Harmonic vibrations in Jeeps: A new theory (please read!)

OP
psrivats

psrivats

TJ Addict
Supporting Member
Jun 4, 2018
1,232
OR, USA
@psrivats

Any updates on your Jeep?
I had Dave order the UCF skid (heavier and stiffer with a built in bracing). Shipping was delayed due to the holiday season, but it finally arrived and is getting powdercoated now. I'm going to have this installed in the same location as stock (ie no body lift/tummy tuck). I should know the results later this week. Will update here.
 

Drizit

TJ Addict
Feb 18, 2018
1,126
Canada
I talked to Dave via text about this, and Dave said he can easily build his own crossmember, but he is worried about increased overall NVH due to limited choices in bushings (i.e trading one problem for another). He sent me a photo of one of his designs (pic below).
I run a tmr customs crossmember that's quite similar to the genrite one. I had massive issues with vibration transfer when stopped at lights unless I shifted to neutral (42rle) in order to solve this I flipped the plate from the top of the tube to the bottom and then used another plate to bolt a 1/2ton chev trans mount between the trans and crossmember. Solved that issue but not the droning at speed.

--
There must have been a time
when we could have said no.
 

Beach_vibes

New Member
Feb 12, 2019
12
Melbourne Beach, FL
Hi all,

I am a new member here. I recently picked up a 2003 Rubicon manual with 33's and a Rough Country (junk, soon to be replaced) 4" lift (going with Savvy short arm). Long story short, is that wifey has wanted a Jeep and we finally went to look at this one after I did a bunch of research. Showed up at the previous owners house, checked it out (no frame rust), so we went for a ride with our two girls 5 and 7 in the back. The girls absolutely loved riding in it and had the time of their life in our 5 minute test drive. We made an offer and it was accepted- this is our first Jeep and we are totally loving it! Its just so dam fun, makes you smile!

I have been cruising the tjforums threads and wanted to thank everyone for the wealth of information (esp. Mr Blaine, Jerry, Chris), it is very helpful and my list of mods is increasing daily, lol. Don’t worry I won’t tell my wife it’s your fault !

I found this thread very interesting (I am a dynamics engineer/geekaroid in the aerospace field during the day, and have been wrenching on stuff since I was a kid). I looked through quite a few threads on this subject and hopefully I can add something to it.

From what I see, the TJ has an engine, transmission, and transfer case that are isolated from the fame/tub, with one isolator on each side of the engine and a single isolator below the transmission. Isolators act as damped springs that will attenuate (reduce) higher frequency engine vibration and other vibration sources from the powertrain so that they are not easily transmitted to the frame and tub. The isolation frequency is related to the square root of the isolator stiffness divided by the sprung mass. There are actually six rigid body degrees of freedom for an isolated system- three translations and three rotations or a mix of them. These are called rigid body normal modes, meaning that the engine/trans/transfer case move as a rigid system. So at a certain frequency, the sprung mass will deform in a certain shape- for the TJ, the lowest rigid body mode is likely roll about X (X being fore/aft axis, so roll is rotation about that axis). This is readily observable by looking in the engine bay with the engine running or revving it. All isolators have a tradeoff- they amplify inputs at their own resonant frequency (roll) and isolate at frequencies that are an octave above the highest isolator modal frequency (2x the frequency of isolation). The isolators must be carefully designed in stiffness and damping by engineers to attenuate the higher frequency vibrations (material stiffness and damping). The isolation frequency of the TJ is likely around 5-15 Hz, meaning any vibes above ~30 Hz would ideally start to be damped/attenuated.
However, the engine/transmission/transfer case is not an ideal isolated system as there are additional load and stiffness paths acting on the TJ sprung mass- the drive shafts. The drive shafts are coupled to the differentials torsionally and weakly coupled in other directions through the u-joints. The differential/driveshaft/engine/transmission/transfer case will have (many) flexible body modes, which are related to the structures elasticity and mass. As the same for the rigid body modes of the isolated system, the flexible body modal frequencies will be the square root of generalized stiffness divided by the generalized mass. There will be bending and torsional modes of the structures and combinations therof.

As a first look, of particular interest in this TJ issue is probably the driveline torsional modes and balance, and possibly the transfer case skid flexible body bending mode (Z-direction, up-down). The driveline torsional modes are related to the rotating mass stiffness and mass distribution. The skid plate mode is related to its bending stiffness and the mass of the sprung mass. One thing to note is that since the skid attaches to the transmission through an relatively soft isolator, they are likely weakly coupled at higher frequencies. Additionally, the skid plate structural mass itself is not significant as compared to the sprung mass, so excitation of its own flexible body mode is not likely to have significant impacts unless there are more dynamics going on there that I am not thinking about.

Looking at torsional loads and modes and how it relates to the taller gear change would probably be a better place to look. Putting in a taller ring gear means a smaller pinion diameter, and possibly a change in stiffness of the carrier since the either the ring gear must be translated closer to the smaller pinion gear with a spacer or a thicker gear needs to be installed, which could have an effect on torsional resonances. What I think would be more significant would be the gear backlash of the smaller pinion gear. Is the gear backlash of the smaller pinion greater than the gear backlash of the original? If so we have another issue that would be different with the taller gears, which is a limit cycle type of oscillation due to the higher backlash of the UNLOADED front driveshaft. In the unloaded driveshaft, the pinion may bounce back and forth cyclically and impact the ring gear each time. These impacts themselves could also excite ring gear resonances. We see this in aircraft flying surfaces (rudder/ailerons/elevators)- if the slop or freeplay in the control linkage is too much, it can cause limit cycle oscillations that can quickly turn unstable and you could lose the control surface or more (this has happened to us). The higher the airspeed, the less freeplay that is allowed. In some designs, the control surfaces are toed-in aerodynamically to preload them so they are less susceptible to this phenomenon- similar to how we toe in the front wheels.

Speaking of preload (front driveshaft), does the issue still exist in 4WD at the same speed? Or alternatively has anyone tried to put a stock gear ratio up front but taller gear in rear only (and not engaging 4WD, lol)? If the vibrations go away that could be the issue. That is likely why installing the hub locks works- the front driveshaft is no longer actively transmitting torsional loads and traversing through the front shaft backlash ranges. If the unloaded front driveshaft is spinning through its limit cycles it could be vibrationally exciting significant portions of the driveline, enough to cause concern. Is the backlash of the new gearing being controlled to be equal to or less than the original gear? From what I have read, when only the front driveshaft is installed the issue goes away, likely since it is under load and the backlash is taken up. However, as the pinion gear diameter gets smaller, the backlash angle necessarily gets larger. One thread I read also stated that the problem got worse as the gears wore in- higher wear is likely going to increase backlash. There is no active preloading of the pinion to the ring gear in the stock design, which would help.

Perhaps it is the combination of the higher shaft speed and backlash. At some speed perhaps even the stock backlash causes the same issue, but since the backlash is less(or maybe effective ring/carrier stiffness is more), the excitation frequency is higher and we are not driving our TJ’s at those higher speeds. And maybe there are some diffs that have very tight backlash that are ok?

One other possibility could be the coupling of the two driveshafts at their higher angles due to lift. I think that was probably already debunked by someone who had lifted before going to taller gears.

Oh ya, forgot to mention. I haven’t had my TJ up above about 55mph since my rough country lower control arm balljoint is shot... would be completely unsafe. Can someone tell Savvy to expedite my lift kit orderJ, it got delayed due to King of Hammers apparently.

Discuss!!
 

k-huevo

TJ Enthusiast
Jul 16, 2018
165
Houston
Something else to consider @Beach_vibes, the body/tub is isolated from the frame at 11 points. The transmission mount is less "soft" than the engine mounts. A healthy transmission mount allows very little movement. OMX-ADA's transmission mount is made of very hard material for example.
 

S.McArthur

TJ Enthusiast
May 31, 2018
262
Greenbrier, TN
Hi all,

I am a new member here. I recently picked up a 2003 Rubicon manual with 33's and a Rough Country (junk, soon to be replaced) 4" lift (going with Savvy short arm). Long story short, is that wifey has wanted a Jeep and we finally went to look at this one after I did a bunch of research. Showed up at the previous owners house, checked it out (no frame rust), so we went for a ride with our two girls 5 and 7 in the back. The girls absolutely loved riding in it and had the time of their life in our 5 minute test drive. We made an offer and it was accepted- this is our first Jeep and we are totally loving it! Its just so dam fun, makes you smile!

I have been cruising the tjforums threads and wanted to thank everyone for the wealth of information (esp. Mr Blaine, Jerry, Chris), it is very helpful and my list of mods is increasing daily, lol. Don’t worry I won’t tell my wife it’s your fault !

I found this thread very interesting (I am a dynamics engineer/geekaroid in the aerospace field during the day, and have been wrenching on stuff since I was a kid). I looked through quite a few threads on this subject and hopefully I can add something to it.

From what I see, the TJ has an engine, transmission, and transfer case that are isolated from the fame/tub, with one isolator on each side of the engine and a single isolator below the transmission. Isolators act as damped springs that will attenuate (reduce) higher frequency engine vibration and other vibration sources from the powertrain so that they are not easily transmitted to the frame and tub. The isolation frequency is related to the square root of the isolator stiffness divided by the sprung mass. There are actually six rigid body degrees of freedom for an isolated system- three translations and three rotations or a mix of them. These are called rigid body normal modes, meaning that the engine/trans/transfer case move as a rigid system. So at a certain frequency, the sprung mass will deform in a certain shape- for the TJ, the lowest rigid body mode is likely roll about X (X being fore/aft axis, so roll is rotation about that axis). This is readily observable by looking in the engine bay with the engine running or revving it. All isolators have a tradeoff- they amplify inputs at their own resonant frequency (roll) and isolate at frequencies that are an octave above the highest isolator modal frequency (2x the frequency of isolation). The isolators must be carefully designed in stiffness and damping by engineers to attenuate the higher frequency vibrations (material stiffness and damping). The isolation frequency of the TJ is likely around 5-15 Hz, meaning any vibes above ~30 Hz would ideally start to be damped/attenuated.
However, the engine/transmission/transfer case is not an ideal isolated system as there are additional load and stiffness paths acting on the TJ sprung mass- the drive shafts. The drive shafts are coupled to the differentials torsionally and weakly coupled in other directions through the u-joints. The differential/driveshaft/engine/transmission/transfer case will have (many) flexible body modes, which are related to the structures elasticity and mass. As the same for the rigid body modes of the isolated system, the flexible body modal frequencies will be the square root of generalized stiffness divided by the generalized mass. There will be bending and torsional modes of the structures and combinations therof.

As a first look, of particular interest in this TJ issue is probably the driveline torsional modes and balance, and possibly the transfer case skid flexible body bending mode (Z-direction, up-down). The driveline torsional modes are related to the rotating mass stiffness and mass distribution. The skid plate mode is related to its bending stiffness and the mass of the sprung mass. One thing to note is that since the skid attaches to the transmission through an relatively soft isolator, they are likely weakly coupled at higher frequencies. Additionally, the skid plate structural mass itself is not significant as compared to the sprung mass, so excitation of its own flexible body mode is not likely to have significant impacts unless there are more dynamics going on there that I am not thinking about.

Looking at torsional loads and modes and how it relates to the taller gear change would probably be a better place to look. Putting in a taller ring gear means a smaller pinion diameter, and possibly a change in stiffness of the carrier since the either the ring gear must be translated closer to the smaller pinion gear with a spacer or a thicker gear needs to be installed, which could have an effect on torsional resonances. What I think would be more significant would be the gear backlash of the smaller pinion gear. Is the gear backlash of the smaller pinion greater than the gear backlash of the original? If so we have another issue that would be different with the taller gears, which is a limit cycle type of oscillation due to the higher backlash of the UNLOADED front driveshaft. In the unloaded driveshaft, the pinion may bounce back and forth cyclically and impact the ring gear each time. These impacts themselves could also excite ring gear resonances. We see this in aircraft flying surfaces (rudder/ailerons/elevators)- if the slop or freeplay in the control linkage is too much, it can cause limit cycle oscillations that can quickly turn unstable and you could lose the control surface or more (this has happened to us). The higher the airspeed, the less freeplay that is allowed. In some designs, the control surfaces are toed-in aerodynamically to preload them so they are less susceptible to this phenomenon- similar to how we toe in the front wheels.

Speaking of preload (front driveshaft), does the issue still exist in 4WD at the same speed? Or alternatively has anyone tried to put a stock gear ratio up front but taller gear in rear only (and not engaging 4WD, lol)? If the vibrations go away that could be the issue. That is likely why installing the hub locks works- the front driveshaft is no longer actively transmitting torsional loads and traversing through the front shaft backlash ranges. If the unloaded front driveshaft is spinning through its limit cycles it could be vibrationally exciting significant portions of the driveline, enough to cause concern. Is the backlash of the new gearing being controlled to be equal to or less than the original gear? From what I have read, when only the front driveshaft is installed the issue goes away, likely since it is under load and the backlash is taken up. However, as the pinion gear diameter gets smaller, the backlash angle necessarily gets larger. One thread I read also stated that the problem got worse as the gears wore in- higher wear is likely going to increase backlash. There is no active preloading of the pinion to the ring gear in the stock design, which would help.

Perhaps it is the combination of the higher shaft speed and backlash. At some speed perhaps even the stock backlash causes the same issue, but since the backlash is less(or maybe effective ring/carrier stiffness is more), the excitation frequency is higher and we are not driving our TJ’s at those higher speeds. And maybe there are some diffs that have very tight backlash that are ok?

One other possibility could be the coupling of the two driveshafts at their higher angles due to lift. I think that was probably already debunked by someone who had lifted before going to taller gears.

Oh ya, forgot to mention. I haven’t had my TJ up above about 55mph since my rough country lower control arm balljoint is shot... would be completely unsafe. Can someone tell Savvy to expedite my lift kit orderJ, it got delayed due to King of Hammers apparently.

Discuss!!
77106


I mean, damn, that's the best response I've ever read on a forum. Promote ahead of peers.
 

bobthetj03

vibrajeep
Supporting Member
Ride of the Month Winner
Feb 3, 2017
7,655
NorCal
Lol. Sorry guys, it's hard to resist as I have lived in this world of shock and vibration most of my life! I enjoy it almost as much as wrenching on our new to us TJ and throwing back the beers!
What are your thoughts about those that have done a SYE and have removed the factory damper that is on the TC output? I've read threads of folks that did not re-gear but did a SYE and developed a harmonic, cyclic vibe above 50 mph.
 

Beach_vibes

New Member
Feb 12, 2019
12
Melbourne Beach, FL
What are your thoughts about those that have done a SYE and have removed the factory damper that is on the TC output? I've read threads of folks that did not re-gear but did a SYE and developed a harmonic, cyclic vibe above 50 mph.
The torsional vibration damper was likely added to absorb torsional vibrations at a certain frequency (sometimes designed to also absorb axial vibrations). Its really just an inner hub, spring/damper (rubber typically) and outer disk/mass sandwich which can be torsionally excited and dissipate (through the damping element) energy at a frequency (and nearby freqeuncies) proportional to the square root of the torsional stiffness of the rubber damper divided by the torsional moment of inertia of the outer weight.

Removing the damper will not allow absorption/energy dissipation at the frequency it was designed to attenuate and could allow potentially harmful (or maybe just annoying) torsional vibrations (and axial if it is also of concern) into the drivetrain that engineers were specifically trying to attenuate (for longevity or maybe just driving experience/noise reasons). It is not a coincidence that a vibration would show up after removal- especially if everything else remained the same dynamically.

Removing it also changes the system dynamics as the rotational mass is less (although downstream driveshaft has likely changed also). So really its a balancing/iterative act as if you need a damper, you have to add mass, so you have to account for the mass you need as it impacts the torsional dynamics.

Significantly changing driveshaft mass (moment of inertia) could also have impacts on the damper's ability to absorb energy as the problematic frequency could change (system dynamics change). I do not know if the damper was specifically designed for the stock setup, but I would guess it was specifically tuned for it...
 
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bobthetj03

vibrajeep
Supporting Member
Ride of the Month Winner
Feb 3, 2017
7,655
NorCal
Well, you're adding mass in the effect of a longer ds with a cardan joint, so maybe that added mass is enough to quell the frequence in the absense of the damper that is now removed, but not in all cases.
 

Beach_vibes

New Member
Feb 12, 2019
12
Melbourne Beach, FL
Well, you're adding mass in the effect of a longer ds with a cardan joint, so maybe that added mass is enough to quell the frequence in the absense of the damper that is now removed, but not in all cases.
That's right, you have changed the system dynamics with the longer and likely heavier driveshaft- so there may or may not be vibrational consequences with the new mass and shaft length (and resultant stiffness).
 
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Beach_vibes

New Member
Feb 12, 2019
12
Melbourne Beach, FL
In layman's English please. ;)
I can try: :)
When switching to taller gears, the rotational "slop" in the front driveshaft may increase due to the smaller driveshaft gear (more backlash). Since the front driveshaft is not loaded (since it is not typically in 4WD at highway speeds), the front driveshaft rotates about in the slop (slapping back and forth) when your chugging along at highway speeds and could be causing high vibrations of the drivetrain.

Has anyone or can anyone with the issue try to use 4WD at the problematic speed to see if it goes away? Maybe even use a touch of e-brake to load up the front driveshaft if the issue does still appear?

"Oh the Crunch enhancer? Yeah it's a non-nutritive cereal varnish. It's semi-permiable. It's not osmotic. What it does is it coats and seals the flake, prevents the milk from penetrating it." - Clark Griswold
 

bobthetj03

vibrajeep
Supporting Member
Ride of the Month Winner
Feb 3, 2017
7,655
NorCal
I get no change in vibration in 4Hi up to 65mph. Don't want to go any faster than that in 4Hi. If I remove the front ds from the equation, my 65mph+ vibes go away, but my 50-60mph harmonic is still just as bad as with the front ds connected. My vibes are the same whether I'm at coast or under load.
 

JMT

The Jeep Guy
Supporting Member
Ride of the Month Winner
I get no change in vibration in 4Hi up to 65mph. Don't want to go any faster than that in 4Hi. If I remove the front ds from the equation, my 65mph+ vibes go away, but my 50-60mph harmonic is still just as bad as with the front ds connected. My vibes are the same whether I'm at coast or under load.
Beach Vibes = Hope 4U.
 
Reactions: bobthetj03