13 Jun 11 10:01, Mike Luther wrote to Roy Witt:

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

 ML> In this case absolutely the same at rest level as the air bag system.

Which isn't good or bad, unless the angles have changed to make things
worse.

 ML> For all practical purposes this makes the shaft angles at both ends
 ML> of the shaft the same as before.  One thing most don't know, I think,
 ML> is that if the pinion shaft and drive/transmission shaft planar
 ML> alignment is perfect, per what I have learned is that the U-Joints
 ML> will wear out!

Just the opposite of what most driveline people say.

 ML> That is because if the planar angle is perfect in alignment, the
 ML> needle bearings will never rotate and will stay in the same spot ..
 ML> thus creating intents in the U-Joint crosses and yokes!!

I think that this would be true, if the driveshaft were straight from the
trans to the differential. Since the shaft isn't straight but at an angle
to each other, the needle bearings will rotate, actually, oscillate back
and forth in the cup as the pitch of the angle changes the position of the
bearing shafts.

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

 ML> Well, you are correct as to what you propose.

My thinking before reading the tow truck test. Which no longer applied
after I read it.

 ML> However in this case, since the vibration frequency absolutely is
 ML> *NOT* at wheel revolution speed, this makes the above wheel roundness
 ML> and so on sort of not important.

Since they've been proven (tow truck) to be good enough to eliminate them
from the problem.

 ML> Unless, it could be a phase cancellation or amplification at some
 ML> precise speed that is part of the shaft speed.  And in this case the
 ML> ring gear - pinion gear ratio would have to be a precise 2 to 1 or 3
 ML> to 1 gambit I'd think. Which it is not.

3.42:1 most likely.

 ML>> OK, this is an alumninum shaft vehicle.

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

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

As is the Camaro. The difference in rear suspension is the torque arm
between the differential and transmission. Both cars are 4 link coil
spring suspensions, perhaps the Buick only has 3 links. Plus the Camaro
has a cross link, sometimes referred to as a panhard bar/rod.

 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
 RW>> differential end.

 ML> No not perfectly.  But again, go back and read my post on that above.

I think this is where I would do some further investigation. Both angles
should be as near to each other as possible, with the differential yoke
pointing down, not up. Under torque it will tend to rise anyway.

 ML> I used to think that same thing.  But I was taught by a very serious
 ML> bearing professional that it absolutely CANNOT be perfect because
 ML> that ruins the U-Joints!!

Suit yourself.

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

 ML> They did that at the transmission end.

The transmission's yoke could have been splined off center. Not very
likely, but not impossible.

 ML> We still have not, as best I can tell, done that at the pinion end.

A bit harder to do.

 ML> However, if it was actually a pinion error, there would have been
 ML> vibration there well back into the less than 40,000 mile range.  And
 ML> there was not.  The vehicle was perfectly quiet.

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

 ML> I don't disagree with you at all here.  However in that the vibration
 ML> is absolutely only there at pinion speeds.  Plus, again as noted,
 ML> this was not there at all when the vehicle was towed with no shaft in
 ML> it and checked. Now, the pinion vibration COULD have still been
 ML> there.

Thus proving that the pinion angle isn't correct. It wouldn't vibrate
without a shaft in it, at least not noticably. When you put the shaft back
in, it vibrates.

 ML> And with that in mind, without the driveshaft to FOCUS that
 ML> ripple into place at whatever reasonance is there to do this,
 ML> frame/shaft or otherwise we wouldn't know, would we?  And who gets to
 ML> ride under the vehicle at 90MPH to touch the third member with a
 ML> finger to feel what?

I knew a cat that did that once. He ruined a perfectly good driveshaft.

 ML> Wince .

                R\%/itt


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