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

jjvw

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.... BTW, @jjvw did you go see if you still have the two parts of the balancer in your Jeep? I confirmed that mine does even with the aftermarket driveshaft. ....

I crawled under mine. The 241 has a similar type of harmonic balancer as the 231 with the rubber isolator. I can move it around a tiny bit by hand. The drive shaft mounting flange is just in front of the balancer.
 
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jjvw

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The factory motor and transmission mount arrangement is a triangle. I've read that the TMR style cross members that move the transmission bushings to the frame, resulting in four isolated mounting points, can create vibrations.

The Savvy cross member maintains the factory tripod arrangement and probably did so for good reason.
 
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psrivats

psrivats

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Thanks @bobthetj03. That's quite hefty indeed. Your photo and @jjvw's observation that it has rubber has led into another area of searching.

@bobthetj03 @Chris the damper design seems pretty specific and I think it's really where our problem lies. I am doing some research on the design details of and I found an interesting paper. Please take a look at attached pdf. It looks pretty complicated but just notice the frequency range they are talking about, and simply look at the figures. This will give you a general idea of what we are dealing with here. I am reasonably confident now that this is the root cause of the vibrations and the fix lies in having a properly weighted damper on the driveshaft.

I will continue further research on my side. I am so very glad I spotted the 55Hz frequency post from that engineer.
 

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Joeker67

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So I have been following numerous threads on this harmonic vibration. Mine started after my gear change from stock to 4.56 and immediately noticed the harmonic vibration. After trying to adjust the pinion angle and many other things, I took it back to where I had the installation done. They did a couple of the same things I had done with no luck, for some reason they pulled the rear pinion yoke and notice there was some play, not much but decided to swap it out AND...... wahlaaaaa the vibration is gone. So as many other post's, it seems once again it can be many different things. Just wanted to share what fixed mine.
 
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psrivats

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I also found this on a mechanics's forum - the "It" is the damper.

@Chris I am not a manual trans guy, but is the top gear in manual transmissions similar in function to the OD gear in autos? That would explain why Jeep put this on all manual transmissions + 4.0L jeeps.
.
upload_2018-11-3_21-27-20.png


Another find;

The problem is not unique to jeeps.

https://www.dieseltruckresource.com...er-can-i-remove-never-reinstall-272888/page1/
 
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Chris

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So my thought is that if this is indeed harmonic balancer related, that part would be very easy to have CNC machined. We could have one machined where it has a stack of ring weights on it (like a stackable bump stop almost) and you would be able to add or remove the weight to fine tune it. The weight would be attached via simple bolts I imagine.

I'm getting ahead of myself here of course, but if this theory proves itself to be true, this might be a good solution.
 
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psrivats

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OK, now I have a decent theory for what's happening.

In the top gear in manual trans or the OD gear in 42RLEs where the torque converter locks up, we have additional vibrations on the driveshaft (see my post further below for why this happens). The damper acts to absorb this extra vibration at that combo of speed+rpm ranges (which doesn't happen in the penultimate gear), without which we get resonant harmonic vibrations in the TC skid/frame/whatever due to the design choices Jeep made. The 32RH jeeps did not have a overdrive gear, so they did not need a damper. All manuals therefore got the damper with the 4.0L engine. Perhaps the vibrations were not bad enough to induce resonant harmonics in the 2.5L engine Jeeps (lower torque?) with any transmission, so Jeep did not put a damper on those. The 42RLE with this stupid 0.69 ratio is really prone to this on 4.0L jeeps.

A good test for my jeep would be to drive it with OD turned off and see what happens. I am going to call Dave kishpaugh first thing on monday to test this. If the vibrations reduce a lot we know we are on the right track to solving this problem (which is having a correctly weighted damper).

how's this theory sound @Chris @Ranger_b0b ?
 
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zebra12

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The factory motor and transmission mount arrangement is a triangle. I've read that the TMR style cross members that move the transmission bushings to the frame, resulting in four isolated mounting points, can create vibrations.

The Savvy cross member maintains the factory tripod arrangement and probably did so for good reason.

Good to know, thank you.
 

Chris

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OK, now I have a decent theory for what's happening.

In the top gear in manual trans or the OD gear in 42RLEs where the torque converter locks up, we have additional vibrations on the driveshaft (I will look into the why of this shortly). The damper acts to absorb this extra vibration at that combo of speed+rpm ranges (which doesn't happen in the penultimate gear), without which we get resonant harmonic vibrations in the TC skid/frame/whatever due to the design choices Jeep made. The 32RH jeeps did not have a overdrive gear, so they did not need a damper. All manuals therefore got the damper with the 4.0L engine. Perhaps the vibrations were not bad enough to induce resonant harmonics in the 2.5L engine Jeeps with any transmission, so Jeep did not put a damper on those. The 42RLE with this stupid 0.69 ratio is really prone to this on 4.0L jeeps.

A good test for my jeep would be to drive it with OD turned off and see what happens. I am going to call Dave kishpaugh first thing on monday to test this. If the vibrations reduce a lot we know we are on the right track to solving this problem (which is having a correctly weighted damper).

how's this theory sound @Chris @Ranger_b0b ?


It sounds dead on to me! To further prove @psrivats theory, the one thing I noticed with mine on the highway is that when I got up to 65 mph it was very, very obvious there were vibrations. However, when I flipped the switch to turn the OD off, the RPMS shot up (obviously) but the vibrations were almost entirely gone... literally, it was night and day.
 
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psrivats

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To further prove @psrivats theory, the one thing I noticed with mine on the highway is that when I got up to 65 mph it was very, very obvious there were vibrations. However, when I flipped the switch to turn the OD off, the RPMS shot up (obviously) but the vibrations were almost entirely gone... literally, it was night and day.

Chris, I think it's all starting to make some sense. I will ping Dave and have him test this as soon as he can.

@jjvw quick question for you. If you take your jeep to 75 in the penultimate gear (if it's even possible at all), do you still have the vibrations at 75mph? Would it be possible for you check this for us sometime?
 

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I see it's already been pointed out that the 2.5 in 2001 didn't come with the damper. Here's some more info;

Last week, i drove up the 97C Connector just west of Peachland. Were talking a 4000' drop @ 6.5% grade over 17 miles from the Pennask summit down. It allowed me to coast at 62-65mph in neutral for extended distances. I also was able to put the transfer case in neutral and coast from 0 to 55mph. I had the wife drive and I jumped in the back seat on the RH side armed with my mechanics stethoscope for the trip down.

So guess what? Zero detectable vibrations. Seems my 175lb ass was enough rearward ballast to make the difference. So in my case, it's possible a driveshaft angle thing but I'm not going to rule out the rear springs and isolators. I've put a digital angle meter before on the rear pinion and Xfer case O/P and they were under a degree. I may try to take apart the rear springs and inspect everything.

Edit: What I didn't mention, was that the harmonic vibrations didn't change when I clutched or put it in neutral. The TJ drops speed so fast, the hill allowed me to prolong the evaluation. I'm not going to rule out body mounts. As I said in another post, they can look 100% but have diminished clamping force. Remember - they are hollow like a pumpkin and the insert inside will not allow them to compress unless you grind them down. Better to replace the set (Crown or whatever). On my the list.

TJ Rear Suspension.PNG
 
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Chris

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Here's a video @David Kishpaugh took while under Sri's Rubicon with it going 65 mph using the cruise control. See that slight vibration near the bellhousing towards the end? The OD is on, and that's got to be the torque convertor causing that.

 
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psrivats

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@RaymondT please see all my recent posts. Your issue is perhaps unrelated since you have the 2.5L

@Ranger_b0b @glwood @ElectricWizard @jjvw hope you are all following this thread. I think I know what's happening. I would really appreciate if you guys read through and see if there is anything that is not consistent in what I am saying.

Time to test my theory and I already sent Dave a message. I will talk to him in person sometime next week.
 
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psrivats

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Here's a video @David Kishpaugh took while under Sri's Rubicon with it going 65 mph using the cruise control. See that slight vibration near the bellhousing towards the end? The OD is on, and that's got to be the torque convertor causing that.


You can bloody hear how rhythmic it is in that video. This is the OD gear as explained in the article above. The damper absorbs this extra vibration (without which the resonance harmonics in the skid frame gets set and you feel it inside everywhere) and right now the whatever weight is on there is not sufficient post the regear since the rpms are higher.
 

RaymondT

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@RaymondT please see all my recent posts. Your issue is perhaps unrelated since you have the 2.5L

Perhaps, but my experience tells me to look beyond the driveline (not to be confused with driveshafts). Otherwise, we would have collectively solved it by now. And if it was just a 2.5 thing, then why would the harmonic vibrations be consistent with the engine shut off coasting down that hill with the transfer case in neutral? And lets not forget that the front carrier was removed. I've yet to read any reported details beyond speeds and and lengthy lists what others have tried. Where are the specific details on the vibrations? Patterns, vehicle load (weight), temperature, engine rpm, engine torque load, anything that alters the vibrations... Hard to solve a puzzle with most of the pieces missing.

See my edited post above about body mounts. I still have this inclination that there's something about the body or rear suspension that's allowing the imperfections in the driveline to act like a tuning fork.

A friend of mine gave me a tool that may have been helpful and also illustrates what I am talking about with the TJ. It was a (German designed) vibration analysis tool that consisted of a plastic round case (about 3-1/2" dia) with frequency markings on the face. You turned a dial and a thin metal wire extended with a tiny ball on the end. With the tool pressed up against anything metal, you extended the wire until maximum movement was achieved and then looked at the window to read the frequency. It was given to Chrysler shops to hand out to their mechanics. Unfortunately, the one I got was broken. Turned out, he had dropped that one but wasn't sure which one he gave me. I may borrow his good one.
 

jjvw

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....

A friend of mine gave me a tool that may have been helpful and also illustrates what I am talking about with the TJ. It was a (German designed) vibration analysis tool that consisted of a plastic round case (about 3-1/2" dia) with frequency markings on the face. You turned a dial and a thin metal wire extended with a tiny ball on the end. With the tool pressed up against anything metal, you extended the wire until maximum movement was achieved and then looked at the window to read the frequency. It was given to Chrysler shops to hand out to their mechanics. Unfortunately, the one I got was broken. Turned out, he had dropped that one but wasn't sure which one he gave me. I may borrow his good one.

This thing?
Briggs & Stratton 19200 Tachometer

https://www.amazon.com/dp/B0044AV5VK/?tag=wranglerorg-20

I have one. It works pretty well, but would be hard to pay attention to as the driver.

An interesting thing about it. My 4.0 is so smooth that the B&S tach won't read the rpms.

 

Mike_H

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@psrivats I've kept up, though I haven't read the paper you posted in its entirety. Interesting thing about that bell housing inspection plate...I have an 04 with a manual, and the inspection plate (the part that is vibrating in Dave's video of Sri's jeep) has a bolt at the bottom to hold it to the bell housing.

You also asked earlier in the thread if the 4 cylinders are different that the 6 with regard to the disturbing frequency. You are correct. That formula is simply calculating the number of explosions that happen at a given RPM, and converting to seconds (to put the units in Hertz). A 4 cylinder will have two less explosions at any given RPM, so the disturbing frequency will be 2/3's that of a 6 cylinder at any RPM. Basically, what is happening in the engine when the air fuel mixture ignites (the controlled explosion that drives our jeeps) is it creates an acceleration pulse on the crankshaft...which will then transfer through the drivetrain. The fact that ALL manuals got a harmonic balancer makes sense, because a manual is "locked" into the drivetrain at all times (well unless you push the clutch). The torque converter on an automatic will allow for some "slop" in the system so it may or may not need a balancer.


I'm going to have to spend some time working out how harmonic balancers are designed.
 
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glwood

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@psrivats I've kept up, though I haven't read the paper you posted in its entirety. Interesting thing about that bell housing inspection plate...I have an 04 with a manual, and the inspection plate (the part that is vibrating in Dave's video of Sri's jeep) has a bolt at the bottom to hold it to the bell housing.

I'm going to have to spend some time working out harmonic balancer design...We may have a product idea here guys...
If you come up with a harmonic balancer design that is adjustable, fixes this issue, and can be manufactured, you won't be able to make enough of them...and maybe win the Nobel prize...which you'd have to share with @psrivats...