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

When I rebuilt my TC trying to see if that was the issue I did go with the larger chain and the upgrades to the planetary gears. And also Tom Woods built two shafts for the front of my Jeep trying to solve the vibration. Ended up refunding me my money because it didn’t solve the problem

That's a good data point to have and answers @bobthetj03 @StG58 questions.

Did you ever do anything to the balancer or the TC skid? Hope you read the thread in it's entirety to get the context of the question.
 
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Yes I read through the entire thread. I didn’t have a balancer on mine. I did not replace the skid but I did take another skid I had and reinforced the center section with some square tube. Added a few sections at a time to see if it would reinforce the skid. Also bolted a vibration damper that is used on a Ford Escape frame rail if I remember correctly to the skid. I read that article before about the skid potentially causing the vibration so that’s when I started to try to change the frequency of the skid. Nothing I did changed it at all.

One thng I always wonder about is the 42rle has a slip on shaft that extends the tail shaft of the trans through the bolt on section of the housing that is added to a 4x4 trans. That shaft actually has quite a bit of movement in it. And it is inside a 5-6 in diameter hollow tube essentially. I can’t see how that DOESN’T cause some type of vibration.
 
Yes I read through the entire thread. I didn’t have a balancer on mine. I did not replace the skid but I did take another skid I had and reinforced the center section with some square tube. Added a few sections at a time to see if it would reinforce the skid. Also bolted a vibration damper that is used on a Ford Escape frame rail if I remember correctly to the skid. I read that article before about the skid potentially causing the vibration so that’s when I started to try to change the frequency of the skid. Nothing I did changed it at all.

One thng I always wonder about is the 42rle has a slip on shaft that extends the tail shaft of the trans through the bolt on section of the housing that is added to a 4x4 trans. That shaft actually has quite a bit of movement in it. And it is inside a 5-6 in diameter hollow tube essentially. I can’t see how that DOESN’T cause some type of vibration.

Thanks for sharing that tidbit about the skid. I knew you had tried a lot to curb those vibrations in your jeep but good to have the details spelled out. But sad to hear it did not work on your jeep, damn.

You lost me on the 42RLE internals ... I have no idea whatsoever (and I am not a mech engr, so it will be quite the effort for me to even begin to understand transmissions!)
 
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One thng I always wonder about is the 42rle has a slip on shaft that extends the tail shaft of the trans through the bolt on section of the housing that is added to a 4x4 trans. That shaft actually has quite a bit of movement in it. And it is inside a 5-6 in diameter hollow tube essentially. I can’t see how that DOESN’T cause some type of vibration.

Are you referring to the tail section that gets replaced when you put in a RubiCrawler that sits right against the front of the transfer case?
 
Are you referring to the tail section that gets replaced when you put in a RubiCrawler that sits right against the front of the transfer case?


Yes, that short section of shaft is just pushed on to the main shaft and is retained by a internal clip.

B5BEC310-20FA-4840-A166-B234D7AC62E0.jpeg
 
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Thought I'd look through the 2004 Parts List, see if that added any clarity. Here is mention of the damper for the 231 T-case. Its not shown on the diagram, and only listed as an option for the DDD transmission, which is the 5 speed HD transmission (NV3550 probably).

upload_2018-11-2_20-32-48.png


For the 241 Tcase, they don't list the balancer seperately. Its part of the yoke, and applies to all vehicles with the 241 (so all rubicons have a damper).

upload_2018-11-2_20-36-2.png
 
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Thought I'd look through the 2004 Parts List, see if that added any clarity. Here is mention of the damper for the 231 T-case. Its not shown on the diagram, and only listed as an option for the DDD transmission, which is the 5 speed HD transmission (NV3550 probably).

View attachment 61309

For the 241 Tcase, they don't list the balancer seperately. Its part of the yoke, and applies to all vehicles with the 241 (so all rubicons have a damper).

View attachment 61310

Thanks for checking! Much appreciated.

I see this diagram in the 2004 FSM (all FSMs 2001 onwards have this figure). Doesn't list the codes for the trans like in your screenshot from the parts list. Interesting that they list the part for only one of the manual trans. I would trust the parts list more, I suppose. More reading for me to do!



upload_2018-11-2_17-48-14.png
 
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Yes. No mention of it in the manuals with respect to autos. I really want someone with a 32RH from 2001-2002 to check and confirm there is no damper. If that's the case, then we can conlcude that all jeeps from 2003 had the damper and rest assured that Jim Frens is 100% accurate with the '03 onwards having the design change prone to vibrations.

He most likely is 100% right on this topic.

No damper for sure. And also no vibrations, mine is a 2001 32RH Auto with 4” lift. I just swapped to an SYE and still no vibrations.


Sent from my iPhone using Tapatalk
 
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No damper for sure. And also no vibrations, mine is a 2001 32RH Auto with 4” lift. I just swapped to an SYE and still no vibrations.


Sent from my iPhone using Tapatalk

2nd data point! Thanks!

Can you reply in my other thread here too, for documentation?
 
Anyone a member at linkedin and would send Jim Frens a message?

https://www.linkedin.com/in/jim-frens-497a5211

I requested to add him to my network just this afternoon (not holding my breath though). If he accepts my request, I'll point him to this thread and ask for his thoughts.

Would LOVE to pick his brains on this topic. He would surely know a lot about TJs. He was with Chrysler 1992-2001.
 
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Bravo! So the 03-06 TJ might be the automotive equivalent of the old Tacoma Narrows bridge.

Just to add an avenue to explore...not all harmonic vibrations pulsate. Also, harmonic balancers damp torsional vibrations.

I was thinking the exact same thing when I read that. Ole Galloping Gerdie
 
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Let me throw this in . Been wanting to mention this somewhere .

Prior to a long trip recently , my pinion nut had been left loose or under torqued during a seal change .

So I 'm driving down the interstate with no clue , as this thing works loose .

At around 65 mph , this weird pulsing vibe starts ...reminded me of the JAWS theme..waaawm........waaawm......real far apart too..like 4 seconds .

Anyway , this may not have anything to do with anything , but that loose yoke was sending some vibes via the drive shaft .

So , yes , I can definitely see anything on either end starting a vibe .

Great job "Priv", somewhere in all this we all stand to learn and benefit from your efforts.
 
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@AndyG that's the perfect description of a harmonic. I remember reading the fix on your thread. Thanks for your comments, I hope I get somewhere with this.
 
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2001 SE 4cyl, AX5, 4.10 ratio,231J, no output dampener. BONE STOCK! with harmonic vibrations. Mine isn't all that bad but I'd still like to reduce them further. My vibrations start faintly around 50mph and get quite noticeable over 60mph. Temperature and tire pressure do play a part. In the morning when everything is cold, I can drive the first 5 minutes with virtually no vibrations. If I were to describe them, I would say a high frequency overlaying a rhythmic pattern. The pattern would be 82 hz @ 62mph.

It would be difficult to describe everything I've done to this TJ without putting most of you to sleep. Suffice to say the entire suspension and drivetrain has been gone over. I've measured run-out on everything that rotates. I've had two different sets of tires of my own plus a good 3rd set from a Jeep shop. The current tires have been balanced 4 times by 3 different shops. The driveshafts were both balanced recently. A brand-new AX5 and the 231J rebuilt by myself with no affect.

Day before yesterday, I added a transverse stiffener to the skid. A 29" x 1.5" sq tubing with .125" wall thickness. Pretty stiff and unyielding. It runs through the side to side recess in the skid. It's attached to the two rearmost nuts for the trans mount using 3/4" access holes that I could fit a deep socket into. 4 - 3/8 X 2-1/2" grade 8 bolts/washers/nuts, 2 on each side, suck it tight. All this accomplished was to change the low freq pattern from vrrm-vrrm-vrrrm-pause-vrrm-vrrm-vrrm-pause (etc) to vrrrrrrrrrrrrrrrrrrrrm-pause-vrrrrrrrrrrrrrrrrrm (etc). I also changed my relatively new Mopar trans mount to a cheap aftermarket Anchor mount with a higher durometer (stiffer) which didn't make any difference.

Two sets of front hubs, Timken and SKF - no effect. All new ball joints, control rod bushings, tie rod ends, track bars, sway bar bushings - no effect. New Mopar motor mounts - nada... New rotors, drums, rebearing both diffs... nothing.

I pulled the front carrier and axle shafts, reinstalling the stub shafts and ran it as a 2WD just to eliminate that rotating mass from the equation. It didn't help.

Now, even though both driveshafts were balanced and zero runout detected on the transfer case o/p shaft, I may still play around with worm-gear clamps on the rear driveshaft. The other thing that nobody looks at is the rear suspension springs, upper and lower isolators. I may pull those for inspection. If they're wonky, they could allow the rear axle to wiggle/vibrate. If I ever have cause to pull the tranny again, I'll get the flywheel/pressure plate combo balanced. I'm running Goodyear Trailrunners 225/75R15's on perfectly true Grizzly mags. No lift! So vibrations can happen to any TJ.
 
Great detailed post @RaymondT. I came to post something else, but I simply had to reply to you. The fact that you were able to modulate and actually percieve the change in vibration frequency is a great observation.

Based on my research, your Jeep should have the damper per the manuals (see the FSM for your year). However, Like @ElectricWizard your Jeep could be an inbetweener Jeep and I strongly think you would benefit from having a damper from what I've seen. The fact that you are stock and still have vibrations is a big clue, and a bigger one is that you are able to change it with changes to the skid. You've done good work already and made it better by sharing it here.

The part is $139 brand new from Morris 4x4. You can try it if you don't mind experimenting. Or maybe perhaps borrow @bobthetj03's to test out if he's willing to send it out to you. I think @Chris will agree with what I'm saying. I'll understand if you are skeptical, but it looks like you've already checked everything else including the skid.

If you have questions please ask us here, we are collectively trying to figure it out. @StG58 lit the spark by posting a photo of his stock Jeep without the damper and it really got me going. Your post is lighting the next one.

What started as an idea in my head is slowly getting to be a strong argument. @Chris and I are still discussing this heavily between ourselves before I even post here, trying to poke holes in the model. We need data and proof of concept and your case is very strong to test it.

https://www.morris4x4center.com/damper-np231-transfer-case-5017819aa.html





And now what I actually came to post ...

This is a question for @Chris and @Fouledplugs. I dug up this discussion on a skid made by a company called TKD from @Chris' build thread. Hope you guys recall the discussion. I've somewhat convinced myself the either the skid or the damper or both may need to be changed to curb these post regear harmonic vibrations. We've been talking a lot about the damper, but the TKD skid is actually quite promising not only from clearence viewpoint, but also from vibration standpoint since the design is actually different and well thought out.

$400+shipping is not too bad either. Thoughts on trying a skid like this in the vibration context? Worse case I'll have a nicer skid.

https://wranglertjforum.com/threads...oring-and-family-fun.8537/page-20#post-137666
 
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As I have been running all the things I have tried through my head one of the last things I did prior to installation of the hub kit was re shimming the front gear set. Unfortunately I didn’t write down the exact numbers but at the time we were focusing on the front shaft rotation causing the issue when not under load. We reduced the backlash on the front gear set from .8 to .5-6. It did actually change the frequency of the vibration but it was still there and by the time I was done the hub kit was sitting on my front porch so I pulled it back apart and did the conversion. There may be something still with the tolerance on the gear setup.
 
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That is the answer! 35's on EVERYTHING!
I've been thinking about this statement as it relates to driveshaft speed. My cyclical vibration starts just before 75mph. I assume it is originating from the rear driveshaft.

These tire sizes are adjusted to match the actual rolling radius, not the sidewall. The rpms/speed match real life. The first is mine with 33s, followed by 35s and then a regear.

NV3550/30"/4.88=2992rpm @ 75mph (33s)
NV3550/32"/4.88=2805rpm @ 75mph (35s)
NV3550/32"/5.13=2949rpm @ 75mph (35s + regear)

Even with a 35s and a regear, mine would be running 75mph with ~43rpm fewer than today. That equates to a mile an hour increase in road speed until the vibration starts because the drive shaft is spinning less at the same speed.

I suppose the point here is that a larger tire (even with deeper gears) could actually improve a situation depending on how the math plays out.
 
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I have been reading a little bit about this too. I have a Mech Eng degree, and while I spend all my time in manufacturing and plastics now, a good engineering problem always gets the mind spinning. I read an article this morning about controlling vibrations in mechanical systems.

ARTICLE

Its a pretty nice primer on mechanical system vibration. We've been talking a lot about the effects of the vibrations. Generally speaking, when these types of vibrations happen, you blame driveline angle. However, in some cases, it seems that the pinion angles are good and vibrations still happen. What to do in that case? I think the first thing to figure out is what is actually causing the disturbance. There is a formula to calculate the frequency of "power pulses" from a 4 stroke engine.

Fd = (N x S)/ 2 x 60 where N is number of cylinders and S is the RPM.

I have a 2004 Rubicon with a 5 speed, 241 T-case, stock gears (4.11), 285/75R17 tires and the 4.0L.

The Formula for me at 60 is (using Grimm Jeeper to calculate RPM at road speed)

(6x1788)/120 = 89.4 Hz. This is the disturbing frequency. That is pretty close to the 55 Hz @psrivats was talking about with respect to the T-case dampner. If I were to do the same calculation on a factory tire...

(6x1985)/120 = 99.25 Hz which gets me further away from the 55 Hz...Unless that 55 Hz is designed to cancel on 2x Frequency. I gotta admit, I don't know much about harmonic balancers.

If I were to go to 4.88 gears with my current tire size...

(6X2123)/120 = 106.15 Hz...

What am I saying...I think there is something to this harmonic balancer thing. Deeper gearing will increase the RPM of the engine which in turn pushes the Disturbing Frequency higher. In @tworleys case...He had vibs, but going to 35s fixed it. Well, going to 35's lowered the Engine RPM with a corresponding decrease to the disturbing frequency.


I dunno though, random musings on a Saturday morning. Kinda feel like I'm creating a theory based on incomplete knowledge of the system at hand. In other words, Grasping at straws. I do know I want to regear my rig to 4.88 someday...and I don't want to fight vibrations, so I have a bit of vested interest.
 
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