Questions regarding vehicle vibrations and help with data anaylsis

[gt86]

Hello,
First, I am so glad I found this forum! I am a mechanical engineer/auto enthusiast and I am trying to diagnose vibration problems. I've done a bit of research so not entirely starting from scratch, but have a few questions:

1. When it says to determine if the vibration is engine speed or vehicle speed, you're told to get the car to rpm that causes the vibration and then put it in neutral and see if the vibration continues. If it does, it's vehicle speed related. Question: When putting in neutral, do you maintain the rpm or do you let it go to idle (while the car the continues to coast)? I'm guessing you'd let it go to idle so you're only getting movement (and therefore vibration) from the driveline elements. Please correct me if I'm wrong

2. I have an 8 cylinder van that I am trying to diagnose. I used an iPhone app and placed it on the floor (no carpet) in front of the drivers seat. (the vibration can be felt while seated). At 2400-2500 rpm (40-41Hz), I get highest amplitude vibrations at 41 Hz, 39.8 Hz, and 38.6 Hz. After reaching 2500 rpm and putting the car in neutral and not maintaining rpms, so ~800 rpm, I got readings of 22 Hz, 1.95 Hz, and 20.3 Hz. In neutral and maintaining 24-2500 rpm, I get 21 Hz. Finally, to make it even more confusing, in neutral,at 2400 rpm not moving (so parked in the driveway I get 40 Hz again.

2a. My interpretation of this is... there's a first order engine vibration at 40Hz - does that mean something is vibrating once per crankshaft turn? and there's possibly a 1/2 order engine vibration at 21Hz - is that related to cylinder or other timing issue?

2b. I suspect there may be multiple issues going on because the vibration is intermittent and when getting to 2500 rpm and putting it in neutral, vibration seemed to be reduced (although looking at the data above, they might not be, so that is why I think it may be more than one source). Am I on the right track? Does anyone have any suggestions or advice for me to further isolate this issue/s?

3. Say in a perfect world there were no abnormal vibrations. If I ran an EVA on the vehicle at 2500 rpm, what values would I expect to see? 0 Hz? Lots of low amplitude vibrations, (ie, no peak amplitudes)? (With this question, I am trying to understand what an abnormal vibration would show up as, as opposed to a "healthy" vibration)


Appreciate any and all insight and thank you very much for reading!

 

[GregLocock]

1 idle
2 engine out of balance
2a yes
2b what vehicle speed is this? what gear ratio, what axle ratio what diameter tires
3 doesn't happen

Cheers

Greg Locock


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[gt86]

Hi Greg,

Thanks for the reply! Question about your response to #3: What do you mean it "doesn't happen"? An engine that does not have any anomalous vibrations would still vibrate - I'm wondering how this would look on an EVA? I'm guessing the amplitude of the vibration is what would be low(er)? Because when looking at EVA data, we identify problem vibrations based on their amplitude, right? Just making sure I understand things correctly.

2b - the vehicle speed is approximately 55mph and these data were mostly taken in 3rd gear.
Gear ratios are as follows: 1st: 2.45:1, 2nd: 1.45:1, 3rd: 1.00:1, 4th: .69:1
axle ratio: 3.32:1
Tire size: 235 75 15


New information: at 2500 rpm in 4th gear, about 90mph, the vibration goes away and the engine hums nicely, as it should. Vibrations also occurred at 1700 rpm, 40mph, 3rd gear: 26.6Hz, 46.88Hz, and 25 Hz.

Also, the vehicle is RWD.

And, last update. I just did the tire/rpm calculations and summarized all the data collected so far. See attachment.

https://files.engineering.com/getfi...file=Screenshot_2024-05-05_at_12.33.46_PM.png

The "EVA Reading" column has the top 2 vibrations identified based on amplitude.

My analysis:
1. looks like there is a 2nd order tire vibration when the car is in N and on the road (4th row on the spreadsheet)
2. there is also something going on related to 3rd gear (see rows 1, 2, 5 on spreadsheet)
3. something with the engine that rotates at same speed as crank, per last row

Questions:
A. Is my analysis correct? Am I overlooking anything?
B. What could be causing issues 2 and 3?
C. New tires or something related to other parts that spin at same rate as tires?

 

[GregLocock]

There is no case where a running engine doesn't vibrate. An I6 at idle will have very low vibrations, but they are still measurable. A conventional V8 will always vibrate.

I don't understand your EVA reading column, it should be amplitude and frequency. Assuming those are just frequencies of peaks, it is rather strange to see 2nd order tire, normally they vibrate at first and 3rd order.

Normally we run 3 dimensional sweeps, that is rpm vs frequency vs amplitude, this makes identifying what is going on much simpler.

In this case there's a resonance at 100 hz, 150 hz and 250 hz



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[gt86]

Understood that an engine always vibrates. I was more trying to understand how to differentiate the "good" vibration from the "bad". I assume it would be done by amplitude when you take a reading, so a normal vibration that is of no concern would have a much lower amplitude than a vibration from a suspected issue. Otherwise, how else do you isolate a vibration that needs to be investigated vs one that is acceptable as part of the engine?

Re: EVA column, yes, there was an output of amplitude and freq, but I (stupidly) did not record the amplitude. They were ID'd by the software as peaks of interest and I logged only the associated frequencies. Is it necessry to have the amplitudes? Since all the analysis involves frequencies, I didn't think it would be important.

The tires are old and worn so maybe they have 2 bumps? Or, if not tire-related, some other component also moving with the tires?

 

[GregLocock]

"I was more trying to understand how to differentiate the "good" vibration from the "bad"."

It's only bad if it causes a problem. Typically when investigating a noise and vibration problem about half of my time is spent coming up with instrumentation and a test procedure that accurately reflects the user's complaints.

The frequencies are more important, for sure.

"The tires are old and worn so maybe they have 2 bumps? "

Maybe. I haven't come across that as an issue, but it isn't impossible.




Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[gt86]

Thanks for the insight. Do you think I may have a 3rd gear issue and/or crankshaft-speed related issue? Is it best to change the tires first and then retest to see if other vibrations resolve?

 

[BrianPetersen]

I think the instrumentation has done its job. It is always going to show that there is "a" vibration at once per crankshaft revolution (and twice per revolution due to secondary imbalance, and once per firing event in a multi-cylinder engine). "Is the vibration annoying" answer that question:
(a) - In neutral, vehicle stopped, engine held at the problem RPM? (What about somewhat higher or lower than the problem RPM?)
(b) - At whatever road speed, 3rd gear, at the problem RPM? (What about somewhat higher or lower than the problem RPM?)
(c) - At whatever road speed, 2nd gear, at the problem RPM?

The engine, and the flex-plate and the outer enclosure "pump" of the torque converter, are all spinning at engine RPM, always. The torque converter "turbine" and transmission input shaft may or may not spin at engine RPM depending on whether the lock-up clutch is engaged. I gather that this is an old skool 4 speed automatic, most of which don't lock the torque converter in 2nd gear, hence the inquiry about whether it happens in 2nd. In neutral the turbine and transmission input shaft are probably spinning so close to engine speed as makes no difference even without lock-up engaged.

What's the history of the onset of this problem? Has it been there since you purchased the vehicle (and do you know anything of what happened before that)? Is this something that developed suddenly and if so, did that sudden development coincide with "somebody doing something" and if so, what did they do? Or is it something that just gradually seemed to develop over time?

Given that 4 speed automatic transmissions went out of favour quite a while ago ... this is an older vehicle. Engine mounts seen better days?

If this vehicle is what I suspect it is (and it would be nice if you told us ...) it is prehistoric in design and dates back to an era where these vehicles were strictly utilitarian work vehicles and as long as the engine mounts and body mounts didn't shake the vehicle apart, that was good enough, nevermind the driver's ears. Refinement and luxurious low levels of NVH were never, ever, in the top 100 of design criteria to be satisfied by the design. On the other hand, "being built as cheaply as possible so that we can sell as many as possible to fleet managers while still making at least some money" was a really high design priority.

Engine mounts. Exhaust mounts. Transmission mount. Unless someone screwed up on a transmission job and improperly installed a flex-plate of an externally-balanced engine or picked the wrong flex-plate with the incorrect external balance weight. Or likewise with an externally-balanced front pulley. Or maybe "that's the way it is".

 

[gt86]

Thank you, I really appreciate the response, Brian! I'll try to answer your questions the best I can. The vehicle is indeed "prehistoric", it's a 45 year old Dodge van. While it was certainly no luxury ride to begin with, this vibration is definitely new and not normal. It has had a replaced transmission to 4 speed. (stock is 3 speed).
a. in N, vehicle parked, it happeend around 2400 rpm. There is a range for the problem, but I did not log all the RPMs that caused the vibration
b. yes, seems to happen in 3rd gear at around 2400 rpm and 55mph and goes away in 4th
c. haven't collected data in 2nd gear, so I think that's because it hasn't happened in 2nd.

As per history of the problem, ran fine until a few weeks ago. no known issues since the transmission/engine work was done, which was several years ago.

Also checked engine mounts (that was one of the first things checked). Trans and exhaust mounts OK too

 

[BrianPetersen]

OK, so this is something that developed on its own, as opposed to something that got put together wrongly.

hmmm 1979 ish. Carbureted 318 with electronic ignition? Do you have a misfire, or the beginnings of one? Does it still have the "electronic lean burn" stuff on it and is it working? How old are the plugs and wires and distributor cap and rotor?

 

[gt86]

Yes, I believe it is something that developed on it's own, but there has been work constantly being done to it, though not necessarily to the engine/trans at this time. I thought it might be related to firing because there was a 1/2 order vibration when the vehicle was parked and revved. (Did you get a chance to look at the data I've collected? this particular 1/2 order vibration isn't on that spreadsheet because I thought it might be a fluke measurement, but perhaps this is still a valid contender).

It's a 518. How would I know if I have the beginnings of a misfire? The distributor is a bit "off" (not sure how to best describe this) in terms of it's rotation, but the timing is correct so I thought this may not be a big deal. I honestly don't know about some of the details you asked, but I can find out and get back to you. It's almost entirely been gutted though so may not have all those components.

Also, it goes away at 2400 rpm in 4th gear (appears in 3rd gear at 2400 rpm, i don't think it happens in 2nd tho) *puzzling*

 

[gt86]

Update! Checked almost all of the things mentioned in this thread and it turned out to be a transmission mount that had been checked previously, but clearly not thoroughly enough. I appreciate all the replies! I think the EVA helped in narrowing down what to focus on. It wasn't the tires/axles but something spinning with the engine.

Follow up question: After fixing this issue, the van runs quietly (well, as quiet as a prehistoric beast can). I reran the EVA curious to see what it would be like now with no "known" issues. I was surprised to find that there is still a peak frequency showing up, one of the same ones as from before: 2400 rpm, 55 mph, 3rd gear.

This time I remembered to note the amplitudes too:
Amp Freq
0.035 39.45 Hz
0.028 1.56 Hz
0.021 39.84 Hz
Prop shaft Hz at this speed is 35 HZ.

And another, new data point is in 4th gear at 1600 rpm and 60mph, I get 1.17 Hz, 1.96 Hz, and 35.55 Hz as the main frequencies (this time I forgot to get the amplitude data). Prop shaft Hz at this speed is 38 Hz.

Does this mean there could be something with the prop shaft that needs investigating? Or if there is no discomfort to the passenger and driver, then is it OK to ignore/overlook?

 

[BrianPetersen]

There's probably something that resonates at that frequency. If the torque converter is not locked, engine RPM won't match propshaft RPM in direct drive (3rd). If it isn't bothersome, consider it fixed.