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

[freedom_in_4low]

Is it a component or system that needs to be altered?

technically, it could be either, but due to the frequencies involved, my inclination would be to rule out high rigidity, high mass things like axles, engines and gearboxes and look into more flexible components like coil springs, transfer case skids, frames or frame crossmembers.

But as I said in my long post, resonance is not a front runner for this in my eye because a resonance issue should come and go within a well defined range of speed, and that is not how this issue has been described.

 

[bobthetj03]

I was hoping that changing the weight/thickness/composition of the t-case skid might help, so I went with a 1/4" thick Barnes skid. Couldn't notice any difference, at least none I could feel.

 

[anarce]

technically, it could be either, but due to the frequencies involved, my inclination would be to rule out high rigidity, high mass things like axles, engines and gearboxes and look into more flexible components like coil springs, transfer case skids, frames or frame crossmembers.

But as I said in my long post, resonance is not a front runner for this in my eye because a resonance issue should come and go within a well defined range of speed, and that is not how this issue has been described.

Yeah, resonance will typically happen in a rather narrow frequency band, but if the system is partially damped (e.g. by springs and shocks) this tends to reduce the amplitude of the resonance but spread out the frequency range where it can happen.

We have worked on a number of hotels with Milnor extractors (giant commercial washing machines for the sheets and towels), and they ramp up to a high speed during the extraction cycle (spin the drum really fast to get the water out) and then spin down. If the load is imbalanced a little bit, this can create some really nasty vibrations because you have a lot of mass spinning really fast. And if that vibration aligns with the resonant frequency of the building structure, look out! In one hotel the laundry was on a suspended slab above the parking garage, and the floor would vibrate up and down so severely that people were afraid to be in the room. It didn't happen the whole time though; just at a certain rpm as the spinning increased and then again when it slowed down. At higher and lower speeds it wasn't an issue. That is typical of a resonance condition.

For a resonance problem there can be a few solutions:
1. Stop the thing that's generating the vibration
2. Isolate the vibration source from the rest of the system (e.g. we put a complex isolation system on the extractor)
3. Change the natural frequency of the system (we have done this before by changing the stiffness of the floor, shortening the structural span, etc. - probably harder to do in a vehicle)
4. Add damping to reduce the amplitude at resonance (limp and heavy materials work best, or you could try tuned mass dampers for a fancy solution. Chevy put them in the first gen Camaro convertibles and people took them out because they were heavy and they wanted to go faster: http://www.chevy-camaro.com/faq/chevy-camaro-convertible-cocktail-shakers.shtm)

Anyway, just some more food for thought...

 

[Captain Phoenix]

I need to regear mine soon and I'm waiting to see the result because being to convert to locking hubs would be a pain.

What would the drive shaft speed difference be between running in 3rd gear at 3,000-3,200 RPM on the highway with my current 4.10 gears vs 2,500 RPM in OD with 5.38 gears?

 

[Mike_H]

More likely with NP241OR: I can't think of a good reason the transfer case should play a role in this short of some significant imbalance on the front output shaft assembly which seems highly unlikely. However, the TJ's that came from the factory with this case also came with a Dana 44 front axle, meaning they always have the same tooth count in both axles. In a perfect world (that mythical frictionless plane in a vacuum they always talked about in physics class) the front and rear shaft should spin in unison like those perfectly tuned guitar strings, but in reality, any slight steering angle to correct for road crown, or a slight difference in rolling radius of the tires due to variances in inflation pressure or vehicle weight distribution is going to cause those driveshaft speeds to shift slightly off each other. A Dana 30 with 5.13 or higher will be in the same position, but a 4.88 or numerically lower ratio will have a slightly different tooth count than it's counterpart in the rear, which will create a larger difference in driveshaft speeds and may simply move the frequency of the wave pattern into a range that isn't perceptible or can't excite whatever component is responding to the more closely matching speeds. I don't know where it was, but I feel like I read a post where someone said the pulse went away or froze at a constant hum at whatever intensity it was at when the case was shifted into 4wd. A hall effect tachometer on each t-case output yoke would help explore this, but in theory I would think one could manipulate the pulse interval with tire pressure. A half inch decrease in rolling radius of one tire ought to make enough difference to notice.

This is a new thought, and very interesting. I have noticed it seems to be Rubicons that are most apt to have vibs after regear, aside from @bobthetj03 and his unicorn. The Dana 44 front being the exact same ratio as the rear could be setting up some weirdness. It's also an easy test by just deflating tires.

The solution at that point really is to install a front hub kit, or, I guess, swap axles! HP30? Lol

The other thing related to the NP241OR t-case is the giant planetary set they have to give the 4:1 ratio. That is a lot more inertia than the standard set, and if the case is flexing, I can see run-out or something causing a vibe issue. That's is just a guess tho, as I'm not sure if that while thing is doing all the time or just when you're in 4x4.

 

[jodomcfrodo]

I need to regear mine soon and I'm waiting to see the result because being to convert to locking hubs would be a pain.

What would the drive shaft speed difference be between running in 3rd gear at 3,000-3,200 RPM on the highway with my current gears vs 2,500 RPM in OD?

Driveshaft speed isn't dependent on the gear the transmission is in, only the tire size, speed, and gear ratio.

 

[Captain Phoenix]

Driveshaft speed isn't dependent on the gear the transmission is in

That's not exactly true.

the tire size, speed, and gear ratio.

Yes, that's a simpler way of looking at it, thanks. I was over complicating things in my head.

So going from 4.10 to 5.38 will be a significant driveshaft rpm increase for the same highway speed and tire size. Maybe if I get vibes I'll go with a new, quality front drive shaft and see if it's a balance problem.

 

[jodomcfrodo]

That's not exactly true.

Nope. Shifting gears changes the speed of your engine, not the speed of your driveshaft. Your tires are mechanically connected to the driveshaft via the differential ratio. Assuming the tires are the same size, the driveshaft speed cannot change unless you change the speed of the vehicle or the differential ratio.

So going from 4.10 to 5.38 will be a significant driveshaft rpm increase for the same highway speed and tire size. Maybe if I get vibes I'll go with a new, quality front drive shaft and see if it's a balance problem.

Going from 4.10 to 5.38 will increase your driveshaft speed by 1.31. If you have vibes at 78.73 mph with 4.10's, your vibes will now occur at 60 mph, assuming those vibes are driveshaft dependent.

 

[Captain Phoenix]

Shifting gears changes the speed of your engine, not the speed of your driveshaft.

Your engine and transmission are what turn your driveshaft. If you do the calculations you can calculate driveshaft rpm based on engine rpm and transmission gear ratio. Driveshaft speed is what turns your tires at a certain rpm to propel the vehicle.

It's just that the math is easier to think about going the other way and I hadn't thought of that until you pointed it out.

 

[Captain Phoenix]

Going from 4.10 to 5.38 will increase your driveshaft speed by 1.31. If you have vibes at 78.73 mph with 4.10's, your vibes will now occur at 60 mph, assuming those vibes are driveshaft dependent.

I don't recall vibes at 75-80 mph when I drove it home in the interstate. A soft top is so loud that I'm not sure I would have noticed.

 

[jodomcfrodo]

Your engine and transmission are what turn your driveshaft. If you do the calculations you can calculate driveshaft rpm based on engine rpm and transmission gear ratio. Driveshaft speed is what turns your tires at a certain rpm to propel the vehicle.

It's just that the math is easier to think about going the other way and I hadn't thought of that until you pointed it out.

Correct. But again, when you shift gears the driveshaft speed does not change. I understand how all the calculations work. The change in engine speed is directly proportional to the change in the transmission ratio.

 

[Captain Phoenix]

But again, when you shift gears the driveshaft speed does not change.

Only if you have the same differential gears. My question was 3,200 rpm in 3rd gear with 4.10 diffs vs 2,400 rpm in 4th gear with 5.38 diffs.

I was overthinking it because as you pointed out, since the highway speed should be the same the driveshaft rpm should be the same.

 

[jodomcfrodo]

Only if you have the same differential gears. My question was 3,200 rpm in 3rd gear with 4.10 diffs vs 2,400 rpm in 4th gear with 5.38 diffs.

I was overthinking it because as you pointed out, since the highway speed should be the same the driveshaft rpm should be the same.

Gotcha. I was confused on the question.

 

[Captain Phoenix]

Gotcha. I was confused on the question.

Because I didn't complete my thought in my first comment.

 

[bobthetj03]

I don't recall vibes at 75-80 mph when I drove it home in the interstate. A soft top is so loud that I'm not sure I would have noticed.

This. The hard top amplifies the vibes.

 

[JMT]

Harmonic vibes under acceleration at 62+mph after full TT. 6-speed, 4.56 gears,

I haven’t done much diagnosing yet, hopefully I can follow sone of the good advice in this thread and dial them out.

 

[fuse]

I'm joining the club.

2005 Sport
Dana 30 Front (new hubs and u-joints), Dana 44 Rear (new bearings)
4.56 Gears
E-Lockers
Adams Driveshafts front and rear, both have been properly balanced
Savvy control arms, so the pinion angles are dead on (but the control arm bushings are poly)
NP231 transfer case with Advance Adapters SYE (rebuilt and runout on shafts checked)
42RLE transmission
Motor mount lift with new Crown motor mounts
Stock transfer case skid with a new transmission cushion

Sitting on 33s now, but the vibrations started while I was still on 31s.

Like @bobthetj03, my vibrations are only related to the speed of the driveshafts and everything directly connected to them. Engine RPMs and transmission shifts don't have any effect on the vibrations at all. Also like others here, my vibrations go away if I pull out the front driveshaft.

While I was diagnosing the problem, I got some vibration data from the Spectroid app on my Android phone. Here's what it looks like.

The peak at 64 Hz is ordinary engine noise. The peak at 32 Hz is the driveline vibration hum. It actually starts at a slightly lower frequency as I accelerate into the range where the vibrations pick up, then moves up to 32 Hz as the vibrations get worse. If I accelerate more, the amplitude increases but the frequency stays at 32 Hz.

Also, for what it's worth, I tried swapping in a Prothane transmission cushion, thinking that the polyurethane might make a difference. It did. Instead of a warm, sine wave hum I got a harsh, louder square wave vibration. That was not an improvement, so I switched back to the rubber cushion.

 

[fuse]

Like most people on this thread, I think I've gone through all the obvious fixes for the vibrations. I still wish I could make things better, and that's got me thinking about some unconventional fixes.

Maybe you guys can talk me out of these crackpot ideas before I waste my time and money on things that aren't going to work.

Crackpot Idea #1: Switch back to Clevite bushings in the lower control arms.

I have Savvy control arms in right now with Johnny Joints at both ends. I know from when I installed them that they do transmit more driveline vibrations than the stock control arms with Clevite bushings because I originally installed them at stock length and noticed a little more vibration from the (now replaced) stock driveshaft. I was able to fix those vibrations with some pinion angle adjustment, though.

Maybe if I went back to stock lower control arms and kept the Savvy upper control arms, the Clevite bushings could soak up some of the current vibrations.

Is anyone on this thread running control arms with Clevite bushings?

Crackpot Idea #2: Change the transfer case output angles.

This one is pretty far fetched, so put on your tin hat. Maybe some (or all?) of the vibration comes from constructive interference between the DC joints at the front and rear transfer case outputs. If that's the case, changing the transfer case output angles could affect the vibrations.

There are a couple of ways I could try to change the transfer case output angles. I could take out my motor mount lift. I could drop the transfer case skid (currently bolted up to the frame). Or I could change front/rear suspension height by taking out the front leveling pucks or adding spacers to the rear springs. That's all kind of a pain because then I have to go adjust pinion angles too.

Does anyone want to compare transfer case output and driveshaft angles? Or is this idea just nuts?

Crackpot Idea #3: Get the transmission rebuilt.

The vibrations are clearly linked to driveshaft speed. I've rebuilt or replaced pretty much everything that spins at that rate except the transmission output. Maybe that's contributing to the vibrations.

Has anyone here rebuilt their transmission recently?

I haven't done any of these things because I think the justification for them is pretty thin. Tell me I should just forget all this and save my money for locking hubs.

 

[bobthetj03]

Like most people on this thread, I think I've gone through all the obvious fixes for the vibrations. I still wish I could make things better, and that's got me thinking about some unconventional fixes.

Maybe you guys can talk me out of these crackpot ideas before I waste my time and money on things that aren't going to work.

Crackpot Idea #1: Switch back to Clevite bushings in the lower control arms.

I have Savvy control arms in right now with Johnny Joints at both ends. I know from when I installed them that they do transmit more driveline vibrations than the stock control arms with Clevite bushings because I originally installed them at stock length and noticed a little more vibration from the (now replaced) stock driveshaft. I was able to fix those vibrations with some pinion angle adjustment, though.

Maybe if I went back to stock lower control arms and kept the Savvy upper control arms, the Clevite bushings could soak up some of the current vibrations.

Is anyone on this thread running control arms with Clevite bushings?

Crackpot Idea #2: Change the transfer case output angles.

This one is pretty far fetched, so put on your tin hat. Maybe some (or all?) of the vibration comes from constructive interference between the DC joints at the front and rear transfer case outputs. If that's the case, changing the transfer case output angles could affect the vibrations.

There are a couple of ways I could try to change the transfer case output angles. I could take out my motor mount lift. I could drop the transfer case skid (currently bolted up to the frame). Or I could change front/rear suspension height by taking out the front leveling pucks or adding spacers to the rear springs. That's all kind of a pain because then I have to go adjust pinion angles too.

Does anyone want to compare transfer case output and driveshaft angles? Or is this idea just nuts?

Crackpot Idea #3: Get the transmission rebuilt.

The vibrations are clearly linked to driveshaft speed. I've rebuilt or replaced pretty much everything that spins at that rate except the transmission output. Maybe that's contributing to the vibrations.

Has anyone here rebuilt their transmission recently?

I haven't done any of these things because I think the justification for them is pretty thin. Tell me I should just forget all this and save my money for locking hubs.

CPI #1: I've swapped out all control arms with MC arms, and IMO the DF bushings are as soft, or maybe softer than factory Clevites. At one point I had Currie lowers and factory Clevite uppers. Switching to the MC arms made little to no difference.

CPI #2: Adding a MML made the biggest gains for my particular rainbow unicorn problem. I've since dropped the t-case skid 1/4" at a time with washer down to 3/4" drop with no perceivable gains, adjusting the pinion angle at each change. I've changed the pinion angle all over the place with no luck.

CPI #3: I've had both axles re-geared a second time by professionals, and every internal component in the rear axle replaced, had the t-case rebuilt when the SYE was installed, including a new chain. The transmission and the engine are the only thing left not rebuilt, so you could have something there.

 

[freedom_in_4low]

I don't think any of your ideas are totally off the wall. #3 is a little flimsy for me just because the transmission output shaft diameter is so small and so precisely machined that it's unlikely to produce a vibration at the speeds it rotates at.

I'm running clevite bushings in the axle ends of my LCA's. I did not have this vibration with 4.56, but I'm mid-regear to 4.88 and in the process of completely switching rigs from a 99 with AX15 to an 06 LJ with a 6 speed, keeping the same axles and 231.