The Official Jeep Wrangler TJ "Vibrations After Re-Gear" Thread

[Chris]

There needs to be a definitive thread for this issue in the TJ Resources section of this site. This issue is so common it seems, that I'd like a thread to be able to direct people towards so they don't have to search endlessly.

The Issue
After re-gearing some Jeep Wrangler TJs, you will find that there will be a "vibration" that wasn't there before. I'd like to start by pointing out that this vibration will be very different than the vibrations you'd feel from an improperly balance wheel / tire, or the vibration you'd feel from an incorrect pinion angle. This is what I would describe as a cyclical or harmonic vibration, that if measured on on Oscilloscope would have a very clear up and down pattern, similar to a heartbeat.

This vibration generally doesn't come on until higher speeds, though some members (such as @psrivats) have reported it at much lower speeds, which I can attest too personally (I rode in his Jeep, and it was terrible, even at low speeds).

The best way to test for this issue is to get on the highway and get going about 65 MPH. Turn the radio off and roll the windows up (make sure you have the top on as well). If the issue exists, you should be able to hear (and feel) a very distinct vibration that moves like a pulse, in-and-out.


Why does it happen?
I can't answer that. If someone has some actual science as to why this happens after a re-gear, I'd love to hear it. It's possible that the issue was there before the re-gear, but wasn't happening until much higher speeds (i.e. 90+ MPH), in which case no one would usually know the issue existed to begin with. Perhaps after the re-gear it just moves this vibration to a much lower speed.

Another thing that has crossed my mind is that maybe there are issues with the gears themselves, though others seem to have ruled this out (@bobthetj03 installed multiple sets of gears I believe, and they all had the same issue).

An interesting thing is that when you remove the front driveshaft on the TJs that do experience this issue, it seems to cure it. There are cases like @psrivats where removing the front driveshaft didn't make the vibration go away, but more often than not, it seems that removing the front driveshaft fixes the issue.

Either way, I'm still very curious as to why this happens.


Does it happen to all TJs?
No. I had both of my 2005 TJs re-geared (both were 42RLE transmissions as well). The first one was re-geared to 4.88 gears on 33" tires and had no issues after the re-gear whatsoever. I could take it up to 80 MPH easily without any vibration at all.

My second TJ I re-geared to 5.38 on 35" tires, and at around 60+ MPH, the vibration was so bad it was almost unbearable.

So the bottom line here is it's just luck of the draw. Some experience the vibration, others do not.


What's the fix?
The only sure fire way to fix this that I know of is to do what @toximus, @Mr. Bills, and many others have done, and install a manual locking hub conversion for the front. By unlocking the front hubs and taking the front driveshaft out of play entirely, this seems to completely cure the issue. The only downside to this is that if you do need to engage 4WD, you're going to have to get out of the vehicle and manually lock the front hubs.

I personally don't see that as being a big deal though, so this is what I would recommend to anyone who is experiencing this issue.

Another potential fix is to do what @psrivats did and take the vehicle to a shop that specializes in nothing but drivelines. Six States is the place that Sri took his, and they were able to dial out almost all of the vibrations. Actually, Sri thought they had gotten rid of them completely, but he reported a month or so later that they weren't in fact gone entirely, but had been greatly reduced.

He has been meaning to take it back to them as there is a very good chance they could dial out the rest of the vibrations, but he hasn't had the time


Resource Threads
Here's some different threads on this subject, all of which are loaded with great information:

https://wranglertjforum.com/threads...ations-after-re-gear-to-4-88-or-deeper.12218/
https://wranglertjforum.com/threads/harmonic-vibrations-in-jeeps-a-new-theory-please-read.15144/
https://wranglertjforum.com/threads/re-geared-now-i-have-a-vibration-above-50-mph.4235/
https://wranglertjforum.com/threads...ions-at-higher-speeds-after-re-gearing.32425/

Have anything to add to this thread? Please let me know. If it's good info, I'll add it to the original post.

 

[Alex01]

While I didn't experience an issue after my regear, after raising the belly, installing the atlas, and reattaching my reworked shafts I experienced a harmonic vibration above 60mph in the rear. After adjusting the pinion in 1/2 degree increments from -3 to +3, logging each change, and taking notes on the results (Double adjustable arms made this extremely easy to do) I found that -2 and +2 gave the best results but the vibration/harmonic was still present. I called Tom Woods, the company that made/reworked my drivelines, and they said it sounded like a driveline balancing issue. TW sent me a return label, free of charge, and rebalanced the rear shaft. At the same time, I had them upgrade my S spline to an X spline. Once I got the shaft back I reinstalled it in the exact position it left in, DS and Pinion were equal, and the vibration/harmonic was gone.

A thing to remember is after a regear the driveshaft(s) are moving at a much faster rate than they were previously. A minor imbalance on the DS could become a much larger issue now that it is turning so much faster. For example with my NV3350 and 4.88 gears if I am running at 2,500 RPM in 5th my speed is 73mph. If I was running on my old gear set, 4.10, at 2,500 RPM my speed would be 87mph. If I still had my original dana 30/35 combo in 3.07 at 2,500 RPM I would be cruising at 116mph. In short at 73mph, my shaft is moving at the same rate as it would have stock at 116 mph which is a 159% increase in rotational speed.

In my opinion, if you are getting a new vibration right after a regear you should check 3 things first.

1. Take a look at the yokes and make sure that the u-joints are properly seated. An improperly seated u-joint can cause vibrations and the shop may have missed the mistake during reinstallation.

2. Pull the Driveshafts and make sure the Yokes are clean. A dirty yoke can cause the u-joint not to seat correctly and cause vibrations.

3. Pull each shaft, at separate times, and figure out which one is causing the issue. (use 4 high when the rear is out) If the vibrations stop take that shaft to a driveshaft specialist to check the rebalance or order a new shaft from your preferred vendor. (The X shaft is a $20 upcharge from TW and I would highly recommend adding it if you go that route.)

*I was assuming the vehicle rolled into the shop in with zero vibrations and the tire balancing had been ruled out.
*RPM to speed calculations done at Grimmjeeper.

 

[someguysjeep]

when i called Woods after my 5.38 install about some harmonic vibrations the 1st thing they suggest is replace or re-balance that stock front drive shaft.

 

[Mike_H]

The front driveshaft is of relatively small diameter for it's length, due to packaging. Combine that with the higher speed it sees with a deep regear, such as when you have the 42rle, and I believe your getting close to critical speed for that shaft.

I believe that is why the front locking hub kit "cures" the issue. Not to mention the other benefits of a locking hub set (caster angle).

There are a couple of outliers to my theory though, @psrivats and @bobthetj03. Sri has the auto, but only 4.88 gears I believe. Jim has a manual... Which is about the only case of a manual I think that has been persistent.

 

[bobthetj03]

My particular case is a bit of a unicorn compared to many others who have experienced this. I can remove the front ds and eliminate the vibes. The biggest improvement I gained was when I lowered the t-case skid down 1". As a result of that change, I added a 1" MML to simulate the t-case drop, then put the t-case back up. I still have vibes above 65 mph. This is after multiple pinion angle changes, multiple new drive shafts, new control arm bushings, new trans mount, newish motor mounts, other stuff I'm sure I forgot about. Oh, and it's not engine RPM related. I can ramp it up to 70 on a down slope section of freeway, put it in neutral and coast and have no change in the vibes.
As the jeeps sits now, the drive shafts are at a 3" suspension lift angles. (3" SL, t-case 1" higher than stock, 1"MML).

 

[Chris]

My last TJ that I sold to @toximus had the vibrations bad after the re-gear.

I'm fortunate that this new TJ does not have them at all. Of course it is a manual, so maybe that has something to do with it. Sure seems like the 42RLE transmission plays a big part in this.

 

[1515art]

i went with 5.13 Yukon gear set on 33’s with my 42rle and I got lucky with my re-gear, I’ve been up to 70-75 on the freeway and other than the transmission Issues I can’t detect Anything from the diffs or drive shafts. I think vegas 4x4 did a good job on the gears and eliminating the cam bolts and installing all double adjustable control arms probably helped.

 

[bobthetj03]

Running a hard top seems to amplify the harmonics. I find that with the soft top, or no top, the added noise muffles some of it. Only when I approach 70mph do I start to feel it in my seat.

 

[freedom_in_4low]

After re-gearing some Jeep Wrangler TJs, you will find that there will be a "vibration" that wasn't there before. I'd like to start by pointing out that this vibration will be very different than the vibrations you'd feel from an improperly balance wheel / tire, or the vibration you'd feel from an incorrect pinion angle. This is what I would describe as a cyclical or harmonic vibration, that if measured on on Oscilloscope would have a very clear up and down pattern, similar to a heartbeat.

This vibration generally doesn't come on until higher speeds, though some members (such as @psrivats) have reported it at much lower speeds, which I can attest too personally (I rode in his Jeep, and it was terrible, even at low speeds).

The best way to test for this issue is to get on the highway and get going about 65 MPH. Turn the radio off and roll the windows up (make sure you have the top on as well). If the issue exists, you should be able to hear (and feel) a very distinct vibration that moves like a pulse, in-and-out.

Can somebody approximate the frequency of the harmonic pulse at a particular speed? Like say at 75mph, could you say how many peaks in intensity you're hearing each second, or a fraction or number of seconds between peaks? It's a slow week at work and I'm digging out some trigonometry and acoustics.

 

[Chris]

Can somebody approximate the frequency of the harmonic pulse at a particular speed? Like say at 75mph, could you say how many peaks in intensity you're hearing each second, or a fraction or number of seconds between peaks? It's a slow week at work and I'm digging out some trigonometry and acoustics.

I no longer have a vibrating TJ, but maybe someone who does can.

 

[joelachr]

Can somebody approximate the frequency of the harmonic pulse at a particular speed? Like say at 75mph, could you say how many peaks in intensity you're hearing each second, or a fraction or number of seconds between peaks? It's a slow week at work and I'm digging out some trigonometry and acoustics.

A rough guess for mine is 1-1.5 s peak-to-peak. I'm usually in the 65-70 mph speed range when I get that frequency.

I get some vibrations in the 45-50/55 range as well. It seems to be the same frequency as above but a reduced intensity.

Edit: mine is not re-geared, I still have the factory 4.10 Rubicon gears.

 

[anarce]

There are some free apps available on Android that might be useful for investigations. They might be on Apple too, but I have no idea.

This one uses the accelerometers in your phone to measure vibration and give you a plot of level vs. frequency. If you have harmonic vibrations, this might help you dial in their frequency.

This one does basically the same thing, but uses the microphone to capture airborne sound. It has a live display that you can pause to capture the spectrum at a moment in time and look for frequency spikes created by any pure tones or harmonics. This could be useful if you can hear certain tones when the vibes happen.

This is an interesting topic to me since 1) I just developed vibes after re-gearing and 2) I work in noise and acoustics. I'm a long way from an NVH engineer though. My work is all related to buildings and environmental noise and vibration, but the same physics and instrumentation applies.

 

[freedom_in_4low]

This is an interesting topic to me since 1) I just developed vibes after re-gearing and 2) I work in noise and acoustics. I'm a long way from an NVH engineer though. My work is all related to buildings and environmental noise and vibration, but the same physics and instrumentation applies.

nice. I'm a mechanical engineer in commercial HVAC manufacturing which is probably the major source of the NV you deal with in buildings. I'm not an NVH engineer either but I've picked up enough to be dangerous.

I've alluded to this in other threads on the topic but I'm almost certain what is going on here is the result of two wave sources that differ in frequency only slightly creating an interference/superposition pattern. The audible result of this pattern can be compared to an out-of-tune piano, or using the 5th fret on a guitar string to tune the string next to it. When the pitches are very far apart you simply hear both tones, but as they converge you begin to hear that harmonic pulse, then as the pitches get closer together the pulse slows down until it disappears and the strings vibrate in perfect unison.

Tom Wood has this to say about harmonics, which provides some more detail:

If you were to overlay a graph of each of these vibrations you would find some areas where the wave patterns are synchronous and other areas where they would not be (asynchronous). In the areas where the wave lengths are synchronous, the forces created will be the sum of the amplitudes. In the asynchronous areas the forces will be no greater than the strength of the greatest amplitude.

This overlaid graphing will create a regular pattern of increased (strength) vibrations where one could expect what I'll describe as a rhythmic & pulsating (harmonic) vibration. These will occur when the strength and frequency are in the range of the natural resonate frequency of the vehicle.

A visual example of the overlays he refers to:

So in this example, we have a 60Hz and a 70Hz wave, overlaid with their sum. What you actually hear would be described as the higher of the two frequencies, pulsing in intensity, which oscillates on a frequency equal to the difference between the source frequencies (10Hz in this example).

The things I've read here about when it happens, the configurations it's most likely to strike, and what fixes it should help point to some possible culprits. Keep in mind this is a lot of logical guesswork and more than anything I'm just throwing out some subject matter fundamentals that might help lead someone in the right direction to figuring this out. Even if manual hubs remains the easiest way to take care of it, my natural curiosity wants to know the cause.

Fixed (almost universally) by removing a driveshaft or stopping it using locking hubs pretty clearly points to the 2 sources being a driveshaft and something else that spins at the same rpm as the driveshaft. If it were something downstream of the pinion-to-ring gear interface, I think the speed difference between the axles required to create a harmonic pulse at the described frequency would be too dramatic. (think 1-1.5Hz out of 13Hz vs 67Hz - you'd have to have 32s on one axle and 35s on the other)

More likely on 42RLE-equipped TJs: A 42RLE properly geared for it's tire size runs about 400rpm higher driveshaft speed than a properly geared 5 or 6 speed. The speed alone will make it more sensitive to slight imbalances and u-joint operating angles, but since we're piling 2 waves on top of each other, the tolerated amplitude may really be half of what it normally should be for that speed to end with an acceptable result. A tremendous amount of calculation and testing goes into making sure noise and vibration aren't triggered within the range of rotating parts that could excite it, but they're doing it based on normally driven speeds and factory available configurations. Our desire for more power to climb grades and pass on the highway means that we don't stay within those limits when we re-gear, and very frequently explore driveshaft speeds outside of what the factory would have planned for. This is especially true with the 42RLE and it's borderline absurd OD ratio.

More likely with NP241OR: I can't think of a good reason the transfer case should play a role in this short of some significant imbalance on the front output shaft assembly which seems highly unlikely. However, the TJ's that came from the factory with this case also came with a Dana 44 front axle, meaning they always have the same tooth count in both axles. In a perfect world (that mythical frictionless plane in a vacuum they always talked about in physics class) the front and rear shaft should spin in unison like those perfectly tuned guitar strings, but in reality, any slight steering angle to correct for road crown, or a slight difference in rolling radius of the tires due to variances in inflation pressure or vehicle weight distribution is going to cause those driveshaft speeds to shift slightly off each other. A Dana 30 with 5.13 or higher will be in the same position, but a 4.88 or numerically lower ratio will have a slightly different tooth count than it's counterpart in the rear, which will create a larger difference in driveshaft speeds and may simply move the frequency of the wave pattern into a range that isn't perceptible or can't excite whatever component is responding to the more closely matching speeds. I don't know where it was, but I feel like I read a post where someone said the pulse went away or froze at a constant hum at whatever intensity it was at when the case was shifted into 4wd. A hall effect tachometer on each t-case output yoke would help explore this, but in theory I would think one could manipulate the pulse interval with tire pressure. A half inch decrease in rolling radius of one tire ought to make enough difference to notice.

First vs second order: This is where instrumentation and data acquisition would be particularly helpful, but if you know what a 60Hz tone sounds like you can probably tell by ear. If the source vibrations are around 60-70Hz they are first order (one "peak" per revolution) and will be likely related to driveshaft balance. If they lie more in the 120-140Hz range (which will sound like the same note as 60Hz but an octave higher in pitch) they are second order (2 peaks per rev) and would be more likely from the natural acceleration and deceleration that the u-joints cause as they move through their rotation. Again - operating angle and balance may be well within normal tolerances, but doubling them may not be. It may be that on affected TJ's, the balance and angles have to be so close as to be impossible to attain with available parts and equipment. Clearly a $1200 hub conversion is still the better choice if the alternative was maintaining driveshaft angles less than 5° from the TC output yoke or balancing at 7000rpm.

Resonance/Natural frequency: I'm a little skeptical of the natural frequency theory because it seems to me that the harmonic vibration would be tightly concentrated around a specific speed, rather than starting at a certain speed and continuing as the vehicle accelerated. I'm not super knowledgeable about natural frequencies however, so I can't rule out that that it isn't simply the combination of the 2 driveshafts but that the combination is then exciting something else at it's natural frequency. Natural frequency can be modified with mass and physical dimensions, so I would be curious whether things like an aftermarket TC skid or frame length (LJ vs TJ) seem to have any correlation with the issue. I'm also not sure about whether resonance can be triggered by this sort of interference/superposition wave or by multiples or divisors of the objects natural frequency.

So anyway, I just threw a bunch of stuff up on the wall. Interested in the feedback on what might stick.

 

[anarce]

nice. I'm a mechanical engineer in commercial HVAC manufacturing which is probably the major source of the NV you deal with in buildings. I'm not an NVH engineer either but I've picked up enough to be dangerous.

Thanks for making noisy stuff for me to work on.

Lots of good thoughts in there. I'll throw out another line of thought: what is creating the "pulses" that lead to the tone? Is it an imbalance in the driveshaft that causes 1 pulse per revolution? Is a pulse created by every tooth on the pinion or the transfer case output shaft? Are 2 pulses created each revolution from the U-joints accelerating and decelerating? Are all of the above happening to various degrees at different frequencies and summing/cancelling as you described?

(I'm sure you're familiar with some of this stuff already, but I'll try to explain in more detail for anyone reading this thread)

For example, with air handler fans you can get a strong low frequency tone at the blade pass frequency of the fan. This depends on the rpm and the number of blades. Let's say the fan is spinning at 2,000 rpm. That's 33.3 revs/second (we're changing to seconds because Hertz is just cycles per second - you actually see "cps" in old textbooks before they named the unit after Heinrich Hertz). Let's say the fan has 8 blades, we would multiply that by the revs/sec to get the blade pass frequency (e.g. 33.3 x 8 = 267 Hz) and we would expect to see a tone there and possibly harmonics at intervals of that frequency.

The same thing could apply to this situation. Let's say the driveshaft is spinning at 2,000 rpm just like that fan. Think about the things that could be creating those pulses and what frequencies they could generate.

Driveshaft imbalance (1x/rev): 33.3 Hz
U-joints (2x/rev): 66.6 Hz
Transfer case shaft (21 or 23 splines): 700 or 766 Hz
Pinion teeth (I think 41/8 is standard for 5.13 gears for example, so 8): 266 Hz
Transmission gear ratio might be a factor too? Do the vibes change if you keep the same speed and downshift?

All of these things are multiples of the base rpm of the driveshaft and should stay in phase with each other, but the amplitude of each one might vary based on other conditions, such as whether you are accelerating or coasting could affect the vibrations from the ring and pinion gears, changes in relative angles from suspension movement, etc. That's going to change the effective frequency created by the summation as you showed. And if something is slightly out of sync, such as the front and rear driveshafts or the axle shafts turning at different speeds, you could get a beat frequency like when tuning a guitar as you said. And then you have potential resonance conditions that can magnify things at certain frequencies.

I'm just doing a lot of speculating since I'm new to TJs and this forum, and I don't know how these vibes usually manifest. I was lucky that I never developed vibes with my XJ. My lift height was borderline, but I used a TC drop and never had to add an SYE. The vibes that I'm getting in my LJ after the re-gear don't have a single strong tone. It's more of a broadband vibration I would say, and the first thing I'm going to try is mounting up my new wheels and tires to see if that helps.

 

[freedom_in_4low]

Thanks for making noisy stuff for me to work on.

Lots of good thoughts in there. I'll throw out another line of thought: what is creating the "pulses" that lead to the tone? Is it an imbalance in the driveshaft that causes 1 pulse per revolution? Is a pulse created by every tooth on the pinion or the transfer case output shaft? Are 2 pulses created each revolution from the U-joints accelerating and decelerating? Are all of the above happening to various degrees at different frequencies and summing/cancelling as you described?

(I'm sure you're familiar with some of this stuff already, but I'll try to explain in more detail for anyone reading this thread)

For example, with air handler fans you can get a strong low frequency tone at the blade pass frequency of the fan. This depends on the rpm and the number of blades. Let's say the fan is spinning at 2,000 rpm. That's 33.3 revs/second (we're changing to seconds because Hertz is just cycles per second - you actually see "cps" in old textbooks before they named the unit after Heinrich Hertz). Let's say the fan has 8 blades, we would multiply that by the revs/sec to get the blade pass frequency (e.g. 33.3 x 8 = 267 Hz) and we would expect to see a tone there and possibly harmonics at intervals of that frequency.

The same thing could apply to this situation. Let's say the driveshaft is spinning at 2,000 rpm just like that fan. Think about the things that could be creating those pulses and what frequencies they could generate.

Driveshaft imbalance (1x/rev): 33.3 Hz
U-joints (2x/rev): 66.6 Hz
Transfer case shaft (21 or 23 splines): 700 or 766 Hz
Pinion teeth (I think 41/8 is standard for 5.13 gears for example, so 8): 266 Hz
Transmission gear ratio might be a factor too? Do the vibes change if you keep the same speed and downshift?

All of these things are multiples of the base rpm of the driveshaft and should stay in phase with each other, but the amplitude of each one might vary based on other conditions, such as whether you are accelerating or coasting could affect the vibrations from the ring and pinion gears, changes in relative angles from suspension movement, etc. That's going to change the effective frequency created by the summation as you showed. And if something is slightly out of sync, such as the front and rear driveshafts or the axle shafts turning at different speeds, you could get a beat frequency like when tuning a guitar as you said. And then you have potential resonance conditions that can magnify things at certain frequencies.

I'm just doing a lot of speculating since I'm new to TJs and this forum, and I don't know how these vibes usually manifest. I was lucky that I never developed vibes with my XJ. My lift height was borderline, but I used a TC drop and never had to add an SYE. The vibes that I'm getting in my LJ after the re-gear don't have a single strong tone. It's more of a broadband vibration I would say, and the first thing I'm going to try is mounting up my new wheels and tires to see if that helps.

All good info and commentary, and further emphasizes how complicated issues like this can be.

My primary focus has been chillers and refrigeration so for the most part my stuff is usually at least outside the building but a couple of 150 ton screw compressors running close to 5000rpm can set up a real racket. For the past year or so I've been doing refrigeration rack system controls and have spent a lot of time in a trailer-sized metal box with 8-10 3600rpm reciprocating compressors. I consider earplugs a critical piece of my PPE.

I wouldn't expect much noise from the splines on the transfer case shaft, but there is a planetary set and a chain that would be unloaded but still spinning in 2wd. I'm not sure how many teeth are on the planetary gears but it's enough that it would be a high frequency.
I haven't heard anybody mention that it changed based on what gear the transmission was in, but maybe they could chime in. My TJ is not an affected one, my main interest in this is that I soon will be in the market for an LJ which depending on what configuration I end up with could fall into a higher risk group than my AX15/NP231 Sahara.
There could be something to pinion teeth.

 

[bobthetj03]

My vibes are present no matter what gear I'm in, or no gear at all. I can ramp it up to 70mph going down hill put it in neutral, let out the clutch and there is no change. I'll be doing some more testing in a few weeks when I put the hard top back on for winter. Too noisy with the soft top to test.

 

[AMS417]

I picked up some harmonic vibes after going to 4.88. I have a 2005 Rubicon 6 speed on 33's. I found my front pinion -2 below the drive shaft and adjusted it to 0. Helped maybe 50% of the vibes, I still have a stock front shaft, that will be next for replacement. It's not unbearable, but its there. Also noticed a very small amount of play in the output side of the t-case to the front shaft. Might be time for a bearing in there, but no leaks.

 

[Offline246]

nice. I'm a mechanical engineer in commercial HVAC manufacturing which is probably the major source of the NV you deal with in buildings. I'm not an NVH engineer either but I've picked up enough to be dangerous.

I've alluded to this in other threads on the topic but I'm almost certain what is going on here is the result of two wave sources that differ in frequency only slightly creating an interference/superposition pattern. The audible result of this pattern can be compared to an out-of-tune piano, or using the 5th fret on a guitar string to tune the string next to it. When the pitches are very far apart you simply hear both tones, but as they converge you begin to hear that harmonic pulse, then as the pitches get closer together the pulse slows down until it disappears and the strings vibrate in perfect unison.

Tom Wood has this to say about harmonics, which provides some more detail:


A visual example of the overlays he refers to:
View attachment 194794

So in this example, we have a 60Hz and a 70Hz wave, overlaid with their sum. What you actually hear would be described as the higher of the two frequencies, pulsing in intensity, which oscillates on a frequency equal to the difference between the source frequencies (10Hz in this example).

View attachment 194795

The things I've read here about when it happens, the configurations it's most likely to strike, and what fixes it should help point to some possible culprits. Keep in mind this is a lot of logical guesswork and more than anything I'm just throwing out some subject matter fundamentals that might help lead someone in the right direction to figuring this out. Even if manual hubs remains the easiest way to take care of it, my natural curiosity wants to know the cause.

Fixed (almost universally) by removing a driveshaft or stopping it using locking hubs pretty clearly points to the 2 sources being a driveshaft and something else that spins at the same rpm as the driveshaft. If it were something downstream of the pinion-to-ring gear interface, I think the speed difference between the axles required to create a harmonic pulse at the described frequency would be too dramatic. (think 1-1.5Hz out of 13Hz vs 67Hz - you'd have to have 32s on one axle and 35s on the other)

More likely on 42RLE-equipped TJs: A 42RLE properly geared for it's tire size runs about 400rpm higher driveshaft speed than a properly geared 5 or 6 speed. The speed alone will make it more sensitive to slight imbalances and u-joint operating angles, but since we're piling 2 waves on top of each other, the tolerated amplitude may really be half of what it normally should be for that speed to end with an acceptable result. A tremendous amount of calculation and testing goes into making sure noise and vibration aren't triggered within the range of rotating parts that could excite it, but they're doing it based on normally driven speeds and factory available configurations. Our desire for more power to climb grades and pass on the highway means that we don't stay within those limits when we re-gear, and very frequently explore driveshaft speeds outside of what the factory would have planned for. This is especially true with the 42RLE and it's borderline absurd OD ratio.

More likely with NP241OR: I can't think of a good reason the transfer case should play a role in this short of some significant imbalance on the front output shaft assembly which seems highly unlikely. However, the TJ's that came from the factory with this case also came with a Dana 44 front axle, meaning they always have the same tooth count in both axles. In a perfect world (that mythical frictionless plane in a vacuum they always talked about in physics class) the front and rear shaft should spin in unison like those perfectly tuned guitar strings, but in reality, any slight steering angle to correct for road crown, or a slight difference in rolling radius of the tires due to variances in inflation pressure or vehicle weight distribution is going to cause those driveshaft speeds to shift slightly off each other. A Dana 30 with 5.13 or higher will be in the same position, but a 4.88 or numerically lower ratio will have a slightly different tooth count than it's counterpart in the rear, which will create a larger difference in driveshaft speeds and may simply move the frequency of the wave pattern into a range that isn't perceptible or can't excite whatever component is responding to the more closely matching speeds. I don't know where it was, but I feel like I read a post where someone said the pulse went away or froze at a constant hum at whatever intensity it was at when the case was shifted into 4wd. A hall effect tachometer on each t-case output yoke would help explore this, but in theory I would think one could manipulate the pulse interval with tire pressure. A half inch decrease in rolling radius of one tire ought to make enough difference to notice.

View attachment 194806

First vs second order: This is where instrumentation and data acquisition would be particularly helpful, but if you know what a 60Hz tone sounds like you can probably tell by ear. If the source vibrations are around 60-70Hz they are first order (one "peak" per revolution) and will be likely related to driveshaft balance. If they lie more in the 120-140Hz range (which will sound like the same note as 60Hz but an octave higher in pitch) they are second order (2 peaks per rev) and would be more likely from the natural acceleration and deceleration that the u-joints cause as they move through their rotation. Again - operating angle and balance may be well within normal tolerances, but doubling them may not be. It may be that on affected TJ's, the balance and angles have to be so close as to be impossible to attain with available parts and equipment. Clearly a $1200 hub conversion is still the better choice if the alternative was maintaining driveshaft angles less than 5° from the TC output yoke or balancing at 7000rpm.

Resonance/Natural frequency: I'm a little skeptical of the natural frequency theory because it seems to me that the harmonic vibration would be tightly concentrated around a specific speed, rather than starting at a certain speed and continuing as the vehicle accelerated. I'm not super knowledgeable about natural frequencies however, so I can't rule out that that it isn't simply the combination of the 2 driveshafts but that the combination is then exciting something else at it's natural frequency. Natural frequency can be modified with mass and physical dimensions, so I would be curious whether things like an aftermarket TC skid or frame length (LJ vs TJ) seem to have any correlation with the issue. I'm also not sure about whether resonance can be triggered by this sort of interference/superposition wave or by multiples or divisors of the objects natural frequency.

So anyway, I just threw a bunch of stuff up on the wall. Interested in the feedback on what might stick.

Would changing the mass of the drive shaft or other component(s) change the harmonics sufficiently to resolve the issue?

 

[freedom_in_4low]

Would changing the mass of the drive shaft or other component(s) change the harmonics sufficiently to resolve the issue?

if the problem is due to a resonance at a components natural frequency, then changing the mass (or length, or stiffness) of the component that is resonating would move it's natural frequency; hopefully out of the range of what is exciting it.

The trick is determining whether resonance is the issue and finding which component is resonating.

 

[Offline246]

if the problem is due to a resonance at a components natural frequency, then changing the mass (or length, or stiffness) of the component that is resonating would move it's natural frequency; hopefully out of the range of what is exciting it.

The trick is determining whether resonance is the issue and finding which component is resonating.

Is it a component or system that needs to be altered?