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Originally Posted by wilkin250r
But wouldn't a driveshaft imbalance, whether it be off-balance itself or yokes out of phase, be a CONSTANT vibration? It might be more pronounced at a certain speed, it might hit a resonant frequency, but it would be constant?
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Yes.
We call "off balance" a "first order" driveline disturbance, because you get one "bump" per revolution of the driveshaft. A U joint issue is a "second order" of driveline problem, because you get two "kicks" per revolution of the driveshaft. Number of bumps times RPM divided by 60 = cycles per second, or Hz.
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Originally Posted by wilkin250r
My vibration comes and goes. It's there, and half second later it disappears, and reappears a half second after that. It's there, it's gone, it's there, it's gone. Like I said in my earlier posts, it's got a rhythm to it. A cycle...
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Understood
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Originally Posted by wilkin250r
Logic dictates that if my vibration comes and goes, it is changing.
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Nope. There is another (logical) explanation.
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Originally Posted by wilkin250r
Even if it's a rhythmic change, has to be components that are changing with respect to each other.
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No.
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Originally Posted by wilkin250r
At any given speed, the driveshaft is a constant rotation, exact same speed.
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Yes
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Originally Posted by wilkin250r
How can I get vibration that is changing from a rotation that is constant?
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Here's how:
It's because you have TWO causes going on at the same time.
See the attachment below. Let's say the top trace is engine firing frequency. For a V8 engine that would be 4th order of engine RPM (= engine RPM times four); for a V6 is is 3rd order of engine RPM (= engine RPM times three).
Let's say the bottom trace is second order of driveline (U joints).
Each trace is bumping along, doing it's own thing, independent of the other. If you eliminated the U joint trace you would hear the steady engine firing frequency. If you eliminated engine firing frequency (kinda hard to do, but bear with me*) you would just hear the driveline disturbance.
Since the two steady frequencies are very close together, as they move to the right in the time domain the amplitudes of the two disturbances add and subtract.
This addition and subtraction is what you are perceiving as "coming and going".
If you eliminate either one of those disturbances you eliminate the boom/beat.
K
*on the road, a person could do this by shutting off the engine and coasting through the range of the disturbance. If a person had a chassis roll dynamometer you can "motor" the rear wheels and driveline with the engine off, using the dyno's generator to provide power.