Pulsing vibration on the freeway

[Mrturner1]

Hey guys,

So I have a 1970 C10, 350/400th and stock rearend. When I'm on the freeway (or anytime I'm driving over 50-55mph) there's this vibration that starts off real slow, as in, it vibrates for a second or two sometimes a little longer and then stops for a second or two, then vibrates, then stops and so on. As I pick up speed and get around 60-65mph the vibration is faster, as in, it vibrates for a second then stops for a second, then vibrates for a second, you get the point.

What could be causing this? I thought driveline angle or something like that, but the angle is fine. The truck isn't lowered or lifted at all. Any ideas?

[cleszkie]

Does it get any worse when you let off the gas and decelerate or make turns? Three things I'd check are the condition of all driveline u-joints, the bolts that hold the torque converter to the flywheel, and your front wheel bearings. Make sure all joints are healthy, that all converter bolts are present and tight, and that the bearings are in good order.

[Mrturner1]

Nope doesn't get any worse when I let off or turn. Might even get a tiny bit better when I let off the gas. I know the torque converter bolts are good but I'll check the wheel bearings and u-joints.

[Highlander72]

Trans tailshaft bushing?

[Stocker]

I'm thinking u-joint(s). Can't always tell by looking, or by trying to move them by hand. Look for signs of rust around the cups. Put it on a lift and have someone gradually increase the speed while you look for vibrations or any unusual movement near the u-joints.

[Keith Seymore]

A "pulsing" vibration is what is known in the biz as a "boom/beat".

It is caused by not one but two vibrations that are close enough in frequency so that they sympathize. The "fade in-fade out" aspect is a result of the amplitudes adding and subtracting as they pass by each other. We used to experience this regularly when I was in band class in high school and actually used this characteristic to help tune our instruments (slower boom/beat frequency means the two vibrations are getting closer together; faster means farther apart).

If you can find and fix one or the other of the two offending components then you fix the boom/beat aspect.

Engine firing frequency (4th order of engine rpm) and driveline u joints (2nd order of driveline) are a couple usual suspects. Hopefully this gives you some ideas.

K

[Grumpy old man]

I'll bet a donut it's the driveline , in particular the carrier bearing ! Stock driveshaft carriers don't like high torque burnouts on a regular basis ( that old need for speed rearing it's head again ) switch it over to the HD moldels sold now and throw new u joints installed correctly ( zerk fitting in compression related to trans shaft ) in at the same time I'd also be checking to make sure the rear is in alignment with the frame .
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