

The driveshaft is in pretty good condition (other than the apparent warping). I got a spare front shaft from a part-out for my SYE, replaced the u-joints with greasable Spicers, and I grease everything on every oil change, so short story everything on the shaft is lubed and smooth. I also think the pinion yoke is good--I have other video of that and it isn't obviously wobbling, plus I was just in there (twice) for the D30 bearing and pinion seal replacements and any kind of damage would have been obvious then.

What's interesting is that the driveshaft and the front axle are both different from when I originally had to set caster super high to cancel vibes in the first place. No vibes without the shaft though. Weird.

50/50 chance to rain tomorrow. If I get lucky I'll move the rear driveshaft to the front and drive around in FWD for a bit.

Did you have it regeared? Change a pinion seal?

I had a similar problem - but it was actually a problem that happened when a pinion seal was replaced. The spring within the seal popped loose and was trapped under the yoke.

https://www.youtube.com/watch?v=3G_JDpAkHwA

Just replaced the pinion seal a few weeks ago, I'll check it but I'm sure its not the problem. I don't see any movement in the yoke

Well I moved the rear driveshaft up to the front and no vibes at all. So I guess its confirmed, the shaft is bent. I will try putting the bent one on the rear and see if it vibes at all, then go on the hunt.

Really should have tried the obvious thing first, I guess.

Thanks for the comments.

postscript: The shaft seems to have thrown a weight at some point



Might be salvagable. Otherwise I'll get another

Do you have a local driveline shop that is good? Start with having it checked out/balanced. I would start with that, then pick up a spare.