Harmonic vibrations in Jeeps: A new theory (please read!)

I can try: :)
When switching to taller gears, the rotational "slop" in the front driveshaft may increase due to the smaller driveshaft gear (more backlash). Since the front driveshaft is not loaded (since it is not typically in 4WD at highway speeds), the front driveshaft rotates about in the slop (slapping back and forth) when your chugging along at highway speeds and could be causing high vibrations of the drivetrain.

Has anyone or can anyone with the issue try to use 4WD at the problematic speed to see if it goes away? Maybe even use a touch of e-brake to load up the front driveshaft if the issue does still appear?

I have a stock 4 cyl TJ with stock sized tires (multiple sets) with harmonic vibrations similar to many described in this thread although not too severe. Also, there is no harmonic balancer on the transfer case output like the 6 cyl's were built with. And since there's no lift kit, no oversize tires, and no suspension mods, it takes a lot of variables out of the equation. Everything between the bumpers is as close to new as you can get it. What I haven't extensively replaced, I've measured runout on the rest. The wheels, axles, driveshafts, rotors, drums are all true.

Some of the things I've done with no effect on the harmonic vibration issue;

  • new engine mounts, new trans mount twice - one stock OEM, one jobber (stiffer)
  • removed the front D/S, carrier, and front shafts. Just the stub axles through the front hubs.
  • 3 sets of tires, 2 sets of wheels, multiple tire re-balances
  • rebuilt both driveshafts all Spicer parts, then had them re-balanced by a driveline shop
  • shimmed the trans mount up 1/2", then removed it. Dropped the skid 3/4".
  • body mounts, Centric drums and rotors, new shocks.
  • new transmission, rebuilt the transfer case, new flywheel, clutch, harmonic balancer front.
What did help a little - replacing the upper rear control arms with softer Clevite bushings. And I also noticed a reduction by sitting in the back seat while the wife drove at highway speeds. This didn't change angles enough to make a difference but it may have reduced rear axle vertical movement (felt vibration). This keeps pointing me back to the rear differential. I re-bearing it and set the back-lash from .010" down to .005" a few summers back. So I made a case spreader a few weeks back and will dive into the Dana 35 to check the numbers and pattern the gears. Just have to wait another 4-6 weeks for it to warm up a bit more. With smaller tires and 410 ratio, my pinion is spinning quite fast at highway speeds just like some of the guys who've re-geared.

You may be on to something redirecting focus on to the axle gears.
 
I have a stock 4 cyl TJ with stock sized tires (multiple sets) with harmonic vibrations similar to many described in this thread although not too severe. Also, there is no harmonic balancer on the transfer case output like the 6 cyl's were built with. And since there's no lift kit, no oversize tires, and no suspension mods, it takes a lot of variables out of the equation. Everything between the bumpers is as close to new as you can get it. What I haven't extensively replaced, I've measured runout on the rest. The wheels, axles, driveshafts, rotors, drums are all true.

Some of the things I've done with no effect on the harmonic vibration issue;

  • new engine mounts, new trans mount twice - one stock OEM, one jobber (stiffer)
  • removed the front D/S, carrier, and front shafts. Just the stub axles through the front hubs.
  • 3 sets of tires, 2 sets of wheels, multiple tire re-balances
  • rebuilt both driveshafts all Spicer parts, then had them re-balanced by a driveline shop
  • shimmed the trans mount up 1/2", then removed it. Dropped the skid 3/4".
  • body mounts, Centric drums and rotors, new shocks.
  • new transmission, rebuilt the transfer case, new flywheel, clutch, harmonic balancer front.
What did help a little - replacing the upper rear control arms with softer Clevite bushings. And I also noticed a reduction by sitting in the back seat while the wife drove at highway speeds. This didn't change angles enough to make a difference but it may have reduced rear axle vertical movement (felt vibration). This keeps pointing me back to the rear differential. I re-bearing it and set the back-lash from .010" down to .005" a few summers back. So I made a case spreader a few weeks back and will dive into the Dana 35 to check the numbers and pattern the gears. Just have to wait another 4-6 weeks for it to warm up a bit more. With smaller tires and 410 ratio, my pinion is spinning quite fast at highway speeds just like some of the guys who've re-geared.

You may be on to something redirecting focus on to the axle gears.
Wow, that is a long chase! Thanks for sharing. Since the rear differential (pinion/ring gear/driveshaft) would normally be loaded by engine torque, I'm not sure the "unloaded driveshaft slop" theory would apply. I know you said its stock, but have you checked output shaft and pinion angles to ensure they are parallel within 1 degrees? Looks like you tried to adjust some angles by shimming the trans mount and dropping the skid with no results, which probably changed that relationship.

Interesting that there is no TC harmonic balancer....

Here are some other things to consider:

https://www.machineservice.com/technical-101/vibrational-issues/
I have also read posts of people having driveline vibes that rotated the driveshaft about the yoke 180 degrees and that solved the problem- very strange.
 
I'd be glad to do this, but It'll be next week. My mud terrains are so worn down that I can barely hear myself think from the noise, so new KO2's are on tap for next week. Also curious to see what difference the tires might make on the harmonic, and if the tires played a role.
 
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Wow, that is a long chase! Thanks for sharing. Since the rear differential (pinion/ring gear/driveshaft) would normally be loaded by engine torque, I'm not sure the "unloaded driveshaft slop" theory would apply. I know you said its stock, but have you checked output shaft and pinion angles to ensure they are parallel within 1 degrees? Looks like you tried to adjust some angles by shimming the trans mount and dropping the skid with no results, which probably changed that relationship.

Interesting that there is no TC harmonic balancer....

Here are some other things to consider:

https://www.machineservice.com/technical-101/vibrational-issues/
I have also read posts of people having driveline vibes that rotated the driveshaft about the yoke 180 degrees and that solved the problem- very strange.

Oh, I'm way beyond that. D/S angle .3 degree. Rotated rear d/s 180, tried indexing worm gear clamps on the D/S to "fine tune", replaced input yoke-crush sleeve, nut and precisely set pinion nut, NV231J output shaft has zero runout, axle shafts true, spiders replaced, etc. Reattaching the front D/S changes the vibes somewhat as does putting it in 4WD.

At 50mph, my driveshaft is spinning at 2490rpm and there's almost no detectable vibration. At 55 = 2748rpm, starting to feel it. At 62mph = 3086rpm and it's pretty much peaked. It's the 3000-3100rpm on the driveshaft where problems start. Most guys running larger tires and deeper R&P are getting vibes at these same driveshaft speeds.

Tire pressure can alter the vibrations as can temperature. If I take off in the morning with almost no vibration, it will start to come on after 5-8 minutes of driving at highway speeds. There is no way the steel tub or frame has changed temp that fast so I have to assume the oil temperature in the gears are a clue. When I drive home in the afternoon after the sun has warmed things, I notice more vibration sooner. Altering tire pressure from 24 to 34 lbs doesn't help the harmonic vibration. Coasting down a hill clutch in, neutral, or even engine shut off doesn't get rid of the vibration.

How many threads have we seen so far about vibration after re-gearing? Once you sort out lift kits, tire balance problems, worn out suspension and steering components, driveline angle issues after modifying the vehicle, the rear axle has to be considered.
 
I now refer to myself as beach-vibes online. I also have a 6 pack.
 
Tire pressure can alter the vibrations as can temperature. If I take off in the morning with almost no vibration, it will start to come on after 5-8 minutes of driving at highway speeds. There is no way the steel tub or frame has changed temp that fast so I have to assume the oil temperature in the gears are a clue. When I drive home in the afternoon after the sun has warmed things, I notice more vibration sooner. Altering tire pressure from 24 to 34 lbs doesn't help the harmonic vibration. Coasting down a hill clutch in, neutral, or even engine shut off doesn't get rid of the vibration.
It is interesting that you see temperature affecting it that much. I would think that elevated temperatures after driving would affect tire temperatures and stiffness more than the stiffness properties of steel driveline/differential components.
 
It is interesting that you see temperature affecting it that much. I would think that elevated temperatures after driving would affect tire temperatures and stiffness more than the stiffness properties of steel driveline/differential components.

I was thinking of the oil. I'm almost tempted to pull a quart out and replace it with Lucas Oil Stabilizer for diagnosis.
 
Here's another data point for you vibration sleuths. I have an '05 LJ Rubicon. It has a 2.5" lift, Savvy TT, 1" MML, 1.25" BL. Adam's rear DS with double cardan and properly set pinion angle. Up front is a basically new Tom Woods DS. It's sitting on 33" BFG ATs. Regeared to 4:56. With 4:56 gearing and 33" tires, DS speed is really close to stock.

Until just recently, I had the factory hardtop installed which was on it when I bought the Jeep a year and a half ago.

I would have noticeable cyclical harmonic vibrations starting around 70-ish MPH. About 75 was the limit for keeping the vibes tolerable. Then I installed a soft top.....

Vibes are virtually gone! I drove it to work this morning, 20 miles on the interstate, cruise set at 75. No noticeable vibes. If I put my hand on the rear view mirror, I can still feel cyclical harmonic vibes just like before but they are so muted that the mirror no longer blurs and it's virtually vibration-free at 75. Going home, I'll try going a bit faster and see what happens.

I also noticed when I removed the hardtop that some (3?) of the nuts were missing. All of the bolts were there but some were not actually doing anything.
 
Here's another data point for you vibration sleuths. I have an '05 LJ Rubicon. It has a 2.5" lift, Savvy TT, 1" MML, 1.25" BL. Adam's rear DS with double cardan and properly set pinion angle. Up front is a basically new Tom Woods DS. It's sitting on 33" BFG ATs. Regeared to 4:56. With 4:56 gearing and 33" tires, DS speed is really close to stock.

Until just recently, I had the factory hardtop installed which was on it when I bought the Jeep a year and a half ago.

I would have noticeable cyclical harmonic vibrations starting around 70-ish MPH. About 75 was the limit for keeping the vibes tolerable. Then I installed a soft top.....

Vibes are virtually gone! I drove it to work this morning, 20 miles on the interstate, cruise set at 75. No noticeable vibes. If I put my hand on the rear view mirror, I can still feel cyclical harmonic vibes just like before but they are so muted that the mirror no longer blurs and it's virtually vibration-free at 75. Going home, I'll try going a bit faster and see what happens.

I also noticed when I removed the hardtop that some (3?) of the nuts were missing. All of the bolts were there but some were not actually doing anything.

This would play into Psri's partial remedy of sound deadening the tub. How new are your tires, and what type are they?
 
This would play into Psri's partial remedy of sound deadening the tub. How new are your tires, and what type are they?

As stated, BFG ATs. They are pre-K02 but I would have to looks at the sidewall to answer accurately as to age. Good tread on them and quiet ride on the highway,
 
This would play into Psri's partial remedy of sound deadening the tub. How new are your tires, and what type are they?

The hardtop does make things worse. It makes everything echo and resonate. I will definitely install the hothead headliners once I have the jeep back in my own garage. Did you buy those?

ps: My name is just Sri (short for Srivatsan) :)
 
The hardtop does make things worse. It makes everything echo and resonate. I will definitely install the hothead headliners once I have the jeep back in my own garage. Did you buy those?

Actually, I do have the headliners installed. And the Jeep has full factory carpeting.
 
The hardtop does make things worse. It makes everything echo and resonate. I will definitely install the hothead headliners once I have the jeep back in my own garage. Did you buy those?

ps: My name is just Sri (short for Srivatsan) :)

Not yet. No headliner insulation yet.
 
2006 LJ Rubicon 42RLE
5:13gearing
Stock dampener
Adam's double cardan rear driveshaft
3.5 coil spring lift
Hard top

Cyclical drone vibration starting 50mph on up.
No drone before regearing.
Vibration is not present when the rear driveshaft is removed.

Subscribing to thread. Good information.
Would very much like to reduce/eliminate this cyclical droning. I would have installed the hub kit if my vibes would have gone away with the front ds out instead of the rear.

My next attempt to reduce will be replacing stock transmission skid.

I had used an app to measure accelerometer response about a year ago when adjusting driveshaft and pinion angles but cant locate the raw data. If memory serves,gravity accelerometer seemed to show the most displacement at lower frequencies.
 
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