Have you always had it or did it appear after some modifications? I'm guessing you have had the usual suspects checked already. FWIW, Dave recently got a bone stock 42RLE LJ that he told me had some mild harmonic vibration as well (he hasn't investigated).
Harmonic vibrations in Jeeps: A new theory (please read!)
- Thread starter psrivats
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Have you always had it or did it appear after some modifications? I'm guessing you have had the usual suspects checked already. FWIW, Dave recently got a bone stock 42RLE LJ that he told me had some mild harmonic vibration as well (he hasn't investigated).
My bad, you are correct. Resonate frequency vibrations was what I was thinking...
Another, related, avenue to explore.
At the resonant frequency, the amplitude is a maximum. So if the wavelength is long enough, it'll feel constant (eg an ultra low frequency sound that does not seem to subside from a giant subwoofer for long periods and appears to be a drone sound, but will modulate if you wait looooooooooong enough like in dubstep). In that sense, what you are saying is correct.
There is so much to these vibrations. People both percieve them and describe them differently as well and it's hard to pinpoint what type it is without actually experiencing if for yourself. Scientific terminology is very useful in these cases. "Vibration" only takes you so far!
In my Jeep, it definitely felt like a harmonic vibration that gradually became resonant (ie built up and got to a maximum). I drove it and listened very carefully to try understand what I was hearing and feeling.
FWIW, death wobble is also a vibration phenomenon but I'm not sure if it's resonant. It's definitely not a sympathetic onset vibration, meaning it does not start vibrating like a wineglass starting to vibrate and eventually break from a opera singer singing. Death wobble, from my understanding, builds as a sum of many simultaneous small oscillations (from worn out parts) that builds to a crescendo and eventually gets out of control chaotically and rapidly. More like in an orchestra where every instrument builds up to play a high energy piece of music but timing slowly gets more and more off between the instruments, and nothing is in sync at the end and we have total chaos. It's a control system rapidly getting out of control to put it scientifically. The regear vibrations are different.
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I also have another question for more experienced members. Did the 97-02 TJs come with a balancer on the 231 TC or not? @Chris do you know? That'll give us another clue in the right direction.
Is it only the 03-06 Jeeps that are more prone to vibration, and were 97-02 Jeeps not as problematic after a regear? If latter, did these earlier Jeeps not have these harmonic vibration issues due to the TC skid design being different, and the harmonic balancer, if present, was sufficient even after a regear? I am very curious to know. I'll be doing some digging to see if I can answer these questions.
@StG58 since your Jeep is older, could you take a peek for us? Not sure if you have made any changes to the stock rear driveshaft setup.
Is it only the 03-06 Jeeps that are more prone to vibration, and were 97-02 Jeeps not as problematic after a regear? If latter, did these earlier Jeeps not have these harmonic vibration issues due to the TC skid design being different, and the harmonic balancer, if present, was sufficient even after a regear? I am very curious to know. I'll be doing some digging to see if I can answer these questions.
@StG58 since your Jeep is older, could you take a peek for us? Not sure if you have made any changes to the stock rear driveshaft setup.
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I'm so glad someone (@StG58 ) mentioned the Tacoma Narrows Bridge. Its still taught as a lesson in Mechanical engineering (At least when I was in school in the late 90's). You cannot talk about resonant frequency and natural frequencies without thinking of that particular lesson (at least I can't).
I haven't done much research into the issue, but it seems to be predominately Rubicons with Auto transmissions, right? @bobthetj03 has vibs too, but he has a sport, manual, 231 T-case. The theory of the skidplate acting as a "speaker" is interesting. Would be an easy theory to prove/disprove though. Just fill it with sand and go for a drive. See if the vibration changes. Increasing the mass dramatically should move the natural frequency around.
To me, this is kind of like a fire. You need three "legs" to keep a fire going. Fuel, Oxygen, and Heat. If you remove one of the legs, the fire goes out. If, as @psrivats is saying, you have a cumulative effect happening, can you remove one of the legs and control the resonance?
It sounds like it to me. Will be interesting to note if Chris, with his Tummy Tuck and dramatically different from stock suspension experiences Vibs? If he does, that tells me its 100% drivetrain related.
I haven't done much research into the issue, but it seems to be predominately Rubicons with Auto transmissions, right? @bobthetj03 has vibs too, but he has a sport, manual, 231 T-case. The theory of the skidplate acting as a "speaker" is interesting. Would be an easy theory to prove/disprove though. Just fill it with sand and go for a drive. See if the vibration changes. Increasing the mass dramatically should move the natural frequency around.
To me, this is kind of like a fire. You need three "legs" to keep a fire going. Fuel, Oxygen, and Heat. If you remove one of the legs, the fire goes out. If, as @psrivats is saying, you have a cumulative effect happening, can you remove one of the legs and control the resonance?
It sounds like it to me. Will be interesting to note if Chris, with his Tummy Tuck and dramatically different from stock suspension experiences Vibs? If he does, that tells me its 100% drivetrain related.
I had the same exact experience with my setup when on 33s. Just over 75 I could begin to hear/feel a vibration. It was very subtle and I doubt anyone else would feel it. I havent noticed it since the 35s went on but thats only because I am dealing with another vibe that overpowers the previous one. I am pretty certain both my front axle shaft ujoints need replaced and the unbalanced MTRs are covering it up.
I remember MZ having a similar cyclical vibe with his 42rle/33s/4.88 at higher speeds. I don't think he ever got rid of it.
I had come across a thread on JF not to long ago where someone asked him if he ever resolved the issue. His response was "no, and its been over 20k miles so I have learned to live with it."
My 2000 TJ with 5spd and 231 had a harmonic balancer on the output shaft prior to installing my SYE, same as the one you pictured in your first post. All the TJ 231s I've personally seen have had the balancer, yet I can't recall seeing one on my buddies XJ when I was under there last.
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This is interesting too...XJ's use the AW4 transmission vs the 42rle. Wonder if that has anything to do with it?
I've said this before and I'll say it again... it would be nice to see if a transmission shop could rebuild the 42RLE with a less extreme OD gear. Anything is possible, so I'm sure someone could do it. It would just be interesting to hear how much it would cost and what they would suggest. I'm talking of course about someone that specializes in rebuilding transmissions.
In my buddies case he has an AX15, but I wonder if most AW4 XJs have one too? Not sure, but this could be an example of this being the right path to go down, obviously Jeep wouldn't include a balancer yet omit it on other models for no reason.
In my particular case, with my setup (5speed, 231 TC, 4.56 gears) removing the harmonic balancer on the TC did not change anything. Adding a heavier aftermarket skid also didn't change things. Removing the front driveshaft did improve the vibes quite a bit, but there is still a harmonic at 55mph, and again above 63mph. It's no longer constant, but still there. My tires aren't helping either. they also are creating a resonate frequency beyond the other vibes due to them reaching their life cycle and are quite loud. sadly, there is no way of reattaching the harmonic balancer after installing the SYE to prove that theory. Adding sand bags to the TC skid might be an interesting experiment I may just try this weekend.
Here is my 231 with the harmonic balancer:
And after the SYE.
There is just no where to attach a harmonic balancer on a 231 after doing this mod.
Here is my 231 with the harmonic balancer:
And after the SYE.
There is just no where to attach a harmonic balancer on a 231 after doing this mod.
I have the old balancer in my stash. I can measure diameter and width, but I don't have a scale handy to weigh it. It is actually pretty heavy.
See, it seems odd to me that they would have you remove the harmonic balancer. I mean why would you just toss something that was clearly put there for a reason?
I’m no engineer, but they did put it there for a reason. Makes me wonder what you might experience if you were to put it back on. Though that would be a lot of work.
Interesting. I didn't know there is a rubber isolator in the 231 balancer. I don't recall seeing one on the 241. The balancer is the mounting flange with the drive shaft directly attached to it.
Very interesting @ElectricWizard. Great that you thought to think about the XJ and pointing out it did not have one (likely because it was not needed). I just googled this and XJs do look different and sure don't seem to have one. Do the below photos seem right to you since you have seen both with your own eyes?
TJ (from a YouTube video, you can see the balancer)
XJ
*that brown thing is just a metal cap
Most people regear because they put a lift, have bigger tires and feel the loss of power. Most people with a lift and 231 also have an SYE (ie no balancer). A good number of them also have vibrations post regear. See where I'm going?
All Jeeps are likely prone to this issue and 03-06 Jeeps, esp with the 241/42RLE, have it much worse. If you have adjusted something else that could have compensated, you may not feel it as much. That's my model, at least.
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@bobthetj03 excellent photos. Thanks a lot for sharing. The rubber part is interesting.
@Chris most (if not all) SYE kits have you remove the balancer from what I'm reading. Not sure why this is the case.
@Chris most (if not all) SYE kits have you remove the balancer from what I'm reading. Not sure why this is the case.
Yep that does look right, his particular XJ was setup like the 2nd photo you attached. I agree with them omitting it likely because it isn't needed, the XJ obviously being built much different from our TJs likely doesn't share those same harmonic issues.