Exercising the very slow (me doing the work) TT rebuild, TC bearings and seals, I took note of some ongoing topics w/r to: “to vibration damp or just put in another bearing”.
In this non-technical story of the mother-of-front-engine Porsches the 924: he reminds that the engine-drive shaft-transmission is a rigid truss and he mentions the initial problems Porsche had with vibrational energy supplied to the drive train causing amplification of the natural frequency of the drive train “truss”. He also mentions Porsche had to fix the problem, which I guess is the vibration dampener. To see absolute hyperbole of the natural frequency amplification problem, search for videos of the Tacoma Narrows Bridge Collapse. Of course I haven’t heard of Porsches shaking apart.
there are some really good after-market bearings out there. People seem happy with Constantine’s. 928Intl has a new one as well.
My mechanical engineering was limited to sophomore level courses, quite some time ago. I am aware that vibration is worse than just wasted energy. But I can’t advise on truss design, but Porsche did. I’m inclined to use two good bearings and keep the dampener. This is one of those things in which we only know down the road if we’ve made things worse or better.

 

There's got to be half a dozen threads debating what is correct, on this Forum.
Use the search function and read what has been said.
Then do what you think makes the most sense.

 

Mike,

How come you are talking in terms of "two bearings"?

As I am aware your model was supplied with a 25mm three bearing shaft. The early S4's also came with a three bearing 25mm shaft presumably to use up the ones they already had in their warehouse inventory and then they started using the 28mm two bearing shafts somewhere around MY88 onwards or so it seems.

What discussions about the resonance issue need to consider is that during late stages of original development Porsche discovered during testing that there was a vibration issue. As I can tell they seemed to be more concerned about NVH [noise, vibration & harshness] rather than out and out mechanical damage as a consequence but whatever the case they decided they needed to do something about it. They dampened down the concerned response by sticking a static lump of metal mounted in "knicker elastic" at a critical position within the torque tube. When they migrated to the two bearing auto shaft they seemingly did the same thing- whether they tested it or just "did it" I know not. Whatever the background it seems to work and no one has ever argued about that as I am aware.

When Constantine started his work on the system he knew that the later model automatics needed a clamp that would not let go as many examples such as mine were so doing. Constantine also knew that by fitting bearings with a wider foot print that would also change the dynamic and he figured that with three of his larger bearings fitted the damper was just not needed be it a 25m shaft or a 28mm shaft. As I am aware from discussions with Constantine no 928 torque tube fitted with his super bearings [and no damper] has reported any issues.

For many years the 28mm drive shaft was castigated as a "disaster waiting to happen" given the number of examples like mine that failed at the base of the taper some 21 years ago. I never bought into that mindset and figured that it failed as a consequence of induced vibrations leading to fatigue brought about by the clamp slipping. After replacing the failed torque tube with a brand new unit Porsche could not stop the clamp from slipping, They came up with the "replace the bolt and over torque it" ruse that just did not work. How did i know it was not working- because of a very specific and very annoying resonant vibration that appeared at precisely 3050 rpms. Loosen the pinch bolt and the clamp would spring back about 2mm or so- tighten it the vibration stopped and would reappear about 2 weeks later- rinse and repeat. I applied the Loctite solution and the problem disappeared to this day. There was no problem with the 28mm shaft- it was simply the victim of a front flexplate clamp that could not hold onto the splines. Increase the camping force and the problem goes away. The torque converter bearings are also a "known problem"- maybe they were also the victims of such problem. We have seen 28mm shafts cover 300k miles with no issues- this could not possibly happen if there was an inherent design fault with the shaft/bearings.

Whatever route you choose to go I would recommend that you fit Constantine's clamp or at the very least some other recognised form of secondary reinforcement. My clamp is on its 2nd reincarnation of the Loctite approach. No shaft snapping, no vibrations, no torque converter bearings failed, no torque bearings failed [yet]. The shaft has now covered 100k miles. My S4 shaft failed with 50k miles on the clock at the time and the torque converter bearings were suspect thus changed out!

My personal view is that Constantine came up with a system design that ideally Porsche should have come up with in the first place. Ironically had Porsche retained the original flexplate clamp design I suspect we would not be having these discussions from time to time and my GTS would still have the original engine intact instead of being on the scrap heap due to TBF failure. On the other hand had this been the case then I would not have been able to purchase my current GTS chassis for the bargain basement price I got it for.

 

Hi Fred; I’m using a much fresher donor driveshaft from Kevin Berez that I’m rebuilding anyway. It’s reportedly from an ‘88, mine was an ‘86.5. Both driveshafts are 25mm. I haven’t bothered to gut my old driveshaft. The donor shaft had 2 factory bearings plus the dampener. Far as I can tell, all bearings and dampener were firmly in proper place in the donor, without migration. In my old shaft the dampener fell right out when I picked up and tilted the shaft. The donor is being rebuilt with new complete 928Intl bearings plus new dampener carriers. Yes, I’m using a shaft migration clamp.
I’ve replaced the TC bearings and the pump seals await. Way over my head. I seem to buy a new tool every 2 days. It’ll be 1-2 weeks before I start the pump seals as I try to study and learn what dragons await in that job.

 

Hi Michael,

I am confident you will end up with a great result and do add more input to your thread as and when you attack the work in hand as such effort is inevitable [sooner or later] for all of us looney tunes who own these things.

I am surprised to learn that you have a 25mm shaft with two bearings - I am under the impression those auto shafts have three bearings just like the manual shafts but then I have not seen one in person and I have trouble remembering what I had for breakfast these days!

Regards

Fred

 

Hi Fred,

It is interesting about Porsche using only two bearing units to hold the drive shafts in some of the 928 automatic TTs, but uses three for all 5-speed TTs. I too thought it was only after changing the auto drive shaft to a 28mm, but then found 87MY 928 TTs with just two bearings with the 25mm drive shaft soon after starting my work in this area.

From all my research, I always thought it was a gaffe for Porsche to just use just two bearing units for such a long shaft, which to me exacerbated the NVH problem, even with the use of the vibration dampener. That's why I always use three bearing units in all TTs being rebuilt.

As far as the 28mm automatic drive shafts, I do wish Porsche just increased the size of both the front and rear couplers to accept a 28mm splined shaft at both unions. But then again, they never increased the size of the 5-speed drive shafts throughout the 928s production run. However, that is a discussion for another thread.

 

I've rebuilt several "2 bearing" automatic transmission torque tubes
I've used 2 of Constantine's bearings. I've used 2 rebuilt Porsche bearings. Although I have not done one of these since 928 International had their new bearings made, I would not hesitate to use 2 of their bearings.
I always re-install the vibration damper.
I've never had any issues doing this.

I've also rebuilt several "2 bearing" automatic torque tubes using 3 bearings.
I always re-install the vibration damper.
I've never had any issues doing this.

Pretty obvious, to me, there was quite a bit of "behind the scenes" engineering work going on regarding torque tubes, over the years. While there was an obvious engineering "failure" regarding flex plate preload issues, this problem occurs with all versions of the torque tube.

I do believe that the damper is an essential part and should be reinstalled with new rubber pieces.
However, 2 bearing or 3 bearing does not seem to matter. It will function equally well, either way.

 

Hi Greg,

I have never had any problems with using three Super Bearings and no dampener in our many builds and by owners who have rebuilt their own TTs over the years, as the Super Bearings have been designed.

Just as you and others, I looked at developing products that would be better than the original pieces they replace.

Forgot to mention our products were designed for three to be used to support the drive shaft better during operation, replace the dampener's weight and to control the engine harmonics traveling down along the drive shaft. This holds for all 928 TTs, regardless of year or transmission. They were not designed to just use two in any driveline, dampener or not.

If you are using old bearing carriers or Anderson's, then the dampener should be used.

Cheers.

 

Constantine and I have debated/argued about the use of the damper ad nauseam.

Since neither of us have the equipment to measure harmonics and do not desire to pay huge amounts of money to have our theories tested, we now agree to disagree and live with each in harmony.

Constantine builds beautiful bearings and the best of the aftermarket flex plate clamps, by far!

 

Dang it Greg, we have got to get harmonized on all this stuff!

And thanks for the compliments!

 

Hi Constantine,

Trust all is well your end.

That Porsche only discovered "the problem" late in the day tells it own story and the way it is described in the book Projekt 928 it leaves one with the impression that they were concerned about the cabin comfort impact more than a risk of mechanical failure but needless to say one problem can lead to another.

The 28mmm item with its tapered shaft is by no means unique and with a gradual taper the issue concerning stress raisers pretty much disappears not that I think it was a great idea. Up until today the 28mm shaft made some sense [to me at least] on the proviso that the 25mm auto shaft had three bearings but given it seemingly has only two just as the 28mm shaft does, what on earth was going through the minds of the Porsche engineers that prompted them to change to the 28mm shaft on ostensibly the same model? I find that truly bizarre- it might have made some sense for the transition to the GTS but not 87S4 to 88S4.

Fit three bearings to a two bearing shaft and then fit the damper as well should do no harm but I suspect it is equally possible that it does nothing for it either. If one does not know the answer then fitting the thing is a logical safe approach. Fitting three of your super bearings to a manual shaft and discarding the damper also makes sense given the mass of your setup is equal to the stock items plus the damper weight wise.

As is often the case in life there is "more than one way to skin a cat" and this is very true in the engineering world. Similarly it is not difficult to understand the adage "better the devil you know". I would say we know for sure that by increasing the flexplate clamping force we can prevent TBF but what we still do not really know is why some examples hold and some do not. I tend to put it down to manufacturing tolerances of kit made by different suppliers but who knows maybe the shaft vibrations being damped vary example to example and in some cases induce an otherwise OK clamp to let go- I guess we will never really know.

 

GT (and SE?) were NOT fitted with a damper from the factory?

Perhaps for a more raw driving experience? (Race car feeling also enhanced by the sound of straight cut gears in the transmission?)

 

It was done as a weight saving exercise for the CS and/or SE model, maybe both.

The GT models still had them from the factory.

 

Tends to support the notion that the damper was installed for creature comfort related reasons rather than any mechanical concerns.

 

Hi Fred,

Just to be clear, Porsche did use three bearings and a dampener in the 1985-1986.5 928 TTs. Then starting in 1987, Porsche used two bearings and a dampener with a 25mm drive shaft until the 28mm drive shafts started being used sometime in 1988. As you surmised earlier, probably after they depleted their stock of 25mm drive shafts.

Questions begs why they went from three bearings to just two, just as they pumped up the HP/TQ of the S4 engines. Also interesting that the dampener design was not changed during this transition, only later in 1991 when they used softer rubber holders for the automatic models, although the GT (meaning 5-speeds?) dampeners remained the same as the earlier ones.

They did not change again through the end of production in 1995, although the GTS engines again increased in HP/TQ where one would think Porsche would have made a change, at least in weight, to counter act the harmonics of the larger displacement GTS engines.

Attached below is the explanation of the different dampeners. Had a PDF file for the dampener U.S Patent, but couldn't get it to upload.

Stay well and as always, appreciate your engineering insights.