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

Test #1

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No apparent difference in speed of onset or intensity. It did seem like *maybe* the frequency of oscillation was slower, but not sure. Before I continue I'm gonna have to get on a longer stretch of highway and actually note the time between peaks, and then again without the weight to see if it did make any impact. Whether it did or didn't, I'll be putting it in a few different spots to see if any of them do effect a change, but if it did, that might inform the next spot I try it in.
 
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Back when I was knee deep in diagnosing mine, I recall talking with Blaine about tying the skid plate into the frame with a turnbuckle, or gas strut, to place pressure on the skid and possibly changing the frequency of the harmonic. Sounds like you may be on a similar path here.
 
bobthetj03 said:
Back when I was knee deep in diagnosing mine, I recall talking with Blaine about tying the skid plate into the frame with a turnbuckle, or gas strut, to place pressure on the skid and possibly changing the frequency of the harmonic. Sounds like you may be on a similar path here.
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Yeah. I'm just about as convinced as I can be that what we're seeing is either one or two components with a resonant frequency lying close enough to the driveshaft rpm that normal amplitudes from the driveshaft that are imperceptible to the driver is exciting it, and then either the driveshaft and the component, or two different components with similar resonances, are vibrating just off of each other.

1. The fact that it seems to mainly impact rigs made after the 2003 skid re-design keeps pointing me to the skid or the frame.
2. That it happens to TJ's and LJ's alike points me toward the skid
3. examples of this happening with a skid/crossmember configuration like Savvy would point me away from the skid (anybody?)
4. that it seemed to decrease in intensity with the 80 pound steel skid isn't necessarily conclusive, because it could be due to a stiffer skid better constraining the frame just as easily as it could be the skid itself...but a stiffer skid should resonate at a higher frequency, so if it was the skid I would think the effect wouldn't be less intensity, but a higher speed of onset.
5. something that literally occurred to me as I'm typing this is that the gas tank skid also changed in 2003...does anybody with this issue have an aftermarket GTS?
 
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freedom_in_4low said:
Yeah. I'm just about as convinced as I can be that what we're seeing is either one or two components with a resonant frequency lying close enough to the driveshaft rpm that normal amplitudes from the driveshaft that are imperceptible to the driver is exciting it, and then either the driveshaft and the component, or two different components with similar resonances, are vibrating just off of each other.

1. The fact that it seems to mainly impact rigs made after the 2003 skid re-design keeps pointing me to the skid or the frame.
2. That it happens to TJ's and LJ's alike points me toward the skid
3. examples of this happening with a skid/crossmember configuration like Savvy would point me away from the skid (anybody?)
4. that it seemed to decrease in intensity with the 80 pound steel skid isn't necessarily conclusive, because it could be due to a stiffer skid better constraining the frame just as easily as it could be the skid itself...but a stiffer skid should resonate at a higher frequency, so if it was the skid I would think the effect wouldn't be less intensity, but a higher speed of onset.
5. something that literally occurred to me as I'm typing this is that the gas tank skid also changed in 2003...does anybody with this issue have an aftermarket GTS?
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Mine has no aftermarket GTS FWIW.
 
NOTNSUV said:
Mine has no aftermarket GTS FWIW.
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thanks.

At what speed does yours start, and does it stick around or go away with the front driveshaft removed? (sorry if you've posted this info before already...I did search but didn't come across it)
 
freedom_in_4low said:
thanks.

At what speed does yours start, and does it stick around or go away with the front driveshaft removed? (sorry if you've posted this info before already...I did search but didn't come across it)
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No sweat. Best I can tell so far is about 60 mph onset but haven't driven it beyond that much with ring & pinion breaking in. And have yet to run w/o front shaft but will get to that before too long and will post some accurate numbers. Too busy running trails in the XJ with my buds.. + a few recoveries.
Getting some grief for not taking the LJ out yet. Told them convertibles are for summer.. LOL. It's soo pretty when clean.
 
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My guess is the entire vibration is simply the front and rear driveshaft oscillating together, front up rear down or diagonally or laterally or any combination. Kinda like a standing wave in a string with one end point at the front axle, the midpoint at the transfer case and the other end at the rear axle. Those shafts have more than enough weight to cause the drone all by themselves without really shaking much of anything else with the transfer case like the null point in the center of the standing wave. I think the frame is too isolated to resonate this way.

Edit: even if it works as I described it doesn't help with a new solution
 
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pagrey said:
My guess is the entire vibration is simply the front and rear driveshaft oscillating together, front up rear down or diagonally or laterally or any combination. Kinda like a standing wave in a string with one end point at the front axle, the midpoint at the transfer case and the other end at the rear axle. Those shafts have more than enough weight to cause the drone all by themselves without really shaking much of anything else with the transfer case like the null point in the center of the standing wave. I think the frame is too isolated to resonate this way.
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That may be the case in some. The problem I have is that it doesn't explain the ones like mine, bobthetj03, and I think psrivats (before the rubicrawler) that had it even with the front driveshaft out.
 
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pagrey said:
My guess is the entire vibration is simply the front and rear driveshaft oscillating together, front up rear down or diagonally or laterally or any combination. Kinda like a standing wave in a string with one end point at the front axle, the midpoint at the transfer case and the other end at the rear axle. Those shafts have more than enough weight to cause the drone all by themselves without really shaking much of anything else with the transfer case like the null point in the center of the standing wave. I think the frame is too isolated to resonate this way.

Edit: even if it works as I described it doesn't help with a new solution
Click to expand...

I think what's happening is that there's something, somewhere in the Jeep that likes to shake at a specific frequency that Jeep NVH engineers never designed for (or perhaps intentionally avoided) because the driveshaft never got that fast in a factory configuration.

I think that on some Jeeps, it takes 2 driveshafts spinning to excite whatever it is that shakes. And I think on other more picky Jeeps, it only takes 1. Either that or these two different subsets are vibrating due to completely different mechanisms.


My data collection thread hasn't pointed to anything yet, but it has yielded some interesting info.

There are examples of effective lift (actual suspension lift + tuck height) ranging from 3 to 7.5" both with and without the vibration.
There are examples with every transmission except the 32RH.
There are examples with both transfer cases.
There are examples in both TJ and LJ.

bobthetj03 and NOTNSUV are getting it at driveshaft speeds around 3100rpm and while they are geared lower than any factory configuration, that rpm @ onset is equivalent to a stock Rubi @ 67mph. tworley, psrivats, and myself don't hit it until above 3500 which is equivalent to stock Rubicon @ 77mph. In either case, they are speeds that would have been reachable by a stock rig, but we don't have any examples of stock rigs experiencing it, or any rig with an axle ratio numerically below 4.56.

@tworley is an outlier as the only affected pre-2003 (so far) and the only one with a Savvy skid (wait a second...I thought savvy didn't make one for the early models. Custom?)

I changed tire size, and my vibration onset moved from 67 to 73, staying at the exact same driveshaft speed.
I changed rigs, keeping the same axles, suspension, and transfer case, and the second rig got the vibes, the first didn't.
 
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I must be confused, does anybody with locking front hubs have this problem? Isn't that the same as removing the front driveshaft? I guess it doesn't matter really to me but it seems like something isn't right.
 
pagrey said:
I must be confused, does anybody with locking front hubs have this problem? Isn't that the same as removing the front driveshaft? I guess it doesn't matter really to me but it seems like something isn't right.
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yes, locking hubs = front driveshaft out. But there are still some that have vibes even with the front shaft out, which is a predictor that the hubs won't fix it. That's unfortunately been the case for bob and myself, and it was the case for psrivats until he installed a Rubicrawler.
 
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If the driveshafts are the only source of the vibration causing the harmonics, even the rear one by itself, changing the weight of the center section, the rubicrawler, will change the harmonics. Maybe try bolting your big block of steel to the transfer case since the extra weight is known to change things?
 
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pagrey said:
If the driveshafts are the only source of the vibration causing the harmonics, even the rear one by itself, changing the weight of the center section, the rubicrawler, will change the harmonics. Maybe try bolting your big block of steel to the transfer case since the extra weight is known to change things?
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That's on my list. Gonna need more weight though. I think to make a difference there I'm gonna need to approximate the weight added by a rubicrawler. Probably close to 10 pounds I would think. It's been a bit since I held a 231 planetary.
 
f the weight of the rubicrawler was an issue, a smaller weight anywhere near the transfer case will probably shift the vibes, especially if it is hung outboard in some fashion like on the side. If I was looking for a place to stick a weight I'd probably attach it to the rear side of the transfer case opposite the front yoke.

Balance wheels on the rear transfer case output did randomly appear on TJs and newer Jeeps did get a totally different front shaft design. These things might point to a more difficult to solve resonance issue especially considering Jeep is now going to be offering 4.88 gears for the first time only after including a disconnect for the front shaft from the factory. It all could just be coincidence.
 
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