Harmonic vibrations in Jeeps: A new theory (please read!)
- Thread starter psrivats
- Start date
There was discussion in my thread about the skid plate and it effects on the harmonics. This was why I decided to try the Barnes skid.
https://www.barnes4wd.com/Jeep-TJ-Bolt-On-Skid-Plate_p_867.html
It was 1/4" plate steel, heavier, and more rigid. It also raised the TC up 1". With my focus on the TC front output yoke, I was hopeful that by raising the TC up slightly, I would effectively be lowering the angle of the front yoke a bit. With pinion angles still in my head, the combination of the thicker skid and raising the TC might have bring on a change. After readjusting my pinion angles, no change in harmonics were noticed. This was disappointing to say the least. This led me to move on and remove the front DS.
https://www.barnes4wd.com/Jeep-TJ-Bolt-On-Skid-Plate_p_867.html
It was 1/4" plate steel, heavier, and more rigid. It also raised the TC up 1". With my focus on the TC front output yoke, I was hopeful that by raising the TC up slightly, I would effectively be lowering the angle of the front yoke a bit. With pinion angles still in my head, the combination of the thicker skid and raising the TC might have bring on a change. After readjusting my pinion angles, no change in harmonics were noticed. This was disappointing to say the least. This led me to move on and remove the front DS.
The only thing I would point out is that I don't know what raising the TC 1" did (ie it was another variable). Other than that I'm also not sure why it not have an effect.
Without front DS with factory skid vs Barnes skid, is there no appreciable difference at all? Hope my question makes sense.
Really curious what happens with your new Tom wood driveshaft.
Resonance is resonance, electrical or mechanical. You well know that electrical and mechanical systems have analogues of each other. This is indeed a complex system with many interactions.
But keep thinking and keep grasping at straws, you may be more qualified than anyone here to address this! I'll go read the article you posted.
Jeep figured this out long time ago. They did not make these changes randomly.
I don't know about the most qualified...I'd say barely qualified. IF we can relate it to engine RPM, and calculate the resonance frequency of the drivetrain components...THEN we should be able to calculate and / or design a new harmonic balancer based on that information. That is a BIG if though. How do you calculate the resonance frequency of the transmission, for instance? Its very complicated assembly. Maybe Jim Frens can help us out there. Or Maybe we can back calculate it based on what we already know? I.E. if we can confirm the Frequency of the current balancer, we can apply a ratio, and come up with a new value, based on the new RPM's?
The back calculation is the way to go at this stage. The whole system is very complex obviously. The design of 241 TC vs 231 T balancers tells me that the frequency numbers are slightly different (ie resonance appears at lower rpms).
The 55Hz number we know for the 231TC, so let's starts with that. Let's assume someone with a 5spd and 33s/4.88s sees resonance vibrations at 65mph. Grimmjeeper should tell us the engine rpms.
Now assume a mass of x for the balancer (I am seeing 2.27kg as the weight online, perhaps we can get an approx weight from @bobthetj03 ) . Let's assume the very basic freq inversely proportional to sqrt(m). How much do we need to change the mass to push the resonance freq to say 80mph? That's the question to answer.
Or use @jjvw's real world numbers that he posted in the previous page. BTW, @jjvw did you go see if you still have the two parts of the balancer in your Jeep? I confirmed that mine does even with the aftermarket driveshaft. This is mine (@Chris cleverly screenshotted it from a video that Dave sent).
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@Ranger_b0b I read that article. Very good primer, thanks for sharing. It's more for crankshaft damper looks like but the math perhaps is still relevant since we see dealing with the same problem at the TC rear output.
The reason why your calculations show a different number (~85 or ~100 compared to 55Hz) simply could be because it's the 241 TC+42RLE with it's problematic 0.69 OD. The 55Hz number we have is for the 231 TC. Can you a similar rough calc for a non-rubicon? Pick any trans or tire choice, the resulting rpms will automatically take care of it. Let's see if the number come closer to 55.
The reason why your calculations show a different number (~85 or ~100 compared to 55Hz) simply could be because it's the 241 TC+42RLE with it's problematic 0.69 OD. The 55Hz number we have is for the 231 TC. Can you a similar rough calc for a non-rubicon? Pick any trans or tire choice, the resulting rpms will automatically take care of it. Let's see if the number come closer to 55.
Ok. Here is a calculation for a 4.0L with a 231 Tcase, NV3550 transmission, 33" (32" actual diameter) tires, and 3.73 gears...
(6*1715)/120 = 85.75 Hz.
with standard 29" tires (so basically stock)
(6*1901)/120 = 95.05 Hz
So...We're starting to build a pattern here, even though it doesn't Match the frequency of the harmonic damper. Seems to be that most combinations put you in the 85 to 95 Hz range. I'm in the middle of cooking dinner right now, but I'll look back through the thread and run some calcs on those guys that are having problems. I'm thinking once you break the 100 hz barrier, you have issues.
@Ranger_b0b @Chris One important item in all this ...
The FSM says the balancer is put only on Jeeps with the 4.0L in any of the years they put it on. The 2.5L engine models did not get them in any year. @Chris this should answer your question from yesterday.
@Ranger_b0b looking at that article it seems 4cyl vs 6cyl matters, right? Does what you see in that article and what we physically have in our Jeeps in agreement wrt freq/rpms you calculate?
The FSM says the balancer is put only on Jeeps with the 4.0L in any of the years they put it on. The 2.5L engine models did not get them in any year. @Chris this should answer your question from yesterday.
@Ranger_b0b looking at that article it seems 4cyl vs 6cyl matters, right? Does what you see in that article and what we physically have in our Jeeps in agreement wrt freq/rpms you calculate?
This addition interests me, if it cut down on vibes then I'd do it for sure.
https://www.tmrcustoms.com/axle-dri...-transmission-transfer-case-crossmember-mount
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Yup. There is no mention of the 2.5Ls when it comes to the damper in the FSMs. See below.
I'm at the shop now re-sealing my leaking rear diff cover. I'll grab the balancer and take it home to put it on the scale.
Not that my butt dyno could differentiate. My thought process for raising the skid height is based on the fact that the drivetrain is angled down with the motor mounts acting as a pendulum. The TC input yoke is in line with the drive train. raise the rear of the pendulum and you are effectively changing the angle at which that input yoke is pointing in relation to the H on the cardan joint.
The balancer weighs in at 7lbs. Now that's on a digital bathroom scale that weighs in half lb. increments, so there could be some ounces there. I also noticed there was a hole drilled in the face of the balancer as if a small amount of weight needed removed as if they were balancing it to the assy.