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

What happens when you drive it without the front driveshaft?

I had it out when I dropped it off & picked it up from the shop doing the work but that’s been over a year ago. I don’t remember if it was good or not or even if I had it up to highway speeds. I’ll check that and see how it does.
 
Has anyone had these harmonic/cyclic vibrations on a stock Jeep? My 2003 Rubicon suspension is all stock as far as I can tell. I’ve had these vibrations since I bought it and originally chased wheel & tire balance to fix it. I threw new shocks and front hub bearings at it also in a fruitless attempt to cure it.

More recently, I’ve installed Centramatics and the vibe, while minor, is still discernible above about 65 mph. Both driveshafts are stock but have been rebuilt with new u-joints and balanced. Front axle u-joints were replaced. I clocked the front shaft at the axle and that helped some but didn’t eliminate the vibes.

Front track bar was replaced a couple years ago. I plan to replace the rear and all 8 control arms this year since I suspect they’re all factory original.

Do the vibration stay when you accelerate past 75mph for example?
 
Do the vibration stay when you accelerate past 75mph for example?

If I'm remembering clearly, it stays once I get above 65 but I rarely have it above 75. I'll push it to 80 and make note of it next time I have it up to highway speeds.
 
Im dealing with a harmonic vibe now. It was present with 35s, and 4.56 gearing at about 73mph. I've regeared to 5.13s and rebuilt the driveshaft, then put in a new Tom Wood driveshaft. Pinion angle is within 0.5 degrees (even had it up two degrees). Since the regear the vibe is about 68 mph and its very light and can drive 65 to eliminate it. Cant seem to get rid of it, though with the savvy tuck I may be at the absolute limit of the ujoints.
 
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I have vibrations as well and on a completely stock Jeep, even the tires are the original size. They appear at 65~68 and disappear closer to 75. My previous Jeep's vibrations were a little more pronounced but around the same speeds. It seems to me this is normal, unless it is really bad.
 
1999, 5 speed, 4.56, 32x11.50 tires, JB supershort SYE and a TW driveshaft. Being a 99, it never had a balancer, and it has the older skid plate design.

I feel nothing, but I can hear a pulsating drone at 75mph. Don't know if it goes away with increased speed, because why would I go faster than that?

It definitely was not happening before the regear, but I haven't done much investigation on it. It seems to diminish if I lean forward or back, like it may be slightly differing frequencies from different sources converging on the exact location of my head. It's also pretty close to the exhaust note, which is higher than it had ever reached at that speed. My rusty muffler is coming apart at the seam, so I'll be replacing it soon. It'll be interesting if it changes what I'm hearing.
 
Are any of you running a hard top? Mine is more pronounced with the hardtop on. Of course with the soft top there is a lot more noise, so these harmonics can be harder to hear. I physically feel mine in the seat above 60 mph.
 
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Are any of you running a hard top? Mine is more pronounced with the hardtop on. Of course with the soft top there is a lot more noise, so these harmonics can be harder to hear. I physically feel mine in the seat above 60 mph.
Mine are very noticeable with the hardtop on. Still have the vibs when I take the top off but it doesn't seem as bad.
 
So on a recent freeway drive, I noted the following with my vibrations:

Like many, they pulse slowly. In a straight line at 80 mph, the pulse is about once every 2-3 seconds.

I found out I can "pause" the pulse by putting it in 4WD. If I pause it at the lowest vibration state, it stays that way. If I pause it at the highest state, it stays that way. So I can eliminate the noise by simply putting it in 4WD at the right time...

So that really makes it interesting...
 
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So on a recent freeway drive, I noted the following with my vibrations:

Like many, they pulse slowly. In a straight line at 80 mph, the pulse is about once every 2-3 seconds.

I found out I can "pause" the pulse by putting it in 4WD. If I pause it at the lowest vibration state, it stays that way. If I pause it at the highest state, it stays that way. So I can eliminate the noise by simply putting it in 4WD at the right time...

So that really makes it interesting...

What I was taught in school is when you have that pulsation, it's because you have two wave sources piling on each other that are slightly different. If their frequencies are different by 1Hz then you will hear a 1Hz pulsation (there may be a multiple of 2 in there somewhere...it's been a while). Just like you hear a pulse when you're using the 5th fret on a guitar string to tune the string above it...the pulsation gets slower as the pitches get closer together, until the pulsation disappears and the strings vibrate in unison. It happens because when the waves aren't synchronized, they alternate between double the intensity and canceling each other out.

What you're describing sounds to me like you have a source on the front side of the transfer case and one on the back side, and they spin at different speeds when the transfer case isn't locked together. I suppose you could achieve the same effect if you played with tire inflation pressures to get the EXACT same rolling circumference, but that would be difficult to achieve and annoying to maintain. Plus every time you turned it would change which part of the pulse you were locked in to, so half the time you'd be on the more intense side of it.
 
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Are any of you running a hard top? Mine is more pronounced with the hardtop on. Of course with the soft top there is a lot more noise, so these harmonics can be harder to hear. I physically feel mine in the seat above 60 mph.

I have a hardtop I run in the winter. But I can’t say I notice a difference between tops. I can feel the vibration on mine thru the seat as well.
 
Man I just signed up to comment on this thread! Glad I'm not the only one with some strange vibes.
2006 4L Sahara 6speed manual.
I ran a spacer lift and 32's happily for quite a few years. Eventually went up to 33s and 4.56 gears. I then upgraded to a RE 3.5inch lift and decided to go the whole way and did front and rear adjustable arms, SYE and double knuckle drive shaft.

Just before doing this I noticed that the Harmonic balancer on my output shaft was pretty worn and starting to fail. After putting the new control arms in I noticed a vibration but that was a slightly bent output shaft it turns out.

Did the SYE install and ever since Ive had high speed vibrations. It's super smooth up to around 60mph (100kmh). Anything over that, especially on full throttle i get a harmonic vibe of sorts. I'm going to check my rear pinion angle but it was pretty damn close and I haven't fiddled with it.

Is there a summary for all the ideas in this thread somewhere?
 
To eliminate all drive shaft vibration drive shafts need to be aligned in all planes. The control arms control the horizonal driveshaft angle in reference to the transfer case. The trackbars controls the vertical driveshaft angle in reference to the transfer case. As the axles moves up and down the radius of the track bar arc moves the axle sideways causing the axle to be misaligned to the transfer case.

When installing a lift the axles need to be aligned with the transfer case in all planes at ride height. A well designed lift kit will control the horizonal angle of the pinion in reference to the transfer case. The vertical angle controlled by the trackbar will always change as the axles move up and down. The more angle the trackbar has in reference to the axle the more extreme that change.

Many on the this forum advocate high trackbar angles via the use of stock trackbar pivot locations with lifts but that practice adds to drive shaft misalignment and vibration many owner seek to eliminate.
 
Many on the this forum advocate high trackbar angles via the use of stock trackbar pivot locations with lifts but that practice adds to drive shaft misalignment and vibration many owner seek to eliminate.

So you're suggesting that a drop track bar bracket (combined with a drop pitman arm of course), would actually be a good idea?

Maybe I'm missing something, but I have to imagine that some of these companies with a lot of background in suspension design such as Currie would be using different track bar pivot locations if it was truly helpful in eliminating vibrations.

Hell, I'd imagine if that were the case that someone like @David Kishpaugh or @mrblaine who builds these rigs day-in-and-day-out would be doing the same thing, no?

I'm not trying to be argumentative, so please don't take it that way. I'm merely trying to understand why if it were this simple, more companies and builders wouldn't be doing this?
 
The argument about track bar angles makes no sense. The axle only moves side to side when the suspension compresses or droops. Going down the highway your axle should be just as centered as it would be if you have a drop trackbar and pitman. People also run ford 8.8's down the highway without problems. Driveshafts account for huge angles downward, why can't they do a much smaller angle side to side?
 
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So you're suggesting that a drop track bar bracket (combined with a drop pitman arm of course), would actually be a good idea?

Maybe I'm missing something, but I have to imagine that some of these companies with a lot of background in suspension design such as Currie would be using different track bar pivot locations if it was truly helpful in eliminating vibrations.

Hell, I'd imagine if that were the case that someone like @David Kishpaugh or @mrblaine who builds these rigs day-in-and-day-out would be doing the same thing, no?

I'm not trying to be argumentative, so please don't take it that way. I'm merely trying to understand why if it were this simple, more companies and builders wouldn't be doing this?
Chris,

As I understand it this tread is about drivetrain vibration at highway speeds not rock crawling. What I am saying is if one seeks eliminate driveshaft vibration then aligning the axle to the transfer case or driveshaft, as the design may dictate, in all planes will be necessary. The more angle the trackbar has in reference to axles the greater the misalignment becomes through the axle travel.

We all seek to use our Wranglers for different proposes, some rock crawl, others overland, many use them as DD, etc. The compromises of the Wrangler design needs to be understood so that owners know what to expect as they build their TJ/LJ to meet their personal needs.
 
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Chris,

As I understand it this tread is about drivetrain vibration at highway speeds not rock crawling. What I am saying is if one seeks eliminate driveshaft vibration then aligning the axle to the transfer case or driveshaft, as the design may dictate, in all planes will be necessary. The more angle the trackbar has in reference to axles the greater the misalignment becomes through the axle travel.

We all seek to use our Wranglers for difference proposes, some rock crawl, others overland, many use them as DD, etc. The compromises of the Wrangler design needs to be understood so that owners know what to expect as they build their TJ/LJ to meet their personal needs.

I always default to assuming everyone will be using their Wrangler for off-roading, which is of course an incorrect assumption.
 
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The argument about track bar angles makes no sense. The axle only moves side to side when the suspension compresses or droops. Going down the highway your axle should be just as centered as it would be if you have a drop trackbar and pitman. People also run ford 8.8's down the highway without problems. Driveshafts account for huge angles downward, why can't they do a much smaller angle side to side?
You make my point, thanks. As little as 1 degree of driveshaft misalignment will cause vibration. High trackbar angles and the change in highway speed ride height could lead to more than 1 degree in change.
 
You make my point, thanks. As little as 1 degree of driveshaft misalignment will cause vibration. High trackbar angles and the change in highway speed ride height could lead to more than 1 degree in change.

I absolutely have not proven your point. The driveshaft does not need to be perfectly aligned across. People with ford 8.8’s in TJ’s don’t have vibrations, people without adjustable track bars at all don’t have vibes. Riddle me this, if your driveshaft can handle 15 degrees (or more) downward, why can’t it handle 1 degree side to side?
 
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