Chasing vibration
 

I've got a vibration around 55mph. It's rather high-frequency, much higher than a wheel imbalance, so it's got to be someplace else in the drivetrain.

But it's rhythmic. It's not constant, it comes and goes at a regular rhythm. A fast rhythm, like once a second. It's there, it's gone. Over and over. There, gone, there, gone, there, gone. Bruuup. Bruuup. Bruuup.

A driveshaft vibration wouldn't come and go. It would be constant. Plus, a local driveshaft shop pulled the driveshaft and told me that although the driveshaft is old, and could probably use some attention, they don't believe it's THIS bad.

What else is going to give me a vibration? Especially a vibration that comes and goes.?

Re: Chasing vibration
 

Could be axle or carrier bearings. Put it up on jack stands and have someone in the drivers seat while you listen to pin point the problem.

Re: Chasing vibration
 

Check transmission tailshaft bushing

Re: Chasing vibration
 

So I put it up on jackstands and pulled the wheels off, and revved it up to 55mph on the jackstands. Same vibration, but it had a different "rhythm" to it. Instead of a bruup bruup bruup, it was a constant bruuuuuuuuuuuuuuuup.

I was by myself, so I didn't have an opportunity to crawl underneath it to inspect at 55mph, the best I could do was crawl under at a high idle, in gear (about 20mph with no load). No abnormal sounds coming from the axle. It's a 2-piece driveshaft and the carrier bearing was recently replaced, no abnormal sounds coming from that, either.

Vibration is unnoticeable at any speed below 35, barely noticeable from 35 to 45, but from there it gets REALLY noticeable and peaks around 55mph. Any image in the rear-view mirrors just becomes a blur. And like I said, significantly higher frequency than the tires, about the same frequency I would associate with the driveshaft.

Re: Chasing vibration
 

What truck? Lifted or lowered?

Gary

Re: Chasing vibration
 

Check the motor and transmission mounts. I had a weird vibration in a car and it was the transmission mount. It looked fine but once I unbolted it, it was in two pieces.

Re: Chasing vibration
 

.
In my daily driver I have a very similar vibration but at/around 70mph. The culprit: The flex disc or giubo. We don't have them in our trucks but this giubo is a flexible coupling between the drive shaft and the transmission output shaft. Once it starts to break down, it introduces enough slop to cause the vibrations.

My guess is that you've got a carrier bearing going bad ( failed ), a shaft that needs some balancing and/or worn/damaged U-joints.

Hth,

-klb

Re: Chasing vibration
 

I don't want to over-rule your statement, but what you indicate here is just not accurate.

When the rotational speed of the drive-shaft increases, it tends to allow oscillation due to inertial loading. Just like a tire that has a tread separation with a vibration that shows up once certain speeds are reached.

Did you replace the front and rear u-joints when the carrier bearing was replaced?

There is a possibility that you had a balance weight come off the drive-shaft, and the drive-shaft needs to be rebalanced.

I had a Fiat that developed a vibration very similar to what you are describing. I took it to a shop, and they separated the two-piece unit, balanced each part, then put it back together, which wasn't the problem, because I still had the same vibration when I got it back, only worse. They also, for some unknown reason, took it upon themselves to tighten the nut that the rear yoke secures to the differential. And then blue RTV'ed the seal at the differential yoke, Money down the toilet. It was the rubber donut that Fiat used (Like BMW) which is the equivalent to having a bad front u-joint. I then had to pay another shop to balance the drive-shaft because the first garage was too stupid to know that you have to balance an entire two piece shaft while the shafts are connected.

Maybe when you or whoever replaced the center bearing put the drive-shaft back together, the two halves were reconnected 180 degrees out.

Like already mentioned, you might want to check the tailshaft mount to make sure it isn't broken by trying to lift up the back end of the housing. If it's free to move, that is something else that could cause your problem you are indicating here.

Re: Chasing vibration
 

x2 on potential improper phasing of driveshaft yokes.

Re: Chasing vibration
 

But wouldn't a driveshaft imbalance, whether it be off-balance itself or yokes out of phase, be a CONSTANT vibration? It might be more pronounced at a certain speed, it might hit a resonant frequency, but it would be constant?

My vibration comes and goes. It's there, and half second later it disappears, and reappears a half second after that. It's there, it's gone, it's there, it's gone. Like I said in my earlier posts, it's got a rhythm to it. A cycle.

Duct tape $1.00 in quarters to your driveshaft, that vibration is going to rattle your dashboard and loosen the fillings in your molars, but it will be constant. It won't come and go, it will always be there until either you stop, or the driveshaft decides to exit your vehicle of it's own accord.

Logic dictates that if my vibration comes and goes, it is changing. Even if it's a rhythmic change, has to be components that are changing with respect to each other. At any given speed, the driveshaft is a constant rotation, exact same speed. How can I get vibration that is changing from a rotation that is constant?

Re: Chasing vibration
 

One of the bearings in the differential is the likely culprit. What happens is as the inner race is rotated The bearing rollers and the outer races are turning at different speeds from one another and when the 'bad' areas aline a vibration or noise similar to what you are describing occurs. The main culprit I've noticed for this is rust pits on one side of the bearing from our trucks sitting for extended periods (years) of time. Because the pinion bearings are the highest in the housing (oil starved) they are the most likely to have this issue.

The part of your description that doesn't add up for me is you claim the vibration is bad enough to vibrate mirrors. I've never seen a differential bearing issue bad enough to cause that but it wouldn't be impossible.

Steve weim55 Colorado

Re: Chasing vibration
 

/start rant/ You are entering and exiting a phasing in relation to the wheels while they rotate. Rolling resistance forces loading back into the driveline, which in turn can cause it to flex and rotate in an eccentric. One worn u-joint at the tailshaft could do exactly what you are describing. Especially given that you just replaced the carrier. Why exactly was that done? Flopping around, and making noise?

Logic dictates that when you are exasperated with a problem to the extent that you post the issue in a forum full of expertise, that you take whatever advice is offered, get under your truck with a flow chart and eliminate the many possibilities that have been offered, and not torpedo it or them. Dialogue with you seems to be along the line of when Na'aman the Syrian came to Israel in the hopes of receiving a cure for his leprosy, and instead he found lots of reasons to second-guess the directions of Elisha to simply wash in the Jordan. 2 Kings 5:10:14 Let me ask you, why ask for advice?

Maybe you are from the school that teaches that free advice is worth-less, compared to going to a shop and PAYING for it.

Maybe when you figure out what the problem turns out to be, you will post your findings back in here, Don Quixote. /end rant/

Re: Chasing vibration
 

Wilkin250r, why are you dredging up an issue you had when you posted your comment and all of the discussions you are having here, were already addressed there?

http://67-72chevytrucks.com/vboard/s...85#post8108885

Flow charts, GM Service Manual - Driveline Vib Diagnosis

Thread started on 4-29-2012. You posted on 12-19-2017. To Quote:

For shame, for shame. Shame on you.

You should be reported to the board administrators.

Re: Chasing vibration
 

Wilkin250r -

Cruiser is right.

"Comes and goes" is called a "boom/beat" in the vibration business, and is the result of two root causes adding and subtracting as their amplitudes phase in and out.

If you follow those flow charts shown in the linked thread you can fix anything.

You've got to walk your way through the options.

K

Re: Chasing vibration
 

.....
Just be done with it... Take the shaft out, drop it off at a shop and ask them nicely to balance it and provide you with a new set of U-Joints for it.

Once you've installed that, your troubles will likely be behind you. If not, you have eliminated the hardware involved with your run from tranny to diff. Then you can start troubleshooting the rest of your gear.


Hth,

-klb

Re: Chasing vibration
 

Yes.

We call "off balance" a "first order" driveline disturbance, because you get one "bump" per revolution of the driveshaft. A U joint issue is a "second order" of driveline problem, because you get two "kicks" per revolution of the driveshaft. Number of bumps times RPM divided by 60 = cycles per second, or Hz.

Understood

Nope. There is another (logical) explanation.

No.


Yes


Here's how:

It's because you have TWO causes going on at the same time.

See the attachment below. Let's say the top trace is engine firing frequency. For a V8 engine that would be 4th order of engine RPM (= engine RPM times four); for a V6 is is 3rd order of engine RPM (= engine RPM times three).

Let's say the bottom trace is second order of driveline (U joints).

Each trace is bumping along, doing it's own thing, independent of the other. If you eliminated the U joint trace you would hear the steady engine firing frequency. If you eliminated engine firing frequency (kinda hard to do, but bear with me*) you would just hear the driveline disturbance.

Since the two steady frequencies are very close together, as they move to the right in the time domain the amplitudes of the two disturbances add and subtract.

This addition and subtraction is what you are perceiving as "coming and going".

If you eliminate either one of those disturbances you eliminate the boom/beat.

K

*on the road, a person could do this by shutting off the engine and coasting through the range of the disturbance. If a person had a chassis roll dynamometer you can "motor" the rear wheels and driveline with the engine off, using the dyno's generator to provide power.

Re: Chasing vibration
 

Keith, thank you for your well-thought-through-presentation.

I think maybe a good example of what you are explaining might be along the lines of what a Beechcraft Bonanza pilot hears when the two propellers on either side of the plane phase rotationally and acoustically overlap when they are both running independently of each other. Or maybe what a person would hear while standing between two running electric fans in a quiet room.

Acoustic energy fascinates me. Music and sonic vibrations. Resonance and reverberation. If you get near a piano, or actually get to do this, it demonstrates some of what you are talking about.

Push the damper pedal open. Smack the A below middle C, and then listen to all of the semitones, halftones, and octive notes that will sympathetically resonate, while the damper pedal is held down.

If you then hit an E or a C, you will add resonance, as both those keys are part of the natural A major chord.

If instead you smack a note not part of the structure, you install dissonance, like two keys out of tune with each other. Each note is in tune with itself, but not with each other.

Who knew that having a background in Music helps fix engines?

Re: Chasing vibration
 

Exactly like that.

We had a guy in our group that was blessed with “perfect pitch”. He could hear a noise and tell us the musical note, from which we could assign a corresponding frequency. Based on his input we could calculate what components were operating at that frequency and compile a list of suspects.

He was a handy guy to have around in the days before small portable electronic noise instrumentation.

K

Re: Chasing vibration
 

The picture above looks like a beat frequency due to the modulated waveform. When two forcing functions are operating within a certain rpm of one another they can excite one another causing an increase in amplitude or the vibration felt. Happening at certain rpm ranges and not others would lead me away from a bearing roller or race failing simply due to bearing defect frequencies not being dependent on running speed, as they have a certain order based on inner and outer race diameters and roller numbers, such as 4.2x rpm, no matter what rpm the defect is there. Mechanical looseness and increased clearances follow run speed orders but dont typically have a coming and going noise. Unbalance usually is at turning speed but doesn't normally have a dependance on speed to increase amplitude. I would start with checking the driveshaft and u joints as well. It sounds like you are hitting a resonant frequency at 55 mph. Gear sets can have a modulation depending on load or lack of depending on increased bearing clearances and backlash changes. I would also check the differential condition.

Re: Chasing vibration
 

I'll admit to my shortcomings.

Guilty. Yes, I have posted about this problem before, but that was somebody else's thread. I did not want to hijack it.

Guilty. I have done more work and research than I have previously indicated, but I'm still stumped. I didn't mean to come off as rude, I was intentionally "playing dumb" because I am fishing for new ideas. My next step is a reed tachometer, but most of them I have seen are in the realm of $400

My yokes are in phase, my tranny/driveshaft/pinion angles all seem to be within acceptable tolerance. In full disclosure, I don't have a really fancy angle finder, I'm just ballparking with a plum-bob and a protractor, but they seem to be really, really close. Most places I have seen say 3 degrees is an acceptable tolerance, and I can tell you that I am certainly within that range.

While I haven't disassembled the axle, the pinion bearing, axle bearings, ect ect, all feel smooth. Plus, I believe I have eliminated them as a possibility with my next step.

I have pulled the rear portion of the driveshaft, eliminating that half as a possibility, as well as the possibility of anything in the rear axle, and any kind of rolling resistance forces loading back into the driveline.

The vibration is still as strong as ever, but it occurs at a different rhythm when unloaded.

Re: Chasing vibration
 

I have also tried revving the engine in neutral. And honestly, this test is inconclusive for me. The vibration is seen in the mirrors and heard through the chassis more than it is felt in the seats. With 4.10 gears and little tires on this old camper special, the 55-60mph of this vibration is getting up there in rpms. I haven't hooked up my tach, I'm just kinda ballparking it by engine whine, it's kinda hard to tell what is imbalance, and what is "this engine just isn't happy revving like this in neutral". Sometimes the vibration seems really bad, sometimes it doesn't. But definitely no rhythm, no boom/beat.

Plus, I live in a duplex, and I'm literally revving this engine nearly 3k, with free-flowing exhaust, less than 15ft from my neighbor's front door. I'm willing to do it a few times to try to figure out this problem, but I don't want to be TOO disrespectful.

Re: Chasing vibration
 

So I reassembled everything and took it for a drive. A long drive, under many different condition. I wanted to concentrate on the boom/beat RHYTHM, partially because I think that might be the key in pinpointing the problem, and partially because I have tried concentrating on other aspects and it led me nowhere.

I noticed a SIGNIFICANT difference in my boom/beat rhythm based on engine load, NOT on speed.

I've thought long and hard on this, and my latest theory is torque converter.

Thoughts?

Re: Chasing vibration
 

I will jump on this. Since you took the rear half of the drive shaft off and vibration was still their, now take the front half off and see if it goes away.

I had an old square with a 350 with a fresh rebuild. The Crank shaft literally broke right in half on one of the mains. So instead of rebuilding the whole thing again, I had another crank laying around and installed it. Worked great till I drove it down the road and had a very strong vibration above 2500 rpms. I checked into the crank that I put into it and found out it was from a 305.

So my point is maybe it is farther up the driveline.

Re: Chasing vibration
 

One minor quibble -

In the diagram above there is no cross excitation occurring between the physical components. They are each doing their own thing, steady state.

The pulsing effect occurs downstream as a result of the wave forms adding and subtracting.

You can get the same effect by generating sound waves with no physical components involved.

K

Re: Chasing vibration
 

This video demonstrates complex wave form interactions.

https://www.youtube.com/watch?v=ZtqEWqVn5xY