Thernlund

Hello all. First post. I've lurked here for a pretty long time. Truly a wealth of information. I thank you all for that. This board has helped me answer a ton of questions just by browsing.

Now I have one I just can't get my mind around. Mental block maybe, I don't know. Maybe I'm an idiot. No matter how much I read I can't compute it. I'll just get right to the issue.

Specs
'99 XJ Sport
AX-15 Transmission
NP231J Transfer Case
G2 SYE
Custom made CV drive shaft
Rough Country 3" lift w/ new leaf packs

Preface
The rear diff was wasted, so I had a shop rebuild it. They were going to replace the drive shaft, so I figured screw it, if I need a new drive shaft anyway, just get a CV shaft made and have 'em do an SYE too. I also had them rebuild the (failing) transmission. All in I guess. Cha-ching.

Issue
I have what I think are driveline vibes at around 40mph+. I mean I guess they're vibes. I'm not experienced enough to know what that sort of thing is like. I hear what sounds like a "brrrrrrrr" at the top end of 3rd, and all of 4th and 5th gears. I can also feel it in the stick a little and in the pedals. But only under a load/acceleration. Just coasting produces no noise.

Near as I can tell it sounds like it's underneath and behind me. Hard to tell direction to a complete certainty though.

Diag
I took out the rear drive shaft and hit the highway in 4H. No more "brrrrrr". The whole thing seemed a little quieter actually. So that must be it. So assuming it was the drive shaft, I put an angle finder on it. See attached images. I calculate that as a 6° difference (81.5 - 75.5 = 6). That's gotta be the reason for (what I perceive as) vibrations, right? 6° seems like a lot even with an SYE, does it not?

Fix?
4° shims I'm thinking? That should make the angle difference ~2°. Yes/no? But... I think I'm too dumb to figure out the math on how much just raising the pinion will naturally change the DS angle. I heard someone say 1° for every 4° on the pinion, or something, but that doesn't sound right to me. Pff. I don't f'in know.

Any input on any of this would be appreciated.


(As an aside, when I had the rear drive shaft out I put the vehicle in 2H and ran through the gears sitting in the driveway. Seems fine up through 4th gear. 5th seems to rattle an awful lot though. You'd never hear it at all going down the road. But sitting in one spot it was apparent. Normal? I don't know. Comments?)


Thanks all.


-T.

4.3L XJ

If you are running an SYE and a CV drive shaft, the pinion angle should be 90° from the driveshaft. In other words, there should be no bend in the pinion U joint. It should be as close to a straight shot as possible.

Thernlund

Really? My understanding from reading around (here mostly) is that it should be about 2° lower to account for the axle twist under acceleration. No?


-T

4.3L XJ

Not really necessary. Most of the leaf spring kits people use in lift kits are fairly stiff and don't flex that much. Mine might, because it is so soft, but I have never had a problem with vibes.

aka"dude"

Drive shaft balance. Right after my SYE I ran a WJ front shaft in the rear that gave me terrible vibes around 45mph. I'd say take your front out and put it in the rear and see how that goes. Also check trans mount and motor mounts. If its still there and only under load you could still be having pinion bearing issues

Thernlund

@4.3L XJ Alright. Sounds reasonable. I checked under the vehicle and apparently the RC spring packs have shims already. I did some measuring and quick math and they seem to be about 3.5°. Stacking shims isn't safe, so I guess if I take that one out I'm going to need 9° shims. Ugh. Ridiculous. Biggest I've ever seen is 8° and I'm probably going have shock mount issues at that angle. What a pain.

@aka"dude" I could give that a try. But as mentioned, running without the rear shaft in 4H rids me of the noises. I'd think that would (mostly) rule out tyranny and motor mount issues. I guess it could be a shaft imbalance, but it's a brand new custom made shaft. I'd be absolutely shocked if they didn't balance it before delivery to the shop. As well, the transfer case and transmission were completely rebuilt. The pinion bearing is brand new. Again, I'd be shocked to learn it was bad. I'll check it out though.


-T

Thernlund

I'm actually looking into having the spring perches and shock mounts rewelded now. I don't know which path I'll take. But further input is still certainly appreciated. I'm open to any and all suggestions.


-T.

WyoCherokee

have the perches re-welded by a reputable shop. safest. best way to make sure the pinion is correct.

Thernlund

I took it to a local shop with a very good reputation to have it looked at. I have an appointment to have the perches and shock mounts rewelded. Turns out it really not that expensive (all things considered). We'll see how it pans out. I'll chime in with more as I it all unfolds.
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-T

Thernlund

I just wanted to circle back and update this...

I took it in to have the pinion angle adjusted. New high-clearance shock mounts from Zone Offroad and new Anti-Wrap Spring Perches from Barnes 4WD. I bought the parts myself and took them to a local shop to do the work.

Result
Smooth... as... glass! I had no idea how bad the vibes were until they weren't there anymore. The difference was astounding. I'm so jazzed that I decided to do this that I can hardly stand it. Anyone with drive line vibes, just call or email a local offroad shop and ask if they do this kind of work. If they do, just get it done. If done right, you definitely won't regret it.

And if you're in or around Phoenix and need this done, hit up Absolute Offroad. These guys were incredible. They finished the job in a single day. I dropped off in the AM, picked it up after work in the PM. Done. And it wasn't even that expensive. Nicest people you'll ever meet, incredible customer service, stellar work, more than reasonable prices. Not that I'm hard to impress or anything, but I am usually fairly reserved/stoic on most things. I was certainly impressed here though, no doubt.

***
NOW... I need some other damn thing in the tranny that I couldn't hear before due to the vibes in the rear. Ugh. Fortunately that's free as it'll be the problem of the transmission shop that rebuilt it to begin with, and, fortunately, they're happy to oblige.


-T.