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

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psrivats

psrivats

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Sri, that single crossmember test they did is suspect, in my mind. They removed a whole bunch of material from a stock skid, hoping to improve vibration. It is much less stiff in that cut configuration than stock, so it's not surprising to me that your vibs got worse. You did get a result tho... Less stiff skid, vibs come on earlier. If they were to complete that test properly and bolt in a nice frame crossmember, that was very stiff and properly supported the transmission, something like the Savvy support, or the gen-right one, then you've got something to test.
That's my theory too and I told them that. Let's see what they try next ...
 

AndyG

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No, I understand that ground speed and driveshaft speed are related but the vibes change whether it is in drive or OD on some. It would be nice to find a better OD ratio for these transmissions.

Vibes at 40 is usually indicative of a pinion angle issue.
I run a small business and understand there are certain type jobs you can’t win on....and with overhead , the problem is you don’t know if or when you can turn the job ( complete the job and get paid).....so it sits and you put your energy into what you can win at.


I’m not defending bad service ...but Blaine is telling it truthfully. Business owners must do what they need to do to survive.

Back to the problem....once Jerry wisely posted that you can’t ignore vibes....they will damage other components.
This is important, and I love the tenacity you guys are showing.

Also, I know it’s messing with your enjoyment of the vehicle, and to me, Jeeps are all about fun.

All problems eventually yield to persistence.

Has anyone ran one on a lift and seen what is happening?
 
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psrivats

psrivats

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@AndyG @jjvw

This is my Jeep on a lift with cruise control at 65. Listen with your headphones on. You can hear the resonance vibration and the droning noise pretty clearly.

Warning: video is loud.


@mrblaine do you recall if the other Jeeps with vibration problems that you've seen show something similar?
 
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Rob5589

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That video is interesting. Based on it, is it safe to say in your case, it is purely in the rear and has nothing to do with the front? I was going to throw out the totally out of my ass theory of two different length drive shafts working against one another. But after seeing the video, doubtful.
 
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psrivats

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That video is interesting. Based on it, is it safe to say in your case, it is purely in the rear and has nothing to do with the front? I was going to throw out the totally out of my ass theory of two different length drive shafts working against one another. But after seeing the video, doubtful.
Unclear to me whether it's an issue only in the rear. I was told that when they initially saw the problem, they got the front shaft balanced and the vibes got slightly better (video is after they got the front drive shaft rebalanced and rebuilt). Now with the front or off, I'm told the vibes are not much different but I haven't seen it myself.

I think I look at this problem very differently than the shop and I'm not very sure how to interpret "not much different" or "slightly better". Being an engineer, I tend to think in terms of intensity (amplitude) and the duration (time period or frequency). Things either make a difference or not ... you may not be able to tell yourself, but there are instruments and appliances to use to help you tell that with confidence.

The standalone skid they tried (it was a cut-up stock skid) made things worse (vibes start coming at lower speed), and that tells me that the stiffness and weight of the skid do matter as @Ranger_b0b pointed out. It's not known if the problem can be solved only with a heavier/stiffer skid. That's yet to be systematically tried.

The rear driveshaft is now getting checked. Note the it is quite new (less than 1k miles) and I had no vibes of any sort prior to regear. I've also been told both front and rear pinon angles have been thoroughly checked and are in good order. Note that these were actually set and checked by Dave himself earlier in the summer when I got other upgrades done.

The way the entire thing is having steady pulsed (i.e harmonic) vibrations tells me that this is the entire frame/skid/whatever is resonating at fundamental frequency and is what made me look into the physics more carefully.

I still don't fully understand what's running out of spec (so to speak) that's making the entire structure resonate. Jim Frens has hinted at some design choices but details are not 100% available for us to go make better choices.

Is it the torsional deflections in the driveshafts at increased rpms near half critical speeds that's the source of the problem? If yes, then I need better driveshafts that doesn't distort (stiffer one, either thicker tube or made of Al).

Or is it that we've somehow changed the system response to have these vibes in the skid/frame to be onset sooner, even though driveshafts and pinion angles are good? So do we need other mods to see (heavier/stiffer skid or in the weight fr damper) to stop the system from responding as it is doing now? There are plenty of reports of people just installing an SYE and the get what seem to be similar vibes that don't go away no matter how much they look at pinion angles. Makes me think the damper does play an important role. But why did the factory put them only in the rear?

Amidst all this, why do quite a few people report everything going back to normal with a front hub conversion? Quite a few also have reported the absence of the issue when they remove one driveshaft (front or rear, doesn't matter). What explains that?

So yeah. I have gaps in my understanding and I'm continually thinking and reading about this. I just don't have anything more concrete to add at this point of time.

I had actually emailed the shop saying that I was going to pick up my Jeep day after Thanksgiving. Dave saw that called me today in the morning and he told me that he has had other distractions and that he'll try his hand again at this problem. That's when I learned the front driveshaft is out for checking. There have been multiple un-communicated long delays from the shop side in attending to my Jeep, some of which have been very frustrating. We really need systematic controlled experiments to understand what changes we make elicit what response, but I'm having trouble making the shop understand the importance of being thorough and systematic.

I've been very patient (and @Chris knows this) but I can only take so much after 6+ weeks of waiting with sporadic communication and then learning that they have not really worked on it or even tried anything that I've requested. Even learning what did not work is good progress in a problem like this. I'm still not quite sure what they plan to do and I'm going to be talking to Dave and Chris (the other Chris) again tomorrow.

One way or the other, I'll find a fix that works and more importantly understand why it works, either at that shop or elsewhere. I will say that I'm still very inclined to pull my Jeep from there and go elsewhere.
 
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Fouledplugs

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Installing an SYE on a 231 requires replacing the rear output shaft, the new one doesn't have any provision for a balancer.
You could attach a dampner if you had a companion flange machined. This might take away from the overall length of the rear driveshaft, which is going to add to the problem.

I have heard that the companion flange on the back of the '80s broncos is a 32 spline size that matches the larger/shorter output shaft used on JB conversions SYE kit. I think JB was working on this a few years ago.

I do know that the companion flange doesn't work on the 32 spline Np231/241 output on the dodge t-cases. I believe the splines are different.
 
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mrblaine

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Unclear to me whether it's an issue only in the rear. I was told that when they initially saw the problem, they got the front shaft balanced and the vibes got slightly better (video is after they got the front drive shaft rebalanced and rebuilt). Now with the front or off, I'm told the vibes are not much different but I haven't seen it myself.
Why did no one push on the inspection cover or remove it to see if that reduced the noise? Or has that been done?

I think I look at this problem very differently than the shop and I'm not very sure how to interpret "not much different" or "slightly better". Being an engineer, I tend to think in terms of intensity (amplitude) and the duration (time period or frequency). Things either make a difference or not ... you may not be able to tell yourself, but there are instruments and appliances to use to help you tell that with confidence.
I know, we could use a seismometer app on a smart phone.

The standalone skid they tried (it was a cut-up stock skid) made things worse (vibes start coming at lower speed), and that tells me that the stiffness and weight of the skid do matter as @Ranger_b0b pointed out. It's not known if the problem can be solved only with a heavier/stiffer skid. That's yet to be systematically tried.
This is very surprising to me. In all of our conversations, I never got the impression that a less rigid skid would be the first attempt at changing something. Nothing we discussed leads me to believe that any solution is going to be less stiff or lighter weight in the same material.

The rear driveshaft is now getting checked. Note the it is quite new (less than 1k miles) and I had no vibes of any sort prior to regear. I've also been told both front and rear pinon angles have been thoroughly checked and are in good order. Note that these were actually set and checked by Dave himself earlier in the summer when I got other upgrades done.

The way the entire thing is having steady pulsed (i.e harmonic) vibrations tells me that this is the entire frame/skid/whatever is resonating at fundamental frequency and is what made me look into the physics more carefully.

I still don't fully understand what's running out of spec (so to speak) that's making the entire structure resonate. Jim Frens has hinted at some design choices but details are not 100% available for us to go make better choices.

Is it the torsional deflections in the driveshafts at increased rpms near half critical speeds that's the source of the problem? If yes, then I need better driveshafts that doesn't distort (stiffer one, either thicker tube or made of Al).
If a driveshaft change is made to reduce deflection, it will be larger diameter tube of the same or thinner wall thickness. Increasing the wall thickness without going larger in diameter typically makes it worse. AL is not the answer in this case. You might get the thicker wall to work with how short the rear tubes are, but for a front, the length forces a larger diameter tube.

Or is it that we've somehow changed the system response to have these vibes in the skid/frame to be onset sooner, even though driveshafts and pinion angles are good? So do we need other mods to see (heavier/stiffer skid or in the weight fr damper) to stop the system from responding as it is doing now? There are plenty of reports of people just installing an SYE and the get what seem to be similar vibes that don't go away no matter how much they look at pinion angles. Makes me think the damper does play an important role. But why did the factory put them only in the rear?
Just to add another data point. We have chatted about DW and what causes it and how many things will or can cause it and then those very same things on other rigs don't cause it. I have a rig I'm swapping out the front axle on and we removed the knuckles the other day. The upper ball joints are completely shot, as in over 100 thou lateral movement at the ends of the pins. Anytime I've found lateral play in them on other rigs, they induced DW, not on this one. Now I have to factor that in and I'm not pleased at all to find that much play and no DW.

Amidst all this, why do quite a few people report everything going back to normal with a front hub conversion? Quite a few also have reported the absence of the issue when they remove one driveshaft (front or rear, doesn't matter). What explains that?
I've got to think about that one some more. I'm not convinced your rear pinion angle is correct yet.
 

AndyG

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That rear diff needs torn down and checked , along with the axle bearings and pinion .

That's a pretty constant buzz.
 
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psrivats

psrivats

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That rear diff needs torn down and checked , along with the axle bearings and pinion .

That's a pretty constant buzz.
Andy, the diffs have both been checked. They are good is what I'm told. I did not hear them howl or anything when I drove the Jeep.
 

Mike_H

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I was frustrated by fact that no one decided to put some pressure on that inspection plate either. That metallic buzz sure sounds light a light Gage panel rattling against something more substantial.

I mentioned this the first time I saw the video too... On my 5 speed, there is a bolt to hold that part of the inspection panel tight to the bell housing.

I don't think that is what is causing resonance, rather it's a result of the resonance. But it would help to limit some of the BSR (buzz, squeak, rattle) concerns.
 
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psrivats

psrivats

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Why did no one push on the inspection cover or remove it to see if that reduced the noise? Or has that been done?
I asked and did not get a reply :(

I know, we could use a seismometer app on a smart phone.
That made me chuckle. I did not mean that of course, there are sone off the shelf instruments we use in our labs and factory (essentially an electronic tuning fork) but perhaps it was not wise of me to expect shops to have these speciality instruments.

But honestly good apps do exist. This one for instance is made by a professor at Weber State University (Dr John Kelly). It's expensive at $400 but it looks to have pretty decent capability. Some folks here think this is a fancy calculator but after seeing the videos linked below, I think it's a pretty good attempt at using what's in your phone. Please see the videos if you have have some free time, would like your thoughts.

http://vibratesoftware.com/




This is very surprising to me. In all of our conversations, I never got the impression that a less rigid skid would be the first attempt at changing something. Nothing we discussed leads me to believe that any solution is going to be less stiff or lighter weight in the same material.
I guess they had a spare skid and just tried it. But it's a good data point and the best I have so far. I've asked to see what happens if you go the other direction and that attempt is yet to be made. I asked if the delay was due to them trying to figure out how to do it, but they flat out said they haven't worked on my Jeep after that test other than to send the rest driveshaft out. This is why I'm a bit frustrated ... delay is fine but they are not bring systematic and changing many things at once.


If a driveshaft change is made to reduce deflection, it will be larger diameter tube of the same or thinner wall thickness. Increasing the wall thickness without going larger in diameter typically makes it worse. AL is not the answer in this case. You might get the thicker wall to work with how short the rear tubes are, but for a front, the length forces a larger diameter tube.
Good points and I understand what you are saying. Thanks for the corrections.

Just to add another data point. We have chatted about DW and what causes it and how many things will or can cause it and then those very same things on other rigs don't cause it. I have a rig I'm swapping out the front axle on and we removed the knuckles the other day. The upper ball joints are completely shot, as in over 100 thou lateral movement at the ends of the pins. Anytime I've found lateral play in them on other rigs, they induced DW, not on this one. Now I have to factor that in and I'm not pleased at all to find that much play and no DW.
There seems to be no end of surprises when it comes to Jeeps. I keep reading nee things as well which is why I'm continuing to think about this over and over. I found one example of a guy who eventually tore down his tcase and found that it was his input bearings that we shot and causing his issues. He replaced it and all got well.


I've got to think about that one some more. I'm not convinced your rear pinion angle is correct yet.
I will request them to check it again ...
 

AndyG

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I really think the one in the video is going to have the solution found in the rear differential.

At some point someone is going to have to make a belt driven pulley that attaches to the yoke so that you can turn the differential at speed with the vehicle lifted and no Driveshaft in place

It actually might could be done with a drill on the pinion nut.

You would know pretty quick.

I definitely agree that the slip yoke is also a pretty good likelihood of movement under high-speed just due to the nature of how it's constructed...

I also don't think it's going to be the exact same cause on every vehicle.
 

mrblaine

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I keep forgetting to mention why the balancer isn't on the front of the t-case. I don't think there is room with the shift lever right there next to the DC joint. I'd have to carry a balancer over and hold it up next to it but the proximity to the side of the case and the shift lever would certainly limit the size and likely the effectiveness.
 
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mrblaine

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I really think the one in the video is going to have the solution found in the rear differential.

At some point someone is going to have to make a belt driven pulley that attaches to the yolk so that you can turn the differential at speed with the vehicle lifted and no Driveshaft in place

It actually might could be done with a drill on the pinion nut.

You would know pretty quick.

I definitely agree that the slip yoke is also a pretty good likelihood of movement under high-speed just due to the nature of how it's constructed...

I also don't think it's going to be the exact same cause on every vehicle.
Possible but I don't think so. The cyclical harmonic shown is very common to a whole bunch of rigs and this one is only louder. The rate of the cycle is identical to many I've dealt with including one that had a Magnum 5.9 out of a Durango in it.
 
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psrivats

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I keep forgetting to mention why the balancer isn't on the front of the t-case. I don't think there is room with the shift lever right there next to the DC joint. I'd have to carry a balancer over and hold it up next to it but the proximity to the side of the case and the shift lever would certainly limit the size and likely the effectiveness.
That makes sense, thanks for sharing that bit of information. Essentially it's a limitation of space. Would the lack of a front damper perhaps also explains why it's the front side that comes out as more problematic in many rigs?
 

mrblaine

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That makes sense, thanks for sharing that bit of information. Essentially it's a limitation of space. Would the lack of a front damper perhaps also explains why it's the front side that comes out as more problematic in many rigs?
If I knew that answer, we would all drive vibe free rigs. I'm going to go with if the engineers at Jeep couldn't figure it out to start, all we are going to do is spend lots of time trying stuff that may or may not work. This crap is so far removed from my abilities and skill set that I don't know what I don't know.
 
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Fouledplugs

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Definitely good questions to ponder. Or even an heavy duty conversion like below ... does it work well without the balancer due to the heft of the upgraded components?

http://www.fourwheeler.com/how-to/transmission-drivetrain/1807-super-duty-jeep-transfer-case-conversion/
Would a longer driveshaft cut down on vibrations created by the re-gear? They mention it in their article: "The lift demanded new driveshafts, and a slip-yoke eliminator kit was needed for the TJ’s New Process 231 transfer case to allow a longer-than-stock rear shaft for better pinion and U-joint angles to help eliminate vibrations."
It has always been a "yes" from me when posed this question.
Another reason I really stand behind JB conversions. They build a THE shortest SYE with the main advantage of being able to build a much longer driveshaft than your run the mill SYE from the brands who have abandoned further SYE engineering.

It still doesnt answer the problem below:
Regearing created vibrations that were not there before. But maybe it's just amplifying the vibes that have been there the whole time since its now spinning faster than it was before the re-gear.

I am still reading from the beginning of this thread but has anyone asked if the gear sets are the problem? Being that most are all made in China/Korea. Does the whole,"made in USA," not hold water for gearsets for some reason? I read all over this forum that Chinese made parts x, y, and z are not the same or as reliable as the real thing. (i.e TPS https://wranglertjforum.com/threads/throttle-position-sensor-tps-going-bad.7784/)

This would be the test I would like to see:
Bone stock TJ such as mine do a USA made gear swap vs a Chinese/Korean gear swap using the same gear ratio. 3.07 to 4.56