Thoughts please?   
      
   A friend has a 2004 Buick Rainer which currently has about 70,000 miles on   
   it.  Perfectly smooth car until about seven or eight thousand miles ago. Then   
   it began to show to me what I felt was 'driveshaft' vibration.  At least the   
   vibration was definitely related to shaft rotation speed.  The vehicle also   
   had developed tire vibration as well.  Which, in fact, masked the driveshaft   
   speed vibration enough so that I really didn't pick up on it as early as I   
   should have.  OK, new set of Michelin tires complete and with a very good   
   balance job as well as wheel check.  Interestingly, one of the old tires, in   
   the rear, actually had a double 'hump' out of round issue, each hump being 180   
   degrees away so that it double bumped per tire rotation!  Interesting.   
      
   A few thousnad miles before this issue, this SUV also began to show some   
   irregular overnight rear end drop.  It was an air bag rear suspension model,   
   also only rear end drive and with limited slip differential as well.  That got   
   worse and a tear down revealed air bag leaks in this as-built trailer towing   
   model so equipped.  This issue was looked at AFTER the original complete new   
   tire installation.  And the driveshaft speed related vibration was definitely   
   there following the new tire installation as was done.  It was particularly   
   noticeable up at between 60MPH and about 78MPH, but also very defintely there   
   between just above 30MPH and 40MPH as well.  However, it did show up enough   
   for an aware person to notice it between 40MPH and 60MPH as well.   
      
   In that the air bag issue 'popped up' more severely at that point, this matter   
   was the next thing serviced.  With no need for trailer towing to be used in   
   the future, as well as advice that replacing air bags weould just present more   
   of the same failures in the future, the decision was made to replace the air   
   bags with steel springs.  Which was done very well with NO possibility of any   
   driveshaft bending or damage.  In that the shocks for the air bag design were   
   stiffer than what was told to be for the steel spring design version which is   
   also used for this vehicle, new matching shocks were installed as well.    
   Following that work, the vehicle level matched exactly the same at rest as it   
   had been with the air bags.   
      
   It was a little stiffer along the roadway, but seemed to be just fine.   
   However, now the 'driveshaft' vibration seemed to be stronger and, as the next   
   couple thousand miles rolled up, got MUCH worse.  Especially down right over   
   30MPH and up just above 60MPH.  In fact at 71MPH to about 76MPH it was REALLY   
   bad and just getting worse and worse.   
      
   The complete tire balance and roundness as well as wheel roundness was   
   carefully checked at two different places.  Perfect,  At the next real service   
   shop, they had it up on a lift and could spin it up and very definitely feel   
   it.  They yanked the shaft, lowered the vehicle and spun up the rear end with   
   a friction wheel balancer on first one then the other wheel.  No vibration.    
   However, the technician did find that the front U-Joint was VERY badly worn   
   out.  He replaced both U-Joints and even that done, the vibration was still   
   there, though somewhat softer.  OK, this is an alumninum shaft vehicle.  There   
   is no shop here in the Bryan-College Station, Texas, area which can handle   
   aluminum shaft balance issues, so he got a complete other exact model shaft   
   from a local wrecking yard and put it in the vehicle.  Same problem!  Telling   
   us this had to be transmission problems, the vehicle went to a local   
   transmission shop that I have worked with for over thirty years.   
      
   They serviced the transmission, found almost NO evidence of steel or brass   
   filings in the fluid, nor could they find and evidence of a thing wrong in the   
   transmission.  As well, they checked the motor mounts and as well agreed with   
   me that there was no evidence of vibration from the engine/transmission and   
   even while driving, there was absolutely no change in the vibration from   
   shifting it into neutral during the vibration, nor from brake tapping to pop   
   the torque converter out of direct into fluid mode, nor from even downshifting   
   into the next lower gear during driving.  Now it gets interesting.  We've been   
   together for decades as I mentioned.  So they got a tow truck.  They took the   
   shaft out and lifted the front end, then towed it down the highway up to at   
   least 80MPH while riding in it studying for vibration!  Absolutely no   
   vibration at all.  Smooth as silk ride.   
      
   OK, there is a really decent driveshaft service operation here that has been   
   here for decades as well.  Though they can't handle aluminum shaft work, we   
   all decided that they should build a steel shaft for this less than six foot   
   long driveshaft and balance it.  OK, it went into the vehicle.  Much   
   smoother!  But still there at just over 30MPH and still there at 64MPH and up   
   at 71MPH to 78MPH, it still was totally wrong such that it could not normally   
   be driven and would chew things up if just left that way.  This group does not   
   do rear-end service work and there is another very capable shop here that   
   does.  The vehicle went out there and the shop owner said it was definitely   
   shaft vibration.  But he also noted that the ONLY way you could handle any of   
   these aluminum shaft vehicles, even if you were to use a steel shaft in them,   
   was to do the shaft balancing with a LASER balancing lathe!  And there was   
   only one shop he had found down in Houston to which he had sent or sent the   
   complete vehicle for 55 different shaft jobs which was the only place he had   
   found that could handle this!  Yes, the local shop did fine with new steel   
   shafts and blancing, but with the aluminum shaft vehicles this was the only   
   answer he had ever found that worked.   
      
   OK, I took the vehicle to Houston.  They determined that the complete balance   
   job on the new steel shaft was absolutely wrong, as well as that the new   
   U-joints in this shaft were not good enough either!  As well this shaft as   
   built had a brand new GM supplied front yoke in it which they said was fine.    
   But ... as laser balanced there in Houston, although the vehicle was MUCH   
   better yet, there was still a tiny bit of vibration down at 30MPH which the   
   average person would never have noticed.  As well, there was still the same   
   60MPH+ vibration, and still the really bad 71MPH+ vibration.  At that point   
   the crew there recalled they had a brand new aluminum GM shaft somebody had   
   bought but never picked up,so they crammed the new yoke, new U-Joints into it   
   and laser balanced it and we tested it.  It was even the same part number and   
   source!   
      
   No vibration at all any more in the 30MPH range,  Very little at all in the   
   60MPH range though a TINY bit could be felt at 69MPH and up until 74MPH.  But   
   from 74MPH to about 78MPH the bad vibration was still there, though not at all   
   as jaw shaking as it had been.  So told me there is no question but the   
   transmission is clean.  This definitely being felt at the rear,  But there is   
   no pinion seal leakage and though there is a little more rotational slop than   
   expected, there is no felt pinion irregulartity.  The only thing that looks   
   like it can be done next will be to yank the third member and attempt to trace   
   this to pinion problems, one way or another.  And there is no rear suspension   
   angularity problems or damage.  Now I am familiar with the very rare issue   
   with a rear pinion bearing problem which does not result in pinion seal   
   issues.  But in that there is no really noteable pinion rotation irregularity   
   seen, even if this is a rear pinion bearing crack or whatever problem, I   
   wonder?   
      
   I'm very close to a Texas Aggie graduate friend who is a lot better at this   
   mechanical stuff than I am.  He was a double degree guy in both mechanical and   
   electrical engineering.  The design guy later for the whole hydraulic motor   
   floating anchor and drilling rig precision location centering for sea drilling   
   rigs.  As well as, among lots of other stuff,he actually was in the designer   
   for the Howard Hughs hydraulic stuff that carried up the Russian recovered   
   vessel in the Hunt for Red October. He and I agree.  This has to be a   
   reasonance issue as well.  But that said, how can this be just a pinion   
   bearing or third member problem? What is going on here?  Where is the   
   reasonance and why?   
      
   Thoughts?   
      
   Mike Luther N117C at 1:117/100   
      
      
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    * Origin: BV HUB CLL(979)696-3600 (1:117/100)