> Fellow Sharksters, > > My local mechanic (Zen at Eurocar Repair) here knows of a S4 that > had the dreaded Thrust Bearing Failure recently. The poor guy went to > the dealer who installed a used engine for him. 1500 miles later it > happened again!!. He also said that there seems to be a shortage of > used engines now so the S4 is laid up waiting another suitable > transplant. > > So here's my question for today: Why does the front flange have to > have the pinch bolts on at all? The rear splines of the driveshaft have > a groove machined in to locate the pinch bolt on the rear flange, so I > wouldn't think there would be too much fore and aft movement of the > shaft. Couldn't you weld the split in the front flange closed and allow > the driveshaft to "float" within, the way a transmission input shaft > does in a clutch? Is that too simplistic a solution? > > > Thanks > Tom Lebovic > 89 S4 Black/grey > 67 Cobra Replica > > From: Peter Dodd [petergdodd@hotmail.com] Sent: Friday, January 12, 2001 2:33 PM To: 928 Subject: [928] Re: Thrust Bearing Failure questions Tom, My experience suggests the clamps on both ends of the (torque tube) drive shaft are to reduce noise. Typically, a rear wheel drive car would have a transmission between the engine and the drive shaft. This, along with the clutch / torque converter, acts to damp out (with rotational inertia) torque fluctuations generated by the engine due to individual firing pulses. With the 928 auto, the drive shaft is connected to the crank shaft via a flex plate. This plate is soft in the fore/aft direction but stiff in all other directions so it transmits the firing pulses downstream. These pulses, depending on their frequency, can excite resonances that will "find" any backlash (free play) in the system and cause it to generate noise by opening and closing the gaps. The ensuing rattle is predominant at certain frequencies, typically at or near engine idle (hence the term "idle rattle"). The rest of the system (long, hollow, stiff tube) acts as an amplifier and you end up with a real refinement issue. The only sure-fire solution is to remove the gaps, hence the clamps. My observations stem from personal involvement with the Prowler and 456 GTA (both rear transaxles). The Prowler has a conventional driveshaft (not a torque tube) but instead of the more typical UJ's, it employs rubber couplings at either end. Significant work was carried out with the UJ's to make them work - dual mass flywheels, altering driveshaft inertial and stiffness properties and minimizing clearances between needle roller bearings and their races within the UJ. Even with clearances down to 0.0002" (much tighter than normal) the system generated noise. The only solution was the rubber couplings. As for the 456 GTA, it uses clamped splines. So, welding the existing hub is not to be advised. In addition to the potential noise it would be difficult to control clearances and the material is hardened so not suitable for welding. As for TBF and the root cause of the problem, I'm not sure on that one. The two things I have observed that might make the S4 more susceptible are greater engine power and a different way of supporting the torque converter input shaft within the rear bell housing. Torque converters "ballon" under load which pushes the front face forward. The flex plates should allow for this but I wonder if the TC input shaft on the S4 isn't allowed to move further than previous models. This in turn will load the front flex plate and therefore the crank. It seems that the axial loads in the drive shaft overcome the clamp force on the front spline (possible because of system resonances). This leads to pre-load and relaxation when the clamp is released. Just my $0.02 worth. Pete. 86.5 A/T Garnet Red From: Adam Birnbaum [adamb777@worldnet.att.net] Sent: Friday, January 26, 2001 12:18 PM To: 928 Subject: [928] Repeated TBF thoughts With the TBF issue raising its head again I got to thinking about the situations where TBF occurs, a replacement motor is installed, and a second TBF occurs soon there after. I'm wondering if the following may be the cause, and if so, anyone doing any torque tube work should take note. Typically, the TT is bolted to the front bell housing first on reinstallation, and then the tranny is mounted onto the back. When I did my TT replacement, I did the same, and torqued down the front clamp. A problem occurred when I went to fit the tranny up to the rear of the TT. The rear flex plate hub was too snug of a fit on the driveshaft. (It was a huge pain even getting the tranny off the drive shaft). When I used extra long bolts to "pull" the tranny onto the drive shaft splines, the drive shaft actually migrated forward inside the TT dislodging one of the bearings. It seems possible that when a shop goes to fit the tranny onto the drive shaft, that in some cases there may be enough friction between the rear drive shaft splines and the rear flex plate hub to cause the drive shaft to move forward within the TT bearings. If they torqued the pinch bolt in front prior to installing the tranny, this phenomenon would put a load on the crank. Granted, the movement couldn't be too extreme (as in my case), because the relief in rear of the drive shaft would not line up with the pinch bolt mounting hole in the rear hub. Moral of the story? If you clamp the front pinch bolt down to minimize migration of the drive shaft while installing the tranny, make sure you go back and crack it to release any loads that occurred from mounting the tranny, than retorque to 110% with a new bolt. (Possibly along with Constantine & Co's loctite fix). Any comments on the above scenario? -Adam Birnbaum '88 S4 A/T From: Constantine [dicongo@earthlink.net] Sent: Saturday, December 01, 2001 8:15 AM To: 928 Subject: [928] TBF research and fix update As some of you might remember I undertook a study of the dreaded TBF issue that affects our beloved 928 automatics, especially the later years. I came up with a plausible reason of why it sets up in the first place (windup of the drive shaft) and I had a fix made which was then installed in my 1989 928S4. The fix worked but had a pronounced rattle which I figured out also. I am currently working with the manufacturer to correct the rattle problem and prototype #2 will be installed sometime this early spring. Of interest, I also studied the C5 Corvette drivetrain which copies the 928 drivetrain layout. In a nutshell it seems that they also have a similiar TBF issue crop up in the C5 automatic cars. They now advise that after a torque tube change the mechanic is supposed to run the car up to operating temperature then shut it down and let it cool off. The front pinch bolt is then loosened and readjusted. It seems that the C5 TBF issue comes up from heat expansion issues and not from wind up issues since their driveshaft is an actual tube which does not twist as much, if any, like our own driveshaft. I think that C5 auto owners will start to have more such problems come up since I don't believe that the mechanics will do the runup and cool off procedure like they should since time = $$$. The whole TBF issue really relates to the driveshaft being clamped at the front flywheel connection. I understand now why Porsche did this through my own trials and trbulations, but that is not the correct solution. It must be allowed free movement, like the 5-speed driveshaft. I have also designed a torque tube and driveshaft setup that could be installed and would allow for the torque tube to be changed without removal of the transmission as Paul Jaeger mentioned recently in one of his posts. But, given the low value of the cars, how many of us will want to spend the $$$ for this new drivetrain? That's the quandry. I will continue with the TBF fix as mentioned earlier but after it's done and tested, how many of you will want to buy it? Best regards, Constantine 1989 928S4, Auto-Her daily driver 1986.5, Auto, TBF test mule and track car-currently a garage queen in pieces.