AWR-fix: vibration diagnostics

[andrewwynn]

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.