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Car Periodic Vibration/Modulation Captured with App - Some Questions

  1. Oct 13, 2014 #1
    Hi Folks,

    Prior to leaving my car in for the annual MOT (UK), there were no vibrations to be felt at any speed. However, to pass the MOT I was advised to replace the 2 front tyres as they were worn. Ever since then, there is a periodic vibration which I can distinctly feel at 85mph but it is also occurring at other speeds (at any speed? I am not sure). I cant help but feel its the tyres is responsible but I am not sure how they can induce a periodic vibration.

    So I decided to try a vibration app on my phone which i taped to the headrest of the passenger seat as this position acted like a good amplifier to the frequencies at play.
    I got the time domain response and converted it to the freq domain as attached. I am just trying to see is there a correlation with the wheel speed (18.8Hz - 85mph based on tyre diameter)
    From the freq response, there are interesting peaks at ~ 16, 20, 32 and 35 Hz. They look like sidebands to me and very symmetrical about ~25Hz.
    Any ideas what is happening? I am quite baffled how I can feel a periodic vibration or a periodic modulation, where is it coming from?

    Note that since it passed the MOT, I presume there are no issues with the CV joints. I also had a look myself and did not see any leakage from the boots.

    The wheels are custom and so there are hub centric rings or spigots to locate the wheel to the hub. Some people say these can be responsible for vibrations but I am not convinced especially for periodic vibrations.
    The wheels are balanced and aligned (well at least the OEM maintenance garage say so)

    Any ideas? Cheers
     

    Attached Files:

  2. jcsd
  3. Oct 13, 2014 #2

    Mech_Engineer

    User Avatar
    Science Advisor
    Gold Member

    Its most likely one of two problems:

    1- unbalanced tire
    2- out of round tire

    I would recommend getting a rebalanace, and if that can't fix it you may try to have the tires replaced under warranty.
     
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