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Old 09-21-2010, 10:06 AM   #22 (permalink)
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Originally Posted by Piwoslaw View Post
Um, 9 isn't a prime number.
Prime numbers are desirable here because they have few divisors, and the ones they have are large. A 9-spoke wheel may have resonant vibrations at 1, 3, and 9 times wheelspeed, while a 6 spoke wheel may at 1, 2, 3, and 6 times wheelspeed.

Sometimes, resonant vibrations don't matter. e.g. if there's enough damping for the vibration to die out before the next excitation arrives, if excitations at that frequency are exceedingly unlikely, or if the frequency of vibration is too high for humans to perceive.
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