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!
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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
- The harmonic dampener is added to help curb 55Hz frequencies (as told by the production engineer himself on another forum).
- 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.
- All 2003-2006 jeeps have the same TC skid (Rubicon and non-Rubicon models)
- Forum members have confirmed that 32RH jeeps in 2000/2001/2002 do NOT have the balancer.
- 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.
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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.
