Vibrations

[Keith Seymore]

Originally Posted by Ronw435

Well took the truck on a 200 mile trip. I am still getting a pulsing vibration that is most pronounced at about 60. You can feel it in your seat as well as the steering wheel. It is not constant but is about a 2 second pulse then 2 second off then 2 second on.....etc.
Any ideas?

Go back to the chart and start over.

Also (discussion regarding "oscillating" disturbances):

Originally Posted by Keith Seymore

In the biz, a "pulsating vibe" is called a "boom/beat".

It results from two disturbances, very close in frequency but slightly out of phase, such that the amplitudes gradually add and subtract over time. That's what results in the pulsating effect. The closer the two frequencies are the slower the boom/beat.

At any rate - it means you have two disturbances going on, like 4th order engine firing being very close to second order driveline, for example.

Fix either one of the disturbances (or dramatically change the frequency of one) and the pulsating effect will go away.

Originally Posted by Keith Seymore

Driveline imbalance is a "first order" of propshaft rotation disturbance (one "kick" per rev). U joints are a "second order" of shaft rotation (two "kicks" per rev).

Imbalance by itself usually results in a cabin "boom" (low frequency sound; almost more like a pressure that you sense) or a vibration that you feel, and normally is pretty steady. Large cabin volumes, like a crew or a Suburban, definitely are more sensitive to this. The boom/beat, or the phasing in-and-out, is a result of the driveline finding some other disturbance that is close to it in frequency.