Oscillating Harmonic Vibes

[joelachr]

It should not, but that is not always the case.
There are people here, including me, who started to have vibes after taking ds out and a simple 180 spin at the tcase yoke was the solution.

I'll second this. Doing this on the axle end of the front driveshaft noticeably reduced the intensity. The post with my vibration analysis screenshots is here. If I remember correctly, the factory service manual states to mark the orientation of the driveshafts at both ends prior to removal so it can be installed back in the same orientation.

 

[freedom_in_4low]

I'm jealous. I haven't even got the pulsing vibes out of mine with just the rear installed, let alone the front.

I have 5.25" BS wheels, so the extra space of the hub kit would actually work out pretty well, and I'd just live with the rear being a little narrower, or maybe just run spacers back there. I'd have ordered the kit already if it wasn't still buzzing with just the rear installed.

Since I have a BL and MML sitting on my work bench, I'm not even sure I want to take the time to work my pinion angle anymore if I might have to just do it again when it changes due to the MML.

 

[psrivats]

That makes sense. Replace all 3 with these? I have a 2005X with OEM front driveshaft

https://www.dennysdriveshaft.com/p3...non_greaseable_u_joint_fits_1997_to_2006.html

@JMT, I did all this and more. I checked so many things, put in so many new parts (inl a new harmonic balancer) and got my driveshafts rebuilt 2x and pinion angles checked so many times. Just talk to Dave Kishpaugh and he can tell you how much time we spent on this. I even went as far as swapping a tcase and trans from a known good jeep and testing to see if the vibes went away.

The only things that helped were (a) heavy skid plate (b) noico. One added more mass and one helped muffle the vibration. Nothing else, I mean NOTHING else really did anything meaningful.

I had the vibrations with either driveshaft out. After I put in the rubicrawler, something about the mass/harmonics in the vehicle perhaps changed and after that I had no vibes without front driveshaft. Honestly I don't even know if it was the rubicrawler. I just happened to balance the driveshafts again and test again at the time of rubicrawler install since we had the driveshafts out and I realized I can put a hub kit and put this behind me.

The vibration is structural. It will not go away without major re-engineering that we cannot do. The only viable fix is a hub kit, if your vibes are not in the rear like @bobthetj03's jeep.

 

[freedom_in_4low]

The only things that helped were (a) heavy skid plate (b) noico. One added more mass and one helped muffle the vibration. Nothing else, I mean NOTHING else really did anything meaningful.

this is the first I've heard of the skid plate changing things. Might make a good argument to go steel instead of aluminum, if having this issue.

 

[JMT]

this is the first I've heard of the skid plate changing things. Might make a good argument to go steel instead of aluminum, if having this issue.

I think @bobthetj03 also got slightly better after changing to the Barnes skid plate. It has something to do with the material and the frequency transmitted when all the parts work together, so I'm not sure which would play better with all your parts. Could be steel, could be aluminum, two different materials that transmit different frequencies. I think you're wanting to cancel out a frequency, but seems like an endless guessing game.

 

[eastbloc]

If you do the channel solution, how will the splined shaft out of the transmission fit into the TCase? Seems to me that the engine, tranny, and TCase all move together or don't move at all. Maybe I'm missing something.

Are we on the same page?
This is what i am talking about, make 4 cuts probably no longer than diameter of nuts, it does not need to move much. Everything will shift to the side as one

 

[JMT]

Are we on the same page?
This is what i am talking about, make 4 cuts probably no longer than diameter of nuts, it does not need to move much. Everything will shift to the side as one

View attachment 249629

View attachment 249632

We are now, thanks, I see what you mean. Just not sure how much a small shift like that would help. People do the 8.8 swap all the time, and the shaft shifts over to the 8.8 much more dramatically than the shift you are describing.

 

[Gollywomper]

Dont give up though. Most can track down and eliminate the vibes. It is more rare that they cannot be remedied through the normal course of actions.

 

[freedom_in_4low]

The only things that helped were (a) heavy skid plate (b) noico. One added more mass and one helped muffle the vibration. Nothing else, I mean NOTHING else really did anything meaningful.

I think @bobthetj03 also got slightly better after changing to the Barnes skid plate. It has something to do with the material and the frequency transmitted when all the parts work together, so I'm not sure which would play better with all your parts. Could be steel, could be aluminum, two different materials that transmit different frequencies. I think you're wanting to cancel out a frequency, but seems like an endless guessing game.

I'll be installing a body lift and MML this week, and now I'm toying with the idea of a skid plate. UCF's aluminum extra-clearance has been the plan for a long time but now that I've got this issue it's tempting to go with 1/4" steel instead. I'd probably still go with the UCF extra clearance; as attractive as Barnes price is, I'm not sure I want to deal with having to drill my own trans mount holes and I'm not sure how I feel about the fact that instead of including a support channel in the skid design, they offer weld-in braces sold separately.

At any rate, it's probably premature to start throwing money at it before I've really done everything I can to figure it out. It's just tempting to do every mod that has any impact on my driveshaft all at once so if I end up chasing 20 different variations in pinion angle, I won't have to do it again later.

 

[freedom_in_4low]

The only things that helped were (a) heavy skid plate (b) noico. One added more mass and one helped muffle the vibration. Nothing else, I mean NOTHING else really did anything meaningful.

this is still processing in my head, and I'm gonna use this thread to talk it out with myself.

My understanding of resonant frequencies is based pretty much entirely on stringed musical instruments. When I think about a guitar string - the resonant frequency shifts UP with a reduction in mass, and with an increase in rigidity, which the only analog I can come up with there is the tension in the string (rigidity->resistance to movement->string tension).

So if a skid plate (or any other component) is resonating, you would want to increase the rigidity and/or reduce the mass to move it's resonance upward and out of the range that the driveline can reach (above 70Hz should do it in most cases), or reduce the rigidity/increase the mass to move it downward and into a range where the off-balance forces of the driveshaft aren't intense enough to excite it. Also want to consider that for a given material and thickness, adding mass (such as tacking on some damping weight) might have the opposite effect since it's inertia would mimic a reduction in rigidity. Then the question becomes...which is the right direction to go?

The Barnes skid plate increases mass by being 1/4" thick steel, and who knows about the rigidity. 1/4" thick steel would be more rigid, but it also lacks the stiffening features that the stock skid or the UCF or effectively, the savvy crossmember adds (has anybody with Savvy UA had this problem and experimented with and without the skid installed?). Since it has features placing opposing pressures on the resonant frequency, one is probably winning out and moving it in one direction or the other and none of us probably have the means to figure out exactly which way. If I had the resources, I would love to go for a drive with accelerometers on the skid plate, the transfer case housing, and the rear axle housing.

A 1/4" aluminum skid should reduce mass and increase rigidity, which might move the frequency up and out of range. A 3/16" steel skid would be the opposite and might move it down where the trigger frequency is less intense, but not sure where it would lie compared to a stock skid, which isn't particularly massive but has a lot of stiffening features.

 

[bobthetj03]

I'll be installing a body lift and MML this week, and now I'm toying with the idea of a skid plate. UCF's aluminum extra-clearance has been the plan for a long time but now that I've got this issue it's tempting to go with 1/4" steel instead. I'd probably still go with the UCF extra clearance; as attractive as Barnes price is, I'm not sure I want to deal with having to drill my own trans mount holes and I'm not sure how I feel about the fact that instead of including a support channel in the skid design, they offer weld-in braces sold separately.

At any rate, it's probably premature to start throwing money at it before I've really done everything I can to figure it out. It's just tempting to do every mod that has any impact on my driveshaft all at once so if I end up chasing 20 different variations in pinion angle, I won't have to do it again later.

No drilling required, and gains an inch of belly clearance.

https://www.barnes4wd.com/Jeep-TJ-Bolt-On-Skid-Plate_p_867.html