Geza

Perhaps some may find the attached informative.

FredR

Excellent papers- thoroughly enjoyed the read.

I had to deal with the drive line problem 21 years ago when my late 90S4 bust its drive shaft at 80k km- I was astonished that such could happen on a top end prestige vehicle from a company like Porsche. When complaining about this at the local agents I was advised that I might be able to get a "good will" claim but first I had to repair the car at their prices for which I was quoted silly money by the chap who dealt with clients issues. In the end I purchased a new drive shaft elsewhere for half the price quoted locally and then the fun started. '

I had noted a vibration at precisely 3050 rpms after purchase early 1999 and prior to failure a few weeks later but did not pay too much attention to it at the time. After repair the vibration disappeared for two weeks- then returned- clearly something was not right. I then invited a couple of mechanical engineers in the company for a coffee to discuss the problem. We concluded that the flexplate clamp had probably worked loose somehow. So, I went back to the main dealers, made a mark on the shaft and the asked them to check tightness [it was OK] and then to loosen the clamp- it immediately jumped about 2mm. They re-torqued it and the problem disappeared- for about two weeks- then it returned! Porsche then contacted their central engineering and in a matter of minutes returned with the "replace the pinch bolt and tighten to 110% of specified torque" [now commonly accepted]. Again the vibration disappeared and returned two weeks later. At this point I advised Porsche that I would not take the car back unless and until the problem was fixed- they had no idea what to do. In the mean time a colleague suggested that we try some kind of means to lock the clamp firmer and a couple of products were mentioned.

About this time the internet was introduced to the public over here and one of my colleagues suggesting getting it as I may find a user group- I did and I did! It was amazing just how common this problem seemed to the folks on Rennlist at the time. One of them [Earl Gilstrom] had devised a procedure using Loctite 290- it fitted exactly what I was looking for and after Earl's advice my S4 was the second 928 to have this solution applied - It worked and 20 years later it is still holding my flexplate clamp to the shaft.

At the time some folks reckoned that the shaft failed because of the tapers at each end reducing from the 28mm body acting as stress raisers. I concluded that there was nothing wrong with the 28mm shaft design. I figured that something was going on that was causing the stock clamp to slip- most probably connected with the 3050 rpm phenomena just that initially the vibrations were not felt. As the clamp slips and the shaft length increases there is more excitation and then it is felt. If the clamp tension is marginal or some examples have more exaggerated symptoms the problem manifests itself. This begs the question as to whether the clamp is the cause or the victim. Either way more clamping force is needed to eliminate the problem and this is what the Loctite effectively adds. At the time i voiced an opinion about how the stock clamp is simply inadequate and how a taper clamp would have been much more appropriate. The stock clamp is like that used to hold the kick starter to the gearbox on the 1957 BSA 650 I owned until recently. Upon seeing this I was contacted by my dear friend Constantine who told me he was already designing a clamp along the lines I intimated - his clamp is excellent and what Porsche should have designed in the first place. We then hear about torque tube and torque converter bearings failing regularly. I suspect they are all interlinked, Stop the clamp from slipping and these problems probably disappear altogether for a good long time. My torque tube and torque converter now have close to 100k miles on them and no problems to date [touch wood].

Then Constantine came up with his Super bearings and fitted 3 on the stock 2 bearing 28mm shaft and came to the conclusion that this changed the resonant characteristics completely and thus no need for it. Not everyone agrees with this approach but it might be interesting to get your take on this given the perspectives you have most interestingly highlighted.

I also remember a common perception 20 years ago that motor mounts fail and when they do vibrations are noticed at 1400 and 2800 rpms. Not exactly 3050 rpms but close- maybe thre is a first order resonance at 375 rpms there or thereabouts.

My experience with the stock mounts [and correct under tray] is not good- probably due to our hot climate but..? At the end of the day there is a system and maybe Porsche just did not get it quite right in the case of the later automatics. I eventually tried the solid rubber mounts- they worked to a point but were not as good as the hydraulic mounts. I then tried the Volvo mounts that are currently running at 5 years and seem perfectly OK. At Roger's pricing I can afford them to fail more often and as I am concerned they offer a viable low cost alternative.

I also agree with you about the gearbox mounts in that filing that gap will fundamentally change the performance characteristics or so I suspect. My gearbox mounts went soggy a few years ago. My solution was to make a packer out of a strip of inner tube bound together by some Gorilla tape plonked into the void. Upon landing the car the packer restores the height but still allows for some flexure as per the original design- bottom line it appears to have worked just fine for the last 3 years or so.