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

S.McArthur

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You have no idea sir! I'm going on 3 years and still can't get them completely dialed out, and my setup is mild by comparison.
I do feel guiltily whining, but...I'm more important because it's me and I'm special so everyone has to be aware of my feelings. (Inset sarcasm disclaimer).
I can't wrap my head around why it was damn near perfect for two 15 min trips and overnight it goes back to being all wacky. It was still present but only so faintly that I would notice it, I'm dropping PSI in tires to 25 and run it home, about 15 highway miles so I can run it up pretty god.
 

S.McArthur

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Here is where I’m at:
Thrust angle:-0.37 (I’ve been making 1/4”, some half ass math from the 3 data sheets says each 1/4” has been roughly .25*. I made another just over 1/4” adjustment to see where I’m at.
Front axle:
Pinion: 6*
Drive shaft: 11.3*
Caster: 7.8*

I’m thinking rotate the pinion up 2/3* which should DS/P within 2* of each other and set caster in the 5-5.5 range.

Sound theory?
 

S.McArthur

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Fixed that control arm as best I could, it wouldn’t fit right in that mount so I flipped them, the clevite bushing is in the frame side mount and the adjustable side on the axle mount. Whatever, if rocks destroy them...
Took front DS off, still had vibration. Set pinion to 2* low and feels like vibe is in passenger rear axle.
I’m done for the night but now I’m second guessing if the revolution axle shafts have a bad bearing or something else. What are the chances I got a bad shaft from them? I may have to put the stock shaft in, unfortunately I don’t have a good driver side shaft so they will be mixed.

Aston forgot, after I made a couple adjustments and went for test drive, I heard something clicking like a lunchbox locker except coming from TC or axle, hard to pinpoint whilst moving.
 

AndyG

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I had a bent left rear flange.


I’m in Huntsville . If it comes out down to tires , I willl be glad to bring my mine up on a Saturday and let you put my tires on and see what happens.

That control arm bushing may be an oem dodge 2500 pickup truck bushing , some rough country kits run that .
 
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S.McArthur

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I willl be glad to bring my mine up on a Saturday
Thank you for the offer, not that I wouldn't mind the company, but I wouldn't want to waste anyone time, maybe when I get a little further and desperate I'll take you up on that offer, I also have the 5x5 JK bolt pattern if that matters. I used to ride at a track down in huntsville called JB Motocross, I think it closed down though.
On to the update: Took it to firestone, thrust angle is 0.12! I actually got it in the green zone but I may take it in one turn on the control arm to get it closer to 0.0.
Front DS is removed, vibration is preset, I may set pinion back to 0/-1* and call that good. If my brain memory serves me correct, that was the best pinion setting.
I'm going to rotate the rubber, swap axle shaft and go from there.

I'm having some concerns about my transfer case as well, it's been leaking yet when I check the level it pours out, it is dirty, so a rebuild/fix the RTV seal is needed anyway.
 
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AndyG

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Thank you for the offer, not that I wouldn't mind the company, but I wouldn't want to waste anyone time, maybe when I get a little further and desperate I'll take you up on that offer, I also have the 5x5 JK bolt pattern if that matters. I used to ride at a track down in huntsville called JB Motocross, I think it closed down though.
On to the update: Took it to firestone, thrust angle is 0.12! I actually got it in the green zone but I may take it in one turn on the control arm to get it closer to 0.0.
Front DS is removed, vibration is preset, I may set pinion back to 0/-1* and call that good. If my brain memory serves me correct, that was the best pinion setting.
I'm going to rotate the rubber, swap axle shaft and go from there.

I'm having some concerns about my transfer case as well, it's been leaking yet when I check the level it pours out, it is dirty, so a rebuild/fix the RTV seal is needed anyway.
Awesome consider it a standing offer if I have the bolt pattern.

I remember JB Motocross... We've ridden at Sand mountain mx , but I'm more of a single track woods rider.

I ride almost 100% in Tennessee... You always know you're about to hit a hill in Tennessee.
 

S.McArthur

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Sand mountain mx
I rode with Adam Cianciarulo at Sand Mountain before...well...I wouldn't say "rode with", he was on his last year on 85's and was so damn fast, very cool kid to meet. After breaking my pelvis I stuck to GNCC racing and even then, I got tired of breaking things; I do miss riding though.
 

AndyG

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I rode with Adam Cianciarulo at Sand Mountain before...well...I wouldn't say "rode with", he was on his last year on 85's and was so damn fast, very cool kid to meet. After breaking my pelvis I stuck to GNCC racing and even then, I got tired of breaking things; I do miss riding though.
He's quick.

It will bang you up , no doubt.

I've hit half the trees in your state .
 

Fonz54

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Apr 9, 2018
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Monterey, CA, USA
A lot of us have vibration issues of some sort in our Jeeps. Vibration is primarily of two kinds, harmonic (i.e rhythmic, pulsating with a specific frequency) or non-harmonic (the various rattles and squeaks and non-rhythmic). There is a also "wobble", for example one coming from improperly balanced wheels (yes, that's a vibration you feel at specific speeds).

In this thread, I am going to be talking about harmonic vibrations. In our jeeps, there are two kinds of these. The common one that lot of people have experienced is driveline vibrations from improperly set pinion angles. Watch this video to understand why they happen and why precise setting of the angles and proper balance of the driveshafts are both very important.


The second kind, and the one that has tested the patience of many, many Jeep owners is the harmonic vibration in the tub that happens after re-gearing the axles to a different gear ratio. They check tire balance, driveshaft balance, pinion angles and everything is in order but the harmonic vibration is still persistent after a specific speed.My jeep currently has this problem after a recent regear at one of the best local so shops installed by a Jeep expert. I felt a gradual onset of vibrations and just past 60mph, it felt like the entire Jeep started to vibrate. I felt it in the pedals, steering wheel, seats, console armrest and heard a unpleasant droning sound. Since then, I have been doing a LOT of reading and thinking about this. I have a background in physics and I am approaching this problem from that perspective.

The most important thing to know about harmonic vibration is that all materials have a natural resonant frequency of vibration. Once they start moving near this frequency, vibrations will increase and at the precise natural resonant frequency, the vibration amplitude (ie the amount it moves) will be maximum. Think of guitar strings; all strings are the same length. Each string is of a different thickness (and therefore, mass) and they have a characteristic sound depending on that thickness/mass and length. There is a fundamental (or base) frequency of oscillation and what are called "higher order" oscillations that are numeric multiples of this fundamental frequency (that are higher in pitch compared to the base frequency).

Same applies to parts in our jeeps, like the frame. Yes, the frame will have a frequency and it is fixed. So will the transfer case. So will the skid plate and so on and so forth. Car manufacturers go to great lengths to make the critical engine/trans parts very precisely sized to get the performance and clearance they need. In addition, all engines have a harmonic balancer fitted to the free end of the crankshaft to counteract the resonance vibrations from the moving parts. Note that these are VERY precisely designed, made of a specific shape, size and material depending on what vibrations they are made to reduce.

Our Jeep Transfer cases also come with such harmonic balancers. Both the 231 TC and the 241 TC have these. Below are pics to show them visually (top two are 241 , last one is 231). Removing this balancer while installing a SYE or aftermarket driveshaft could be very risky thing to do, in my opinion. The factory went to great lengths to put it there and we risk vibrations without this component. This is a really important to think about. Better driveline angles and stronger driveshafts are important, but equally important is this balancer and no one seems to think about removing it!


View attachment 61221

View attachment 61212

View attachment 61216

In the last 4 weeks of reading an enormous number of vibration threads post regear on different forums, I have come across just two cases of people solving their vibration issues. One was a guy who found out that his driveshaft u-joints were installed out of phase (easy fix!) and the other guy's shop removed his harmonic balancer and the problem went away. That's it, just two cases out of many, many people who are just living with vibrations having tried whatever they could with driveshafts and wheels and whatever else they could think of.

You might be asking me how the vibrations went away after removing the balancer in that guy's case, or that why you don't have a problem in your jeep with or without the balancer. I will try to explain how. I believe that this is not a magic, unexplainable problem but that each Jeep being different, they each have a variety of things happening at the same time making the problem difficult to communicate and comprehend.

Note that the problem is indeed nuanced. First thing, the gear ratio matters since going to deeper gears you tend to see this problem sooner. Second thing, you may have other aftermarket mods that may be actually helping you. With deeper gears, now the driveshaft is moving at a frequency that is closer to the natural resonant frequency of the system. In our jeeps (specifcally 03-06), the TC skid has a resonant frequency that is already close to the normal driveline vibrations. So, the faster the driveshaft is spinning (with deeper gears), the more prone you are, to setting this resonant vibration in the system which you hear and feel inside the tub. See the comment from Jim Frens (prev owner of Nth degree and ex-Jeep engineer, very knowledgeable guy) on the topic of vibrations in jeeps on a different forum (emphasis mine):

"Vibration problems in ‘03+ Wranglers are mostly due to poor decisions that were made at Jeep several years back (when engineering the ‘03’s started). The stock center skid for ‘03+ TJ’s has a natural frequency that lines up with the normal powertrain vibrations, and so acts as a ‘speaker’ to make this energy very apparent in spite of the rubber isolators. The other problem is that your new 42RLE automatic is derived from the FWD transaxle in the Dodge Stratus/Cirrus cars, and unfortunately has their carryover gear ratios (way too wide with a horrifically tall OD ratio of .69:1). And guess what? The OD ratio sets up a harmonic with the third-order firing frequency of the I-6 engine because they are only 2% apart, so you have a powertrain that just loves to ‘sing’ – and the only part of it you can do anything about is the center skid, which is why it’s been learned that changing out the center skid does help a bit (removing the speaker by changing it’s natural frequency, since the natural frequency of the beefier skid is much higher – this applies to both the RELA skid and our Tummy Tucker). Vibration problems in a lifted vehicle are only about 25% dependent on the balance of the driveshaft itself and it’s operating angles, the rest is natural frequencies, etc. – unfortunately virtually none of the end-users w/ lifted TJ’s know anything about this and so get mad at whatever they think is the culprit – and in fact no one thing usually is – Jeep did some things that didn’t help, and the lifting certainly didn’t help, and incorrect set-up of the lifted suspension only adds to this mess, so there’s much more than meets the ‘eye’ (or in this case your *** feeling the vibes!)."

So how do you fix this? As Jim Frens says, You have to change the mass of the resonant system (since the frequency depends on the mass). You can perhaps do it by going a thicker, heavier skid as mentioned above (without installing a body lift). Note that Dave kishpaugh told me that he has dealt with this problem in the past on some stubborn jeeps by installing dampening material on the inside of the skid plate.

There is also another way. You can mess with the harmonic balancer, which is designed for a specific speed range; either take it away, reducing the previously correctly tuned weight, or by installing a heavier balancer of the right shape and size. In the case of the jeep owner who's vibrations went away, this is precisely what happened. They accidentally removed the balancer and tried it and found out that the vibrations were gone. He never investigated further as to why it went away after his saga, but he does mention it here. To my knowledge, no one has ever tried installing a different balancer. This component is easily made once you know what size and weight you need. One other simple way could be to add a bunch of wheel weights (the stick on kind) all around the periphery and see what happens.

Now, to further strengthen my arguement, here is a post from the very knowledgeable MrBlaine on a different forum (emphasis mine)

"The Rubicon 4-1 T-case does have the option of having a conventional yoke installed in place of the flange mount on the face of the harmonic balancer.

That is done typically for either when the conversion to a Double Cardan style driveshaft is installed and if the yoke is present, it will resemble the conventional SYE on other TJ t-cases since the Rubi case already has the SYE from the factory.

Some folks install the yoke style under the mistaken impression that they are better or that they will wind up with a longer rear driveshaft and neither is true. The 241 has an inherent need for the balancer most especially when certain gear ratios are in play. It is fairly common for those with the 241, 5:13's and the auto to have high levels of difficulty dialing out vibes that cyclical in nature.

Some Rubis I've messed with are obstinately particular about the driveshaft angles and the only way to know if they are correct is to make minor changes starting with the front one removed and keep dialing it up or down until you get it vibe free and then install the front shaft and do it all again.

The rear driveshaft is more forgiving when the angle is lower, the front is more forgiving when the angle is higher. Neither can be ascertained when you get one that is a PITA simply by looking at it and my helper and I set driveshaft angle at least once a week simply by looking at them and we are rarely wrong except when it comes to that small handful that are problems. When we get one of those, we break out the angle finder and we've found a few where the rear driveshaft angle was critical to be adjusted to within 1/2° which is not able to be done by eye"


You can see the flange mount on face of the harmonic balancer in the first couple photos I posted. I wonder if that is part of the harmonic balancer "system" in terms of weight. In my 05 TJR, I had gotten a double cardan driveshaft installed to get rid of the TC drop that the previous owner installed when he lifted the vehicle. I do not know if I have the flange mount currently (guessing not, and perhaps that changed the weight balance).

The reason I'm going to this much detail is to show how these changes all could interact and why one Jeep has a problem and another doesn't, although the Jeeps outwardly have same trans, axle gear ratio and lift. The devil is really in the details, and how your Jeep is set up.

Now, also note that you could be having driveshaft vibrations (recent lift, pinion angles etc) and/or wheel balance induced vibrations, on top of the resonant harmonic vibrations and it'll be very hard to decouple them. So everything needs to be checked and re-checked as Blaine says above.

I am going to try and have Dave check the harmonic balancer idea on my jeep. I am confident we will find a solution. I really think this is what is happening in our Jeeps w.r.t harmonic vibrations after a regear. It took me a while to assimilate and come up with this consolidated thought since information is pretty scattered on the forum threads.

I have been talking to @Chris a lot and he may have more thing to add that I may have missed. I invite your comments and discussion on this topic.

NEW FINDINGS 11/2/2018
  1. The harmonic dampener is added to help curb 55Hz frequencies (as told by the production engineer himself on another forum).
  2. From reading the FSMs for all the years, this is what I gathered:
    • 1997,1988,1999, 2000 jeeps - no balancer in autos or manual trans (they all had 231 TC)
    • 2001-2002 - damper only on AX5/NV3550 manual trans only, auto trans did not get it (they all had 231 TC).
    • 2003-2004 - damper on NV1500/NV3550 manual trans with 231 TC and 42RLE automatic trans with 241OR TC. No damper on 42RLE automatic trans with 231 TC.
    • 2005-2006 - damper on NSG370 manual trans with 231 TC and 42RLE automatic trans with 241OR TC. No damper on 42RLE automatic trans with 231 TC.
  3. All 2003-2006 jeeps have the same TC skid (Rubicon and non-Rubicon models)
  4. Forum members have confirmed that 32RH jeeps in 2000/2001/2002 do NOT have the balancer.
  5. FSM only mentions 4.0L models in any of the years they put the balancer on. 2.5Ls are not mentioned in any of the years.
View attachment 61320

Given this info, I postulate that there are 4 variables to be used in figuring out if your jeep has a balancer (and is therefore prone to resonant harmonic vibration). The combo of Year+engine+TC+trans+axle gearing tells you the information. I will post a good table inside the thread later. @Chris is helping me with it.


BIG UPDATE (6/11/2019)

My post re-gear vibrations are resolved. Yes! Finally!

Been working with a great local drivetrain specialty shop that's finally helped figure out the problem. There was no one root cause.

The on-vehicle balancing I posted a couple pages back helped. But not completely. The shop then replaced 2 items (front driver inner axle shaft and rear driveshaft yoke on the axle side), and adjusted pinion angles, followed by one more round of in-vehicle balancing. The vibes are completely gone as of this point.

I'm waiting for a detailed write-up from them and I'll post it here as soon as I get it.

Big thanks to everyone when has helped me and encouraged me and taught me. I owe you all a beer.

@JDRAM11 is going to try the same techniques at the same shop in a short while. We'll see if he has success as well.
Thanks for this great write up.
 

S.McArthur

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Update:
Pulled all tires off & rotate in an X
Set pinion/DS at 18/19ish
Changed diff oil for the 2nd time after re-gear. 500&1000k miles. It’s cheap and made me feel better.
Axle shafts look fine, I still don’t like my rotors not seating on the lugs tightly, but they are snug on the lugs.
Cussed at rough country for 10min, I can’t wait to rid this Jeep of their control arms.
What else...drank 3 beers (about a 3hr timeframe for the internet popo.

Torqued lug nuts to 100ftlbs, and went for test drive.

Nothing but maybe some cupping/tire vibe until ~70, vibration set in but it was constant and not harmonic (in/out). I’m calling that progress.

I’m wondering if maybe my transfer case or transmission are having an issue. TC has been leaking yet I haven’t had to refill it...very odd .
Transmission has had a slight slip(?) or shudder when taking off sometimes, there have been other small incidents but nothing I’ve really paid attention to.
The DeRale trans cooler I bought from a member here should be here Thursday, I’ll probably install that and drop the TC to fix the RTV leak on the main cases.

That’s all for now, had to make note of it before I forget. I have a better feeling tonight than I did last night, we shall see if the gremlins change how it rode overnight. Front driveshaft is still out until I can get the vibe to go away(ish).
 

AndyG

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You may find tires to be a lot of what you are dealing with . Maybe not all, but a good bit. I realize that comes from across state lines , so I may be way off.
 

glwood

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Update:
Pulled all tires off & rotate in an X
Set pinion/DS at 18/19ish
Changed diff oil for the 2nd time after re-gear. 500&1000k miles. It’s cheap and made me feel better.
Axle shafts look fine, I still don’t like my rotors not seating on the lugs tightly, but they are snug on the lugs.
Cussed at rough country for 10min, I can’t wait to rid this Jeep of their control arms.
What else...drank 3 beers (about a 3hr timeframe for the internet popo.
I'd be very curious to see what difference you'd see (if any) with quality control arms under there...
 

bobthetj03

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I found that tires contributed a lot to cyclic harmonic vibes more than anything.
 

S.McArthur

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Well, I don't understand why things change overnight, but the vibe is back, it seems to be under acceleration that it's worse, maybe I need to swap trans/motor mounts while I'm at it.

You may find tires to be a lot of what you are dealing with
tires contributed a lot to cyclic
I may have to suck it up and get a new set, as much as I do not want toss more money at it, I may have too.

quality control arms under there...
I'm looking at some now, more $$$...Savvy/Currie seem to be the same, I'll have to search the forum for comparisons. I'm ready to get the double adjustable ones, I'm tired of removing bolts to adjust.
 

S.McArthur

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Alright, I'm buying tires, new damn tires...BFG All Terrain T/A KO2 C1 Load Range $1200 installed for 4, I may splurge and get the 5th for the spare.
I can't find a cheaper good tire, I'll keep looking for something though.
 

S.McArthur

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Did a test, took wheels off to see if the vibe is in the tire, still had vibe, had ol lady get it to the vibration zone (hold the comments), and watched driveshaft look wobbly. At Drive without accelerating, it was clearly “hopping”. I watched transfer case and axle for any hop and saw none. I’m making a call to Adams now.
 

S.McArthur

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So, change 57, I put my old DS on, it’s kind of crap that’s why I replaced it, the TC is shaking a bit. So, I think I need to address that first before pointing at the DS.
My bad, it wasn’t shaking as obvious as when the Adams shaft was on it.
Where’s the best to get a transmission mount? Mopar or???
 

BuildBreakRepeat

Garrett Hoover
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So, change 57, I put my old DS on, it’s kind of crap that’s why I replaced it, the TC is shaking a bit. So, I think I need to address that first before pointing at the DS.
My bad, it wasn’t shaking as obvious as when the Adams shaft was on it.
Where’s the best to get a transmission mount? Mopar or???
I got a crown mount, and even though it was rubber it’s stiffer than my old one and I got more engine vibes. Maybe Mopar is a little more compliant