[Keith Seymore]

Thank you. It was/is fun. I did that full time for about 4 years. My technician (RIP Bill) was awesome, so he should get a lot of credit here. General Motors has the finest tools (and teachers) available, plus - you get to use high falutin' words and impress your friends.




So - having given this further thought - I would describe Tom's situation this way:

Ever "twanged" a ruler off the side of a desk?

That's what his powertrain is doing. His engine/trans assembly is vibrating vertically on the mounts (at about 13 Hz, to be precise), based on some to-be-determined input.

For every noise or vibration you have to have three things:
…..a) an input
…..b) a noise transmission path
…..c) a response

If you ring a bell you have:
…..a) a hammer (the input)
.….b) the air around you (the transmission path)
…..c) the vibration of the bell dome (the response)

Take away any of those and...no more noise:
…..a) Don't hit the bell = no noise
…..b) get rid of the air = no noise
…..c) clamp your hands over the bell to dampen the vibrations = no noise

Obviously - some solutions are more practical than others. For example, taking away the air would result in some collateral damage that we would not want to experience, I presume.

So - back to Tom's truck:

The power cube wants to vibrate at 13 Hz (response). It's designed right in, to every truck of this configuration, just as though the engineers meant to do it on purpose (they did, btw).

It's just looking for a reason to light off and the driveline is more than happy to provide that excitation force (input) by kicking the trans tailshaft twice every time the drive shaft completes one revolution (which correlates to the accel/decel generated by the single cardan u joints - that's what "2nd order of driveshaft RPM" means in the order tracked data above). Single cardan U joint disturbances are "load dependent" - so the amount of force applied makes a difference. Ease off on the throttle - it gets better. Add a load to the pickup bed, or hook up a trailer - it gets worse.

The vibration transmission path, in this case, is through the frame/mount structure and up into the seat bracket, into the seat cushion and then into Tom's seat.

So our options are:
…..1) Do something to the vibration transmission path - if you were to disconnect the powertrain from the chassis the vibration would not be felt by the driver. You can actually do this on a chassis roll dynamometer, and it works, but obviously this is not a good solution for real life.
…..2) Affect the response - change the mounts so they are super stiff or super soft. Change the mass of the system by adding weight. These would also both have some affect, as described by the formula attached above - we just don't know if it would move the disturbance enough to be out of the problem range.
…..3) Don't hit the bell. Stop it. Just.....don't. Sounds easy enough. Our options for affecting the input are related to doing "something" with that driveline:

……...Add CV or double cardan joints
……….Single cardan U joint operating angles (maybe)
……...Move the center bearing rearward (maybe)
……….One piece shaft (likely)
……….No shaft
……….Rope drive/chain drive
……….Front wheel drive
……….Shorten the wheelbase 18 to 24 inches
……….Don't accelerate from a dead stop
……...Limit the amount of power available at launch through engine calibrations
……...etc

Here again - all of these are potential solutions but some are more practical than others.

K