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


freedom_in_4low

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Troy called from Tom Wood's. Says the driveshaft was straight and as well balanced as they could have gotten it. Sending it back to me.

He suggested checking the centering tabs on the yoke, and they're sending me new straps. The yoke was replaced with the regear and the tabs are healthy. The straps are the same ones that were on, and weren't causing vibration then, but I'll try the new ones.

Troy seemed pretty confident telling me he didn't think it was pinion angle since the vibration doesn't change with load, but I'll still play with it because I have mostly johnny joints so the axle probably doesn't move much at all under load. I haven't questioned the angle up to this point because it just feels different than the last time I had driveline angle issues....but that was with mostly stock bushings, stock uppers, and at much lower speeds, so it may have just been a different frequency and transmitted differently.
 

bobthetj03

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My rear pinion is sitting at 4.5* lower than the rear DS right now. This is after making small changes over several days and drive cycles, 0.5* at a time, and seems happier there than it did at 0.5* lower than DS. Goes against all conventional wisdom per a DCDS and the rear pinion. I'm guessing I'm getting a lot more deflection with these MC joints vs. a JJ. I'm still not convinced that the DS cannot be balanced better, but that appears to be a losing battle after trying 3 Adams shafts and a TW shaft. I haven't tried going higher than the DS yet.
 

freedom_in_4low

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My rear pinion is sitting at 4.5* lower than the rear DS right now. This is after making small changes over several days and drive cycles, 0.5* at a time, and seems happier there than it did at 0.5* lower than DS. Goes against all conventional wisdom per a DCDS and the rear pinion. I'm guessing I'm getting a lot more deflection with these MC joints vs. a JJ. I'm still not convinced that the DS cannot be balanced better, but that appears to be a losing battle after trying 3 Adams shafts and a TW shaft. I haven't tried going higher than the DS yet.

wow! agreed - totally against conventional wisdom. That would make me start looking for an error in my measurement but I know you've been at this enough to have covered that base by this point.

I think mine is set about 0.5 degree low, I think I'm gonna dry dead-on first and if it doesn't get better I'll start going the other way. I can't imagine how going higher could possibly be better but I wouldn't have guessed 4.5 degrees would work, either.
 
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bobthetj03

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wow! agreed - totally against conventional wisdom. That would make me start looking for an error in my measurement but I know you've been at this enough to have covered that base by this point.

I think mine is set about 0.5 degree low, I think I'm gonna dry dead-on first and if it doesn't get better I'll start going the other way. I can't imagine how going higher could possibly be better but I wouldn't have guessed 4.5 degrees would work, either.
Just to clarify, my vibes still kick in at 65 mph and faster, so still no solution.
 

freedom_in_4low

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The rear shaft showed up already and I have a 120 mile round trip I need to do for work in the morning that I didn't want to do with front wheel drive so I just put it in. Pinion is dead on with the driveshaft, or maybe 0.1 low. I left it there. It had new weights and a new coat of paint on it so it seems even though they told me it was as good as they could get it, they rebalanced it anyway. They also sent an invoice for $0. Can't ask for better customer service than that!

Front pinion was about 1.4 lower than the shaft so I raised it to just 0.2 low, leaving me with 4.5 deg of caster.

I'll see how it goes in the morning!
 
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freedom_in_4low

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no dice. Can't tell that anything changed (other than a noticeable reduction in return-to-center from losing 1.3 deg of caster).

Guess I'm going to disconnect and tie up the front driveshaft and start trying to tune the rear. Might have my wife sit in the rig while I do the measurements to see if it changes much. Maybe the pinion being dead-on with nobody inside makes it too high with a driver, though it's probably a stretch since I haven't noticed any difference between a full and empty tank of gas.
 
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freedom_in_4low

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well this is interesting.

I went to make an adjustment to my pinion angle today. As I've always done, I use the magnetic side in both spots - which means for the driveshaft I get a number around 11 and on the back of the axle I get something more like 79, and subtract it from 90 to get the number to compare.

After loosening my UCA jam nuts, I stuck the angle finder on top of the driveshaft instead of hanging it from the bottom...and the reading was off by as much as a degree or MORE depending on the orientation of the angle finder!!! So I started taking measurements with it in the same orientation, so I would set it on top of the shaft, and then hold the side of it against the axle housing...and my readings indicated that my pinion was over a degree too high. So I've now adjusted it to be about 0.3 low, using consistent orientation of the angle finder.

I went and checked the front, and though I don't have time right now to adjust it, found that it's about 0.2 degrees high instead of the 0.2 low I thought it was. So at the very least, I may be able to get half a degree of caster back.
 

freedom_in_4low

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wow this is frustrating.

I've disconnected the front shaft and still have a pulsing vibration with only the rear installed. I've been through pinion angle adjustments in roughly half degree increments from 0.2° below driveshaft to 1.4°. The best was at around 0.9° but it was still there. Interestingly, there seems to be a peak right around 75mph where intensity is unaffected by pinion angle, but at 71mph, the intensity was almost double at -1.4 than it was at -0.9. This leads me to believe there is something in the rig that likes to vibrate at around 67Hz and I'm picking that up. After upgrading to 35's, this peak should move to 82mph, which I pretty much never get where I live now, but after the move to Oklahoma this summer my wheeling options are going to be a couple hours away and being able to take the interstate with it's 80mph speed limit would be nice.

So I guess I'm gonna take the pinion back to 0.9° low, put the front shaft in and see where I'm at. If I can't tune the vibes out even with the front shaft out, I don't have a lot of hope for the locking hubs option. It makes me sick to my stomach, but I'm already thinking if 35's don't take care of it I may eventually end up re-gearing again, back down to 4.56, which will still be ok with the NSG370 from a highway rpm standpoint, and will bring the driveshaft rpm down so the current 75mph peak would be at 87mph. I just hate the thought of dropping another $3k into something like this that could have gone toward something fun.

Something worth bringing up that I learned in my conversations with the guys at Tom Wood's, is that there's basically one company that's been making driveshaft balancing machines for decades. The vintage determines some of the tech that they have, but they all top out at 3300rpm. That may be something to consider, especially for 42RLE owners, when we give guidance along the lines of "there's no such thing as too much gearing", that we're exceeding the limits of commercially available driveshaft balancing equipment when we pair 35's with anything above 4.56, or 33's with anything above 4.11, and while problems aren't a certainty, we may be setting ourselves up for an elevated risk because of whatever the hell Daimler Chrysler put on our rigs that vibrates at 67Hz.
 
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fuse

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wow this is frustrating.

I've disconnected the front shaft and still have a pulsing vibration with only the rear installed. I've been through pinion angle adjustments in roughly half degree increments from 0.2° below driveshaft to 1.4°. The best was at around 0.9° but it was still there. Interestingly, there seems to be a peak right around 75mph where intensity is unaffected by pinion angle, but at 71mph, the intensity was almost double at -1.4 than it was at -0.9. This leads me to believe there is something in the rig that likes to vibrate at around 67Hz and I'm picking that up. After upgrading to 35's, this peak should move to 82mph, which I pretty much never get where I live now, but after the move to Oklahoma this summer my wheeling options are going to be a couple hours away and being able to take the interstate with it's 80mph speed limit would be nice.

So I guess I'm gonna take the pinion back to 0.9° low, put the front shaft in and see where I'm at. If I can't tune the vibes out even with the front shaft out, I don't have a lot of hope for the locking hubs option. It makes me sick to my stomach, but I'm already thinking if 35's don't take care of it I may eventually end up re-gearing again, back down to 4.56, which will still be ok with the NSG370 from a highway rpm standpoint, and will bring the driveshaft rpm down so the current 75mph peak would be at 87mph. I just hate the thought of dropping another $3k into something like this that could have gone toward something fun.

Something worth bringing up that I learned in my conversations with the guys at Tom Wood's, is that there's basically one company that's been making driveshaft balancing machines for decades. The vintage determines some of the tech that they have, but they all top out at 3300rpm. That may be something to consider, especially for 42RLE owners, when we give guidance along the lines of "there's no such thing as too much gearing", that we're exceeding the limits of commercially available driveshaft balancing equipment when we pair 35's with anything above 4.56, or 33's with anything above 4.11, and while problems aren't a certainty, we may be setting ourselves up for an elevated risk because of whatever the hell Daimler Chrysler put on our rigs that vibrates at 67Hz.
Sorry man. If it's any consolation, I've been through the same mess including a second regear. The math for axle gearing does work. If you decide to go that route, you'll change the speed at which you'll start getting bad vibrations.

My main vibrations are at 32 and 64 Hz, which is pretty close to where you're at. I suspect the "thing" that Jeep put in our rigs that likes to vibrate at that frequency is the motor and transmission. Maybe that much mass on the clevite cushions just likes to vibrate around 65 Hz.
 

bobthetj03

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Feeling your pain brother. I re-geared twice, thinking the setup was messed up the first time, causing the harmonic. I should have re-geared the 2nd time down from 4.56 to 4.10's. Mine kicks in at 65. Front shaft out does nothing. I'm at 4.5* low now, doing the same thing you did starting a 1* high and going down in half degree increments and driving it each time for a day or 3. 4.5* low is the best it's been, but still annoying.
 
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freedom_in_4low

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Sorry man. If it's any consolation, I've been through the same mess including a second regear. The math for axle gearing does work. If you decide to go that route, you'll change the speed at which you'll start getting bad vibrations.

My main vibrations are at 32 and 64 Hz, which is pretty close to where you're at. I suspect the "thing" that Jeep put in our rigs that likes to vibrate at that frequency is the motor and transmission. Maybe that much mass on the clevite cushions just likes to vibrate around 65 Hz.

interesting. I don't have anything at all at 32Hz.

I've picked up some peaks that are definitely tires, because they're ~13Hz or 1/4.88 of the driveshaft frequency. Most interestingly (though not really relevant), is I once picked up something that's exactly 1/2.42 of the driveshaft speed, which would match the planetary gearset in the transfer case.

Now here's where it gets really interesting. There's a 17.99 Hz peak I've picked up at 71mph, 75mph, and 77mph, which I'm thinking must be a resonant frequency of something since it doesn't change with vehicle speed. There's also something that seems to be really close to 29% (1/3.47) of the driveshaft speed or 34% (1/2.88) of engine speed. I haven't figured out what would spin at that speed, but it would match up with the 17.99Hz vibration right around 73mph, which now that I know that, I might start focusing my testing around that point and see if that's where things peak. It's possible I'm just chasing ghosts in the signal with that because the graph just seems very noisy around 18Hz, but who knows. I know that's below the threshold of human hearing but I can't rule out that the vibration is 18Hz and what my ears pick up is the 2nd or 3rd order result of that vibration shaking something else.

Screenshot_20210410-204342.png
 

fuse

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interesting. I don't have anything at all at 32Hz.

I've picked up some peaks that are definitely tires, because they're ~13Hz or 1/4.88 of the driveshaft frequency. Most interestingly (though not really relevant), is I once picked up something that's exactly 1/2.42 of the driveshaft speed, which would match the planetary gearset in the transfer case.

Now here's where it gets really interesting. There's a 17.99 Hz peak I've picked up at 71mph, 75mph, and 77mph, which I'm thinking must be a resonant frequency of something since it doesn't change with vehicle speed. There's also something that seems to be really close to 29% (1/3.47) of the driveshaft speed or 34% (1/2.88) of engine speed. I haven't figured out what would spin at that speed, but it would match up with the 17.99Hz vibration right around 73mph, which now that I know that, I might start focusing my testing around that point and see if that's where things peak. It's possible I'm just chasing ghosts in the signal with that because the graph just seems very noisy around 18Hz, but who knows. I know that's below the threshold of human hearing but I can't rule out that the vibration is 18Hz and what my ears pick up is the 2nd or 3rd order result of that vibration shaking something else.
I think you and I have different transmissions, which would be different gearing but maybe more importantly, different mass. I have the 42RLE.

That's some pretty accurate correlation between vibrations and speed. How are you managing to match up the data?
 

freedom_in_4low

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I think you and I have different transmissions, which would be different gearing but maybe more importantly, different mass. I have the 42RLE.

That's some pretty accurate correlation between vibrations and speed. How are you managing to match up the data?

The app I use lists the 3 highest peaks - and one of them is almost always the driveshaft. From there I can multiply by 60 to get rpm, then use the axle ratio and tire revs/mile to match up the screenshot to what speed I was going. Every time I go out I take multiple readings at different speeds so I can tell if a peak is speed-dependent or independent.

Then I just multiply the driveshaft rpm by my OD ratio of 0.84 to get engine speed and divide by my axle ratio of 4.88 to get tire speed, and see what lines up. At slower speeds, the engine can make it into the top 3 in the 50Hz range. You can see it as a small peak of about .22G just to the left of the 67Hz peak in my screenshot - at that speed, the engine RPM comes out to 56Hz. The tires come through at lower speeds as well - it would have been 13.7Hz in my screenshot but it's dwarfed by the ~18Hz peaks in that specific condition. At that particular pinion angle, the tires (or axle shafts, I suppose) showed up as 13.08 vs a predicted 13.1 at 71.5mph and as 14.1 vs a predicted 14.05Hz at 76.8mph, but the 18Hz peaks drowned them out at 75.

Then, I take any other peaks that don't line up with the engine, driveshaft, or tires and divide them each by each of the engine, driveshaft, or tires to see if they land on a familiar number. That one that I attributed to the planetary set only shows up at certain speeds when some of the other signals are weak enough that it makes it into the top 3, but on that one that it showed up in, it was 25.78Hz which is 1/2.414 from the 62.25Hz that matched up with the driveshaft. So not 2.420000000 on the button, but I think close enough that it seems a reasonable inference.
 
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The app I use lists the 3 highest peaks - and one of them is almost always the driveshaft. From there I can multiply by 60 to get rpm, then use the axle ratio and tire revs/mile to match up the screenshot to what speed I was going. Every time I go out I take multiple readings at different speeds so I can tell if a peak is speed-dependent or independent.
Which app are you using? Maybe i can give that a try.
 

freedom_in_4low

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Which app are you using? Maybe i can give that a try.
It's called vibration isolator pro. I can't take credit though, it was recommended by another member here that I think makes his living dealing with noise and vibration in commercial buildings. @anarce

I'm also open to trying anything else. It would be great to have a live readout instead of just taking aggregated snapshots of 4 second recordings.

Fft spectrum analyzer is another app he recommended that does a live readout, but it does sound, not vibration, and it doesn't nail down the frequency as precisely, rather assigning it to a 4Hz band. When the vibration is particularly intense I can see a spike in the 59, 63 or 67Hz band, depending on my speed, that rises and falls with the oscillation, but it can't hear it over the other noise when just the rear shaft is in.
 

freedom_in_4low

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interesting. I don't have anything at all at 32Hz.

I've picked up some peaks that are definitely tires, because they're ~13Hz or 1/4.88 of the driveshaft frequency. Most interestingly (though not really relevant), is I once picked up something that's exactly 1/2.42 of the driveshaft speed, which would match the planetary gearset in the transfer case.

Now here's where it gets really interesting. There's a 17.99 Hz peak I've picked up at 71mph, 75mph, and 77mph, which I'm thinking must be a resonant frequency of something since it doesn't change with vehicle speed. There's also something that seems to be really close to 29% (1/3.47) of the driveshaft speed or 34% (1/2.88) of engine speed. I haven't figured out what would spin at that speed, but it would match up with the 17.99Hz vibration right around 73mph, which now that I know that, I might start focusing my testing around that point and see if that's where things peak. It's possible I'm just chasing ghosts in the signal with that because the graph just seems very noisy around 18Hz, but who knows. I know that's below the threshold of human hearing but I can't rule out that the vibration is 18Hz and what my ears pick up is the 2nd or 3rd order result of that vibration shaking something else.

View attachment 243197

I think the ~18Hz signals I was picking up may have been an artifact of the center console. I had been taking samples with my phone sitting on the lid. I took some measurements yesterday with my phone sitting on top of the cupholders, with the rubber insert removed, and got consistently clear peaks at the engine rpm, and two very near the driveshaft rpm, though one is 0.08% slower than the other, so I don't know which one is the rear. The difference ends up about 0.5Hz, which seems to align with feeling/hearing about 2 seconds between peaks in the oscillation. What's also curious is that between those two near-driveshaft peaks that are 0.08% apart, the faster one increases with speed, while the slower one peaks around 73mph and starts dropping off after that. I don't know if the faster one might also start dropping off at some point, but I don't really go past 80mph so I probably won't find out. Intestingly, the 0.8% correlation seems to break down at higher speed...between 73 and 77 it grew to 1.4%, which makes me think maybe the lower speed is the front axle, and it's slowing down as increased aerodynamic resistance increases chassis squat. Currently, the front driveshaft is not installed, so if anything does appear related to the front axle, then the only thing that could be contributing is the pinion shaft or pinion yoke.

I've taken a bit of a break from working with the pinion angle, partly because I've been busy getting my house ready to sell, partly because the pinion angle is likely to change after I install the MML sitting on my work bench, and partly because the app is picking up first order frequencies of the driveshaft, and a vibration induced by an angle issue should be a second order frequency.

For my next round I'm going to play with tire pressures front vs rear. If one of the peaks I'm getting is the front axle, then that 0.8% difference should change when I change the rolling radius of the tires on that axle. If it doesn't change, then the source has to be something else. I'm also going to spin the driveshaft 180deg at the transfer case output because I read a post yesterday where that worked for someone.
 
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I have a new crackpot theory. No need to talk me out of this one because I'm just going to test it this afternoon.

I have a UCF engine skid and it creaks when I'm making a sharp turn or flexing the suspension. I've chalked that up to the front end of the frame flexing just enough to cause the skid to shift. So it creaks. I've tried to fix that, but the creaking comes back after a few trail runs. No big deal once you know where the noise is coming from.

What if that frame section is also resonant at some of the drivetrain frequencies? Maybe that's where the vibrations are coming from?

The UCF skid bolts to the transfer case skid and to supports that attach to the engine mounts. It's obviously not strong enough to keep those points from flexing in relation to each other. The stock transmission skid also bolts to the transfer case skid, but it has a crossmember that bolts directly to the frame right in the section that's flexing when I turn or flex the axles. The UCF skid does not have a crossmember. If that frame section were vibrating, the crossmember would likely change the resonant frequency.

I have to take the UCF skid off today for maintenance. I have the OE transmission skid in storage. I'm just going to bolt it back in and see if it makes a difference.
 

freedom_in_4low

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I have a new crackpot theory. No need to talk me out of this one because I'm just going to test it this afternoon.

I have a UCF engine skid and it creaks when I'm making a sharp turn or flexing the suspension. I've chalked that up to the front end of the frame flexing just enough to cause the skid to shift. So it creaks. I've tried to fix that, but the creaking comes back after a few trail runs. No big deal once you know where the noise is coming from.

What if that frame section is also resonant at some of the drivetrain frequencies? Maybe that's where the vibrations are coming from?

The UCF skid bolts to the transfer case skid and to supports that attach to the engine mounts. It's obviously not strong enough to keep those points from flexing in relation to each other. The stock transmission skid also bolts to the transfer case skid, but it has a crossmember that bolts directly to the frame right in the section that's flexing when I turn or flex the axles. The UCF skid does not have a crossmember. If that frame section were vibrating, the crossmember would likely change the resonant frequency.

I have to take the UCF skid off today for maintenance. I have the OE transmission skid in storage. I'm just going to bolt it back in and see if it makes a difference.
I don't think you're far off, but I suspect it's the rear. We don't seem to hear of this happening on 97-02 rigs (rarely enough that when it happens is could very well be a completely different mechanism), which had a T case skid with an asymmetrical bolt pattern. I've been wondering if the back half of the frame is acting like a tuning fork and 03-06 get the oscillations because the two sides are the same length and vibrating at very similar frequency, whereas the older models had more separation.

If my 1/4" steel skid doesn't dampen any of it out, I may bolt some weight onto one side of the back half, and if that works, I might leave it or i might start thinking about moving the passenger side rear skid nutsert a couple inches forward.
 

fuse

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I don't think you're far off, but I suspect it's the rear. We don't seem to hear of this happening on 97-02 rigs (rarely enough that when it happens is could very well be a completely different mechanism), which had a T case skid with an asymmetrical bolt pattern. I've been wondering if the back half of the frame is acting like a tuning fork and 03-06 get the oscillations because the two sides are the same length and vibrating at very similar frequency, whereas the older models had more separation.

If my 1/4" steel skid doesn't dampen any of it out, I may bolt some weight onto one side of the back half, and if that works, I might leave it or i might start thinking about moving the passenger side rear skid nutsert a couple inches forward.
Well, I can confirm that reinstalling the OE transmission skid did not affect the vibrations.
 
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freedom_in_4low

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it's premature to break out the champagne flutes, but I did have my LJ up to 71 after installing the UCF Extra clearance (1/4" steel) skid without discernible vibration. Being that it was coming in around 67 beforehand, I'm optimistic. I need to spend some time on a real highway to know for sure.

Since I changed my rear driveshaft angle by about 4 degrees (set the pinion back to half deg below driveshaft), and changed the front driveshaft by about 3 degrees without changing the pinion angle (now about 3 degrees below driveshaft), I can't for certain say it's the weight or stiffness of the skid vs just blindly stumbling my way into the right combination of angles...but if the vibes are gone at least up to 75, I'll take it. If they're gone up to 80, I'll buy lunch for the first forum member that's had this issue that finds themselves in the same metropolitan statistical area with me.
 
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