Quote:
Originally Posted by andrewwynn
...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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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?