# natural frequency of vibration measurement

• posted on October 24, 2006, 4:51 am
Hi all, It is known that car wheels create vibrations in the car at their resonance frequency when the wheels are not balanced.Is there any method to find the rersonance frequency of wheel unbalance
vibrations?How to know at what speeds the vibrations will occur?Will the vibrations occur only at a particular speed or at all speeds beyond a particular speed? Thanks in advance.

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• posted on October 28, 2006, 10:48 am

I am no expert on this, but it is interesting and I would hope we could have some intelligent discussion, whether or not you like my physics.\
Mechanical systems may have several modes and frequencies of resonance. Just because it is resonant at several frequencies, however, does not mean that it will go into oscillation unless it is driven with energy...something that out of balance suspension components are all too adept in supplying.
If a wheel or tire is out of balance, then it will generate a vibrational frequency which increases with the increasing speed of rotation. The intensity or energy of the vibration due to unbalance conditions will also increase with frequency as a function of centrifugal force.
BUT, the situation will be most noted when the vibrational frequency is the same as one of the undamped mechanical resonant frequencies. When the induced vibration is far different in frequency from one of the natural resonant frequencies, the vibration should be less noticeable... Hence you will seem to go into, and perhaps out of, speeds at which more, or less, severe vibration is detectable.
I dont know whether any of the wheel balancing machines are set up to detect chassis mechanical vibration modes. It wouldnt be a hard thing to do, at all, from a physics standpoint. You could excite the chassis with sound waves, for example, and measure the intensity of the response at various locations.
I have wondered what some automakers have done to improve the ride so drastically on some models of cars. Some of the brands and models in recent years are as smooth as an eel in an oil barrel. On some other models, I can feel a slight annoying jitter. In some cases, this may be due to faults in tires, balance, etc.
For example, some models of Renault, Peugeot, Toyota, and Volkswagen (and others, I am sure) can be so smooth, it makes me wonder how the manufacturers made such a jump step improvement.
Recently I drove a Hyunda Azera, and although it had rather good quality, it had the little bit of annoying jitter. Our 98 Buick drives well, but it is not as smooth (and age must count) as some of the cars mentioned above.
Do the manufacturers tune out vibration by better damping, do they match components better, are tolerances so much improved? Dunno.... maybe someone here does.
Someone complained on one of these groups a long time ago about buying a custom machined set of wheels from a well known manufacturer, only to find them away out of round (but still within the 'specs' of the manufacturer.) You can't remove out of round vibration by balancing, although they CAN be mechanically balanced.
If you were to buy one of Boyd Coddington's special one-off rods, would it drive as smoothly as one of the best above, or would it drive like a home build rod??
Ideas?