A different approach to analyzing cyclic vibrations at highway speeds

RaymondT

RaymondT

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Assuming you've checked all the simple stuff - tires, eng/trans mounts, ujoints, front hubs, etc, and ran it with each driveline removed. And the problem persists...

First off, I'll pass on a tip I recently got from a diff/trans shop we do business with. If you really want to narrow it down, pull the rear axles and carrier. Run it up to the problem speed. There's enough residual lubrication on the pinion bearings for a short test. Right away you'll know if the problem is in the rear axle or forward.

I'm starting to think that the rear axles could be the smoking gun in many of these highway speed vibration complaints. The spec is typically within .003" on axle straightness. You could have a bent flange or the axle is tweaked further in so you have to check both vertically (shaft) and horizontally (flange). These are not things you can eyeball. A machine shop with a lathe is the only way to definitively check the straightness. If it's a minor flange issue, sometimes they can turn it true although the studs would have to be removed.

I've been tinkering with this issue for far longer than most and there are so many variables that affect the severity of the vibration. I will update as I find out more.
 
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RaymondT said:
pull the rear axles and carrier. Run it up to the problem speed.
Click to expand...

I must really be missing something here.

The rear wheels are fastened to the rear axles. How on earth are you driving with no rear axles?

And if the missing part is bolting the axles back in with no carrier and driving at speed down the road, I can't see how you would not have crazy vibrations from the inner axles spinning in the diff with no support except the wheel bearings on the outer ends.

I'm just trying to understand how the results wold prove or disprove anything.
 
The only way I can think of would be to put the jeep on jack stands so the tires are off the ground. You'd have to verify that the jeep still has vibes while off the ground before pulling the axles and carrier. By doing that, you'll still be spinning the rear ds and pinion shaft.
 
Pretty sure where he is going with this if the Jeep is on stands with no axels and no carrier. The pinion is still in and the driveshafts, run it up to speed like this to see if it’s there, if not you know the issue is past the pinion
 
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As surmised by some, the vehicle would have to be on jackstands.

I also downloaded this free app on my Iphone.

https://www.mecanocaucho.com/en-US/

Placing it on the top of the center console today, it record one peak at the revolutions of the wheels and one (4X the amplitude) for the exact RPM of the driveshaft (tire RPM @ 60mph X axle ratio). This app is slick! The analysis only takes 10 seconds.

I'm still going to pull and check/true the axles as my measurements with a dial indicator has a marginal fail. The LH axle also has a slight leak despite a defect-free surface and Timken seals. Enough to slightly soil the shoes over the winter. Makes me think vertical movement. My vibration is barely noticeable at 50 but irritating at 62+
 
@RaymondT From what I have read and understood:

1 vibration per revolution of driveshaft (ie 1st order) = balance related vibration (ie something is running out of round or out of balance in any part of the drivetrain). Anything that is out of balance will not cause a 2nd or higher order vibration.

2 vibrations per revolution of the ds (2nd order) = pinion angle related or due to failing u-joints or improperly phased driveshaft

3 vibrations per revolution of the ds (3rd order) = failed or binding CV joints

You could also have engine and tire related vibrations adding on top of any of the above.
 
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Odd this came up.

I have a slight 67 mph vibe .

Then had a weird left rear low speed brake noise pop up.

Guess what ? I have a bent flange . Hmm.
 
I had the same 65-70 vibration. Had a leaking axle seal so suspected bearing and possible bent axle/flange. Replaced the axle shafts/bearings/seals and it was 80% better. Replaced the rear drive shaft u-joints and it eliminated the issue completely so mine was more than one problem. If you haven't hit something hard with your driveshaft and inspection shows no factory balancing weights knocked off (unlikely) its pretty doubtful that it's the driveshaft unless you installed the pinion shaft 180 degrees off when replacing u-joints
 
coherent said:
I had the same 65-70 vibration. Had a leaking axle seal so suspected bearing and possible bent axle/flange. Replaced the axle shafts/bearings/seals and it was 80% better. Replaced the rear drive shaft u-joints and it eliminated the issue completely so mine was more than one problem. If you haven't hit something hard with your driveshaft and inspection shows no factory balancing weights knocked off (unlikely) its pretty doubtful that it's the driveshaft unless you installed the pinion shaft 180 degrees off when replacing u-joints
Click to expand...
I can see any minor drivetrain tolerance issue doing this . I bet it doesn't take much on such a small, light vehicle.
 
Taken from my Axle shops web page. He did my regear to 5.38s 1ust got it back on the 26th.


The dana/spicer light duty differentials have a problem that’s not always easily fixed.
The carrier bearing on most dana/spicer diffs rides directly against the housing as opposed to some GM and Ford diffs that use a shim between the bearing and the housing. As the bearings wear, the race wears into the housing by spinning. This causes a groove in the housing and also causes the carrier to start to walk side to side, which leads to some sort of humming or grinding noise.
 
Update: Spent a day and a half on the rear axle. Pulled it apart, checking all the specs and painting up the teeth. The carrier came out too easy for my liking after I re-bearinged it a year or so ago. I did a number of tweaks that were probably not necessary but gave me peace of mind. I re-loctited the pinion nut and reinstalled 1/32 less rotation. Still took 160lbs but dropped my pinion rotational force down from 22 in lbs to 14 in lbs. I got to use my case spreader to re-shim the carrier bearings tight. Dropped the backlash from .0075 to .005 which brought the pattern a wee bit deeper where I wanted. Installed two new axle shafts, bearings, and seals. Cleaned up the little bit of diff oil on the one set of shoes and drums.

Had my share of monkey-***ing. I had to remove the track bar to use the case-spreader. Then I had to modify a T-55 socket by cutting it down because the new 2005-06 fuel tank skid has metal crowding the lower bolt where the old skid one did not.

Results: Not a bit of difference to the vibration. The first order vibration was/is the driveshaft but I thought I would rule out the axles being that they measured borderline. Just a reminder, my vibration is totally caused by the back but radiates up through to the front. I totally disabled the front by removing the front D/S and both axle shafts (leaving the front stub axles to keep the wheels attached). I have had both D/S's balanced but I still measured .008" runout on the rear D/S after it was installed. Could be that the yoke ears got tweaked from a previous u-joint install gone wrong. Might tackle that next. Maybe tweak the ears true and re-install the u-joints. Nothing to lose.

Can't rule out the body/engine as the cause. Might tackle the body mounts someday.
 
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