Another Vibe Thread...

99XJ, 8.25 w/ 4.56, JB SYE, TW Driveshaft, Stock (I think...) Shackles, OME CS034RB Leafs, 31x10.5r15 BFG ATs

I rebuilt this entire vehicle over the past year. I didn't drive it much before having it regeared but didn't have any noticeable vibes. Although, I didn't drive over 40mph much... Anyway, I originally had 4deg shims installed. I went from 3.55 to 4.56 and started having major vibes. I couldn't drive it over 45mph without feeling like it was going to explode. No front driveshaft installed...

Finally had time today to install some 6deg shims and measure. Something is way wrong... The vibes are still there, and as bad, but now start around 50mph. Measurements:

Driveshaft: 13.5deg
Flat Area on Rear of Diff Beside Cover: 79deg (I believe this face is perpendicular to the pinion, making the pinion angle 11deg)
Face of JB Tail Section on TCase: 86.5deg (angled slightly down from vertical)

So, there is 2.5deg between the driveshaft and the pinion. The diff bearings/seals were all done when it was regeared, 1500 miles ago. The driveshaft has 2000 miles on it now. I did put in new motor mounts (stock) and a new trans mount (stock) when I put this thing back together. No sag yet from them. From my past XJs, I can't understand why I need 6deg shims with such little lift. Anyway...

The only odd thing that I can see is the shackles keep leaning forward, like /. I thought they should be angled back like \. It's like the springs aren't long enough. Thoughts? Rebalance driveshaft? Tires? 8deg shims?!

I'm stumped.

-NA

Monday morning bump. I ordered 8deg shims last night - so the pinion angle will be .5deg low of the driveshaft angle. Hopefully this "fixes" it. If it does, I'll be cutting the shock mounts off and rewelding them, again.

Next, I'll have the tire balance checked. Then, rebalancing the driveshaft if there are still vibes.

Dude, search . I believe TW has the thing on his site. measure at the u joint on the pinion for the pinion angle.. then, post pics .

also, first and foremost , are your tires balanced, and recently , not last year ?


you want the rear pinion angle to be 0-3* below the pinion angle. so a 13.5 degree DS angle means you need a 10.5-13* pinion angle.

I shoot for 2-3* with soft leaves. your shackle angle shouldnt be inverted with a 3" lift, that is probably pointing your pinion way too high. You need to post pics. if you were running that setup you probably ruined the pinion u joint with axle wrap.

Id examine the pinion u joint. and dont just keep throwing shims at it, measure, see where you need to be.

Yeah, I've searched a million times, hoping for an answer that I've not thought of.

My tires were road force balanced last May, 2000 miles ago.

I put in 8deg shims in this morning. My pinion is the exact same angle as my driveshaft now, 15deg. This is with a full tank of gas, no rear bumper or spare tire. The stock shackles are vertical.

IMO, the next possible culprits are the tires (rebalance), driveshaft (rebalance), or something is wrong with the regear.

Thoughts?

ETA: My springs are CS033RB, not 34's...

have you pulled the front shaft to isolate that?

have you checked all u joints in the rear DS for wear?


Also, youd want the rear pinion 1-2 degrees below pinion

Yes, I have removed the front driveshaft. I'm leaving it off until I figure this out. All ujoints are tight and were well greased. The shaft has 1587 miles on it now. My spring pack is stiff, so I went with 0deg difference. In my experiences with past Jeeps, this works best when cruising at a constant speed and minimizes vibes at WOT or coasting. I had the pinion 2deg before this morning, no difference.

Put your front ds on the back. Test it oot. Let us know.

I had all 5 tires rebalanced this afternoon. I had them do a before (check with the current stick-on weights) and then strip them off and do a balance using regular old clip-on weights. The worst tire (the never used spare) was over an ounce off. They are all perfect now - and the vibes are as bad as before.

I will install the front shaft on the rear and report back. Good idea.

u joints can appear tight and can still be journalled perhaps. did you pull the caps on the pinion u joint to make sure. yah swap the rear to the front and see what happens.

also, how are the tranny mount and the motor mounts? x member bolts are tight?

pics of your setup?

Brand new, OEM motor and tranny mounts. All bolts on the mounts/crossmember are tight. I will examine the rear driveshaft more after I remove it.

ETA: I will post pics later today.

I went out this afternoon and swapped the "new" (2000 miles) Tom Wood shaft with the OEM front shaft - no difference in the vibes at all. Still WICKED above 52mph. You can feel them coming on around 42-45. I checked the TW shaft for play, ovalled ujoints, etc. - nada.

So far, I: went from 4deg to 6deg to 8deg shims, swapped driveshafts, checked both driveshafts for play in the ujoints/etc., currently running without a front shaft (have been for months), checked/retorqued all bolts on the rear axle/leafs/crossmember/tranny mount, rebalanced/rotated the tires, checked the tcase flanges for play, etc.

Keep in mind, this thing was PERFECT with the shaft 4deg low (with the 4deg shims) before the regear. Both the person who regeared and and I thought the vibrations were from the additional shaft speed when going from 3.55 to 4.56. This vibration was very evident immediately after the regear. Now, I'm convinced it is something in the diff now. According to the guy who did the gears (he had done quite a few), the only "odd" thing during the install was that he had to do a bit of grinding on the ring gear to get the crosspin installed (8.25). He said it would probably whine a bit b/c of this - which it does. The whine usually goes away once I let off the gas.

Here are a few pics for your viewing pleasure (as it currently sits). The only odd thing I see is the angle of the driver shackle. They are usually vertical - the rear is very light right now as the spare tire is not onboard, no bumper, tire carrier, skid, etc.

Pinion Angle (0deg difference the shaft/pinion):


Pinion/Driveshaft:


Driver Shackle:


Passenger Shackle:


Front Height:


Rear Height:


Overall Height:

Check the yoke on the 8.25 for up/down/left/right/in/out play. Maybe the yoke wasnt tightened down enough.

Jeep450r said:

Check the yoke on the 8.25 for up/down/left/right/in/out play. Maybe the yoke wasnt tightened down enough.

Click to expand...

I did that. I couldn't get any movement doing it by hand. It does rotate a bit, probably 1/4" (or slightly more).

Could be the transfer case output shaft is bent. Or output bearing going. Have you put it on jackstands and looked for any wobble? Should be able to see without having to put much speed.

I have 6 deg shims with about 5" lift in my 2000 so it's not unusual, but IMO if vibes are bad after a certain speed and only get worse it's likely something else. There's about +- 3deg tolerance back there.

Good luck.

notamos said:

I did that. I couldn't get any movement doing it by hand. It does rotate a bit, probably 1/4" (or slightly more).

Click to expand...

That much rotation is normal.

I would check the yoke at the T-case too for the same play as the 8.25. Id assume youve already checked that though.

When the vibes start, they get worse the faster you go. It isn't like harmonic vibes I've had on past Jeeps, they seem to have "spots" where they occur.

I will try it without driveshafts at all to see about the possibility of a bent output shaft. I doubt this as the JB SYE only had about 200 miles on it prior to the regear and all was fine. But, I will try anything at this point. There is no play in the front or rear output shafts. This tcase was completely rebuilt with a JB SYE, 6-pinion planetary and a wide chain kit. Everything tested perfect in it 2000 miles ago.

Try driving it with only a front driveshaft installed in 4wd. See if that makes any difference.

If you have a dial indicator check for lateral run out on your rear axle shafts too.

Have you measured the yoke to be sure it is not worn out? I don't remember the exact numbers, but I think my old 8.25 yoke was about 11 thousandths wider than the ujoint. The new yoke gave the joint a nice tight fit.

98stocker said:

Have you measured the yoke to be sure it is not worn out? I don't remember the exact numbers, but I think my old 8.25 yoke was about 11 thousandths wider than the ujoint. The new yoke gave the joint a nice tight fit.

Click to expand...

No, I haven't checked that. Boy, that would be too easy. When I pull the rear shaft to try Dutch's suggestion ^, I'll measure all 4 yokes.

-NA

3/7/2013 Update:

I removed the rear driveshaft (the oem front shaft) completely and moved it to the front. I drove it approximately 6 miles, taking it up to 65mph - SMOOTH as butter. No rear vibration at all.

Also, while I had the driveshafts off, I put it in drive so I could watch/listen to the transfer case. There is zero play in the front/rear tcase yokes and they spin perfect - no wabling, etc. I pulled/pushed on both yokes and the front/rear axle yokes (outward/inward) - no play.