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

MarkAlan

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A band-aid repair can have more than one meaning depending on the user of the word. I call it a band-aid because we shouldn't have to resort to an expensive mod to solve this particular problem. Maybe crutch is more palatable?
Bingo! I am not saying that it is bad or anything. But in my world if you cannot determine the root cause it is a band-aid. Yes it works and works very well. It also has other attractive qualities. But a simple gear change should not cause the need for hubs.

Mr.Bills if you know the cause of the problem, there are always multiple ways to fix the issue. I am not saying they would be less expensive or easier, or even better.

And I am running the HP30 in mine.
 
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psrivats

psrivats

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I agree with the exception of the characterization "band aid."

A hub kit doesn't just mask the vibration issue as a band aid temporarily covers a wound, it cures it. Simply because we haven't been able to isolate the specific root cause of the vibration doesn't negate the effectiveness or the permanence of the fix.

Would it be helpful if we knew the actual cause? Absolutely, because it may lead to another fix that is less expensive and less involved than converting to manual hubs. But I'm going to leave that for those with engineering minds who stay awake at night thinking about such things. I'm just happy that the vibrations in my jeep disappeared when manual hubs were installed so that I can drive it without ever thinking about the vibrations again.
I will not say that the vibes are "cured". If you engage the hubs in regular 2WD and drive at hwy speeds, they will come back. They have simply been hidden because you could hide them. I couldn't hide them in mine.

I laughed at your description of "engineering minds who stay awake at night thinking about such things" because that's exactly me. As irritating as this problem is, I still find it interesting at an physics/engineering level. I would love to be the guy that can simply think of the problem and find a cure. But alas, I'm not that type and sometimes difficult problems like these are only solved, if at all, by the process of elimination.

My understanding of the issue has evolved over time and I'm grateful for how much I got to learn about 4x4s in general since my re-gear.
 
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MallCrawlOnTons

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I mean to really break it down, who would you put your money on at making the best gears for a Dana axle? Dana or an aftermarket company?

It dont think its a stretch to say that aftermarket gears are not on the same level as genuine Dana Spicer gears.

Its like buying Mopar sensors for the engine. OEM is USUALLY far superior to aftermarket.
Edit: Double post.
 
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MarkAlan

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Here is something I was wondering, I have not tried this but would like to try it. Everyone that has this issue, has regeared to run larger tires. Has anyone tried putting the next tire size up from what you are geared for? This would be easier for those that geared to run 33's. If so did it move the vibe?
 
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psrivats

psrivats

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Here is something I was wondering, I have not tried this but would like to try it. Everyone that has this issue, has regeared to run larger tires. Has anyone tried putting the next tire size up from what you are geared for? This would be easier for those that geared to run 33's. If so did it move the vibe?
I did. I tried 33s and it moved the vibe up a little bit in speed. But it did not disappear or reduce in intensity once it started.
 

MarkAlan

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I did. I tried 33s and it moved the vibe up a little bit in speed. But it did not disappear or reduce in intensity once it started.
What are you geared for? Mine is geared for 33's and would like to throw a set of 35's on and see how it changes.
 

dudehitt

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Because so many don't have the problem it seems like something must be 0.2 grams out of balance or 0.0005 in. out and then the harmonics kick in. That harmonic thing has to come from resonance. If it was easy to make this shaft diameter 1/4" larger or that part a little smaller you could change the resonant frequency of individual parts to figure it out.

Too bad only unicorns fart rainbows.
 

AndyG

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Because so many don't have the problem it seems like something must be 0.2 grams out of balance or 0.0005 in. out and then the harmonics kick in. That harmonic thing has to come from resonance. If it was easy to make this shaft diameter 1/4" larger or that part a little smaller you could change the resonant frequency of individual parts to figure it out.

Too bad only unicorns fart rainbows.
I have often thought , with all the moving parts going on here , it just wouldn’t take much being out to cause vibes .

As far as a band aid goes , a little light in a dark room beats no light at all.

I think of locking hubs as a “ breaker”, taking the diff and related components out of play .

Keep at it gentlemen.
 

Mike_H

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Because so many don't have the problem it seems like something must be 0.2 grams out of balance or 0.0005 in. out and then the harmonics kick in. That harmonic thing has to come from resonance. If it was easy to make this shaft diameter 1/4" larger or that part a little smaller you could change the resonant frequency of individual parts to figure it out.

Too bad only unicorns fart rainbows.
If that theory is true, then you could slap a hose clamp on the driveshaft and change the vibration significantly. That doesn't happen. But there is something there that's causing the harmonics. Sri says it's first order, so something balance related, but I know he's been through his rig a number of times and they are all dead nuts. I think he's even done the high speed balance thing.

It would be nice to have the tools to run driveshafts up to speed and measure deflection. Also to have accessibility to a bunch of driveshafts. We could then compare critical speed and see how consistent the speed is.
 

dudehitt

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If that theory is true, then you could slap a hose clamp on the driveshaft and change the vibration significantly. That doesn't happen. But there is something there that's causing the harmonics. Sri says it's first order, so something balance related, but I know he's been through his rig a number of times and they are all dead nuts. I think he's even done the high speed balance thing.

It would be nice to have the tools to run driveshafts up to speed and measure deflection. Also to have accessibility to a bunch of driveshafts. We could then compare critical speed and see how consistent the speed is.
It could be in the axle shafts or gears or whatever. We're pretty sure the driveshafts are balanced, right? That's one reason the good ones aren't cheap and there's only 2 sources. It may be easy to put a hose clamp on a driveshaft, but what about an axle shaft or a 0.0005 shim on a gear?

I really have no experience with this, but it seems like something is hitting a resonant frequency. I was just wondering if there was an easy (ish) way to change the resonant frequency of any parts. I don't know if it's possible, other than adding a huge imbalance to a driveshaft by putting a hose clamp on it.

I know Sri's appear to be dead nuts, but how close is close enough? If digital calipers have readings to 0.001 is that good enough?

The only thing i am sure of that Sri's setup is dead nuts by whatever instruments whoever has to measure it. What i don't know is how that compares to whatever it takes to create vibes. Obviously something ain't right.

I'm trying to ask questions that haven't been answered yet since all the current answers appear to be incorrect sometimes.
 

mrblaine

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It could be in the axle shafts or gears or whatever. We're pretty sure the driveshafts are balanced, right? That's one reason the good ones aren't cheap and there's only 2 sources. It may be easy to put a hose clamp on a driveshaft, but what about an axle shaft or a 0.0005 shim on a gear?

I really have no experience with this, but it seems like something is hitting a resonant frequency. I was just wondering if there was an easy (ish) way to change the resonant frequency of any parts. I don't know if it's possible, other than adding a huge imbalance to a driveshaft by putting a hose clamp on it.

I know Sri's appear to be dead nuts, but how close is close enough? If digital calipers have readings to 0.001 is that good enough?

The only thing i am sure of that Sri's setup is dead nuts by whatever instruments whoever has to measure it. What i don't know is how that compares to whatever it takes to create vibes. Obviously something ain't right.

I'm trying to ask questions that haven't been answered yet since all the current answers appear to be incorrect sometimes.
The issue as I understand from all I've gleaned and coalesced into a chunk that I can get my uneducated head around is based on the overall chassis stiffness and the 4.0 firing order. Supposedly it is a first order harmonic that gets going when the engine gets to an RPM that lines up with the resonant frequency of the chassis.

What I don't understand is the engine hits those revolutions in every rig before the re-gear so it should be possible to produce the harmonic in most rigs at similar engine speeds. Why that can't be done is likely a clue to at least one solution. I question whether it has to do with how the wind resistance loads the chassis and what effect that has on the resonant frequency.

What I do know is they are not immune in stock form. I have an 04 that is bone stock. At about 70-75 mph, you can feel the harmonic starting if you are sensitive to it and recognize what it is.
 

jeepndogs

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I question whether it has to do with how the wind resistance loads the chassis and what effect that has on the resonant frequency.
I can tell you that last fall we did a jamboree up in Wisconsin, it was about a 500 mile round trip there and back. On the way home we had absolutely crap weather. 30-40 mph wind gusts and probably a 20 mph headwind most of the drive home. My Jeep that was vibe free with the hub kit had the same harmonic vibe the whole way home. I actually stopped to make sure I unlocked the hubs. With 4:88’s and 33’s I had the Jeep to the mat trying to maintain 65-70. So somehow load makes a difference.

Which makes me wonder does the condition of the engine make a difference and could this be why some do and some don’t have the harmonic vibe. I have two cylinders that I know have lower compression, slightly higher leak down and on a power balance test show less power than the others.
 

mrblaine

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I can tell you that last fall we did a jamboree up in Wisconsin, it was about a 500 mile round trip there and back. On the way home we had absolutely crap weather. 30-40 mph wind gusts and probably a 20 mph headwind most of the drive home. My Jeep that was vibe free with the hub kit had the same harmonic vibe the whole way home. I actually stopped to make sure I unlocked the hubs. With 4:88’s and 33’s I had the Jeep to the mat trying to maintain 65-70. So somehow load makes a difference.

Which makes me wonder does the condition of the engine make a difference and could this be why some do and some don’t have the harmonic vibe. I have two cylinders that I know have lower compression, slightly higher leak down and on a power balance test show less power than the others.
Yours is easier to explain in that scenario. The rear was lower due to the gear you were carrying and combined with the headwind dropped it even lower which altered the rear pinion angle and intensified the vibes.
 

MarkAlan

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@mrblaine have you seen it happen to rigs that you have added a cross member or added mods to the frame that would stiffen or alter the factory frame?
 

mrblaine

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@mrblaine have you seen it happen to rigs that you have added a cross member or added mods to the frame that would stiffen or alter the factory frame?
Not really. Typically the issue shows up after a regear but rarely does a that happen without putting on bigger tires and bigger tires usually follow a lift from near or stock form. We haven't had an issue of his magnitude. I've dealt with quite a few where the owners have done all the adjusting and balancing, threw in the towel and brought it to me to install a hub kit.
 

bobthetj03

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I can run at 70 mph on a straight stretch, coast down a grade, climb a grade under load to maintain 70 mph, coast down a long down hill with the jeep in neutral at idle RPM, doesn't change the vibes. After 3 years of throwing shit at it, I have reduced the vibes, but not eliminated them.
 
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Tend to have a vibration when i first start driving , especially in 4th and 5th gear . vibration comes and goes . the other day it went away all together . going to get it looked at ,but it was nice that it had went away .