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Old 02-18-2020, 04:03 AM
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AWR-fix: vibration diagnostics

Before I start my wheel bearing replacement this week, I really wanted to confirm that it is the source.

Unlike my previous bearing failures, this one has not gotten louder and no growling, just a minor case of "the UFOs" (wub wub wub).

Based on a suggestion from somebody on xo, I grabbed this app:

https://apps.apple.com/us/app/vibrat...o/id1089589547

And did some sampling.

First I set my cruise to an indicated 60=57 mph and put my phone oriented mostly upright in landscape mode in front of the speedo and took a reading:.



This showed me the primary vibration was 11.33 Hz and the second strongest was exactly double so likely a harmonic.

So to confirm its a once per wheel vibration, a little bit of math is involved.

First go to car Bibles and determine the rolling circumference of my 255/55/18s:
.

So that's 2225mm

mm to inches;



Inches to feet:



Feet/rev x rev/sec = ft/sec



And ft/sec x 3600 sec/5280 ft/mile

The results: 56.38 mph

That confirms the vibration is caused by something that happens once per tire revolution. The vibration is not a lot at 1/100 of a g-force, but by taking a reading from four corners of the car I can confirm which wheel and exclude certain other possibilities like CV axle etc.

A missing wheel weight or a bent rim would give similar results so some secondary inputs like having the sound get quiet during cornering etc.

That said it will be extremely helpful finding plenty of issues like for example, I suspect my CSB is failing and now that I know my rolling circumference of my tires is 7.3 feet I just need the final drive which I think is 4.1 and I can calculate the shaft speed. (or I can just ask Mr foxwell the output speed of the transmission) and if I have a solid spike right at the transmission output speed, very likely CSB or giubo is the source.

Finding a vibration at double the RPM and you can suspect something in the valve/cams, exactly RPM and it hints at crank. A little ratio math will get you the rpm of the alternator or ac compressor, I think you get the idea.

It's normally so difficult to confirm a wheel bearing especially which corner but when using the vibration vs sound it's so much easier

Once I replace the bearing I'll do another reading at 57/60 and try to figure out what that 23Hz vibration is about and show how the 11.33 spike is nearly non-existent.
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Last edited by andrewwynn; 02-18-2020 at 01:21 PM.
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Old 02-18-2020, 01:26 PM
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Quote:
Originally Posted by andrewwynn View Post
...Once I replace the bearing I'll do another reading at 57/60 and try to figure out what that 23Hz vibration is about and show how the 11.33 spike is nearly non-existent.
Really cool stuff. Thanks.

I'll bet the 22.66 and 23.05 are actually close to the same thing. Linear analysis of a nonlinear system separates them, but they're the same. And it will be caused by the same 11.33 source - surely from the wheel rotation as you conclude.

If the problem with the wheel were perfectly sinusoidal and there was nothing in between the souce of the vibration and the measurement (between the wheel and your iPhone) that would distort / filter / amplify the vibration, you'd get a pure 11.33 spike and nothing else beyond background noise. But in reality, the vibration source may not be perfectly sinusoidal, and the path through the whole system will change things as well.

So if you were to look at the time-domain signal with the period corresponding to the 11.33 Hz, you would not see a perfect sinusoid. It would be close, but distorted. That distortion is what creates the harmonic at 22.66, and also at 34 Hz, etc. Just like the terms in a Fourier series, there's always something out there in the real world.

How about swapping wheels as another test?
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Old 02-18-2020, 01:46 PM
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If I didnít already have other confirmation the bearing is shot swapping is a great idea to make sure itís not like a thrown belt in the tire.

You are very likely correct about the 23. I noticed on some readings I got some 1.5 Hz readings which are likely some large part like the frame resonance getting energy from wiggles in the road and the 23 is a combination. That will definitely help make sense if the 23 goes away and the 1.5 comes back.

I tried for years to do the same thing with sound FFT analysis but it never generated the clear spikes like the vibration.
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Old 03-21-2020, 02:54 PM
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Thank you for that info andrewwynn Iím about to head down the find the vibration sound path. Mines in the rear.
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Old 03-21-2020, 11:28 PM
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It was so much easier with the vibration vs sound.


Sent from my iPhone using Tapatalk Pro
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Old 05-26-2020, 05:00 AM
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Hi Andrewwynn
Here is my attempt it is at 100 km/hr tyre size 285/45R19 on the rear right hand side. hopefully that comes thru as an image?? Looks a little similar to your bearing one?
What do you think?
Thanks
Danny
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Old 05-26-2020, 05:54 AM
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AWR-fix: vibration diagnostics

2.229*12=26.75 m/sec = 96,300 m/hr is 96.3kph.

If you were using the speedometer not the obc internal value that means the car was going 97 kph.

calculated speed is 99.27% of speedo measured speed.

100% the problem is from a once per rev defect.

Are you trying to confirm or rule out a wheel/tire or bearing as the source?

Swap left & right wheels. Sound swaps itís in the tire. If it stays itís in the hub.

If in the hub, put a spare on that Corner, if itís the bearing it will make a huge difference in my case bearing was smooth as glass with the 255s. Put on some 245s and instantly the bearing sounded like a ufo!
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Old 05-26-2020, 03:29 PM
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Awesome thanks.
I was hoping it wasn’t in the diff.
I will do the wheel swap and see how it goes.
Thanks
Danny
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Old 05-26-2020, 07:33 PM
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Did you try to counterweight the rotating mass to see if that had any effect on your results?
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Old 05-27-2020, 02:09 AM
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Quote:
Originally Posted by Danny woodland View Post
Awesome thanks.
I was hoping it wasnít in the diff.
I will do the wheel swap and see how it goes.
Thanks
Danny
there were two other strong vibrations but as somebody pointed out above, probably a suspension part with that particular resonance frequency

if it sounds like a ufo 'wuh wuh huh' very good chance the bearing. if the sound moves when you swap wheels that usually means you lost a wheel weight or dented the rim from a pothole (i've had both happen) or can be internal tire belt separation i've had that also; Z28 gets pretty shaky at 110-120 when the belts come apart on your Z-rated tire.

if a dif part vibrated at the same frequency as the tire rotation it shouldn't be too bad it would mean a boot or seal more than likely ; dif takes a shit the frequency will be high with a horrible growl sound.
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