RW> 09 Jun 11 16:14, Mike Luther wrote to All:

Thanks Gentleman Witt

 ML> Thoughts please?

 RW> Looks like you've covered all the bases. I'd bet that the problem was
 RW> there all along, just covered up by the air bag system. Once you went to
 RW> springs, that just manifested it more.

Somewhat yes perhaps.

 RW> Not necessarily a good or bad thing. But could effect the driveshaft
 RW> angles at both ends of the shaft.

In this case absolutely the same at rest level as the air bag system.  For
all practical purposes this makes the shaft angles at both ends of the
shaft the same as before.  One thing most don't know, I think, is that if
the pinion shaft and drive/transmission shaft planar alignment is perfect,
per what I have learned is that the U-Joints will wear out!  That is
because if the planar angle is perfect in alignment, the needle bearings
will never rotate and will stay in the same spot .. thus creating intents
in the U-Joint crosses and yokes!!

 RW> Hmmm.

Yes ,,, Hmmmm

 RW> Perfect as in within .005 run-out? Or more?

Well, you are correct as to what you propose.  However in this case, since
the vibration frequency absolutely is *NOT* at wheel revolution speed, this
makes the above wheel roundness and so on sort of not important.  Unless,
it could be a phase cancellation or amplification at some precise speed
that is part of the shaft speed.  And in this case the ring gear - pinion
gear ratio would have to be a precise 2 to 1 or 3 to 1 gambit I'd think.
Which it is not.


 ML> OK, this is an alumninum shaft vehicle.

 RW> Chevrolet had this problem with their early 4th gen (93 and up) Camaros
 RW> with steel driveshafts. They replaced those with an aluminum driveshaft,
 RW> although any rear end noise, such as gear howling, got even more
 RW> pronounced. Some of the 6 cyl cars got a double shaft with a carrier
 RW> bearing in the middle. This cured the vibration problem though.

This is a very short single shaft vehicle . Less than 72 inches total
including the front yoke.

 ML> got a tow truck.  They took the shaft out and lifted the front end,
 ML> then towed it down the highway up to at least 80MPH while riding in
 ML> it studying for vibration!  Absolutely no vibration at all.  Smooth
 ML> as silk ride.

 RW> Question. Has anyone ever checked the pinion angle 
 RW> with all four wheels on
 RW> the ground? Does the angle at the transmission end match the differential
 RW> end.

No not perfectly.  But again, go back and read my post on that above.  I
used to think that same thing.  But I was taught by a very serious bearing
professional that it absolutely CANNOT be perfect because that ruins the
U-Joints!!

 RW> Before yanking the third member, check to see how true the driveshaft
 RW> turns at the yoke. Could be the yoke was machined slightly off center.

They did that at the transmission end.  We still have not, as best I can
tell, done that at the pinion end.  However, if it was actually a pinion
error, there would have been vibration there well back into the less than
40,000 mile range.  And there was not.  The vehicle was perfectly quiet.


 RW> Resonance and vibrations are caused by the centrifugal forces applied to
 RW> those things that rotate at high speeds. Especially those that are out of
 RW> balance or do not run concentric, will cause your problem. i.e. the ring
 RW> gear could have been machined off center where the external diameter is
 RW> running out enough to allow centrifugal forces to be applied to the part
 RW> that isn't concentric with the differential bearings. Although something
 RW> like this would show up when you towed the vehicle without the driveshaft
 RW> in place. I'd check the andle of the driveshaft ends 
 RW> and the runout of the
 RW> yoke on the differential and if that doesn't do it, I can't think of
 RW> anything else...

I don't disagree with you at all here.  However in that the vibration is
absolutely only there at pinion speeds.  Plus, again as noted, this was not
there at all when the vehicle was towed with no shaft in it and checked. 
Now, the pinion vibration COULD have still been there.  And with that in
mind, without the driveshaft to FOCUS that ripple into place at whatever
reasonance is there to do this, frame/shaft or otherwise we wouldn't know,
would we?  And who gets to ride under the vehicle at 90MPH to touch the
third member with a finger to feel what?

Wince .

Mike Luther {at} 1:117/100

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