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The 1982 service info bulletin says the damper is to block twisting oscillation of the whole TT/transaxle. Maybe the degree measurement relates to some parameter of the axial resonance in a given drivetrain? Or perhaps variation in a partcular unibody's dimensions (?) - Wonder whether the multi-mm variation in the rear crossmember shims tell us something about getting the drivetrain not only aligned, but balanced?
That's probably enough uninformed question marks for one post....
Porsche left them out of their Euro 928SEs to save weight.
I left them out a few times during my R&D and didn't really feel any difference in the drive.
But, I have a few of the newer versions that did away with the old school rubber holders that sagged and fragmented over time with the weight of the vibration dampener and with the heat it was subjected to over years.
I'm sure we can work out a deal to get you back on the road. PM or email me (preferred) if interested.
I just sent my TT off to Constantine's and he installed the Super Bearings. I have been very happy with that. It did not take long at all. Give him a call.
Thanks for the kind words and glad it is working well for you!
That excerpt from the WSM is very interesting, Rob. Here are my thoughts. Don't read if you don't like long-winded musings.
A) "In central tube there is vibration damper, which by way of its elastic mounts counteracts the twisting oscillations of the entire transaxle system."
Imagine bolting the forward end of the TT to the floor, with the transaxle mounted to it, all vertical. Twist the transaxle, then let it go. Record the oscillations. You'll get a frequency and a damping rate (time to half-amplitude). Repeat this measurement with the aft end of the TT mounted to the floor, with the engine attached above. In each case, you have a torsion bar spring (the TT) and a oscillating mass. The spring is the same, only the moment of inertia is different. The engine will oscillate at a lower frequency since it's heavier and its mass is distributed further from the centerline. A manual gearbox will oscillate faster than the automatic, assuming it's lighter.
An engineer would not want either of these frequencies to line up with a source of excitation, causing a resonance. The engine produces pulses with every power stroke. For example, 220 Hz at 3300 RPM (but not quite evenly in a V8 - right?). Not easy to change. An easier way is to change the spring rate - change the diameter, wall thickness or material of the TT. Making it torsionally stiffer will increase the frequency, and vice versa, so you might be able to move its natural frequency away from excitation frequencies. A third way is to attempt to dampen the resonance. Metals absorb very little energy as they're strained elastically back and forth, but rubber does - you've heard the term "hysteresis" about the rubber in tires - that's the property in question.
If you suspended the entire engine/TT/transaxle assembly by a cable, put a twist on the TT then let go, you'd get a complicated waveform, a combination of the separate high and low frequencies.
Of course, the engine/TT/transaxle is actually supported by four rubber mounts, which can dampen vibrations by absorbing energy. But they are elastic as well, and will change the natural frequencies. Not very much if they are very compliant - soft - but if perfectly hard, the stiffness of the chassis enters the picture.
A TT damper will be designed so it resonates at the frequency of concern (choose one of the two). With it's weight flopping back and forth on its rubber mounts at this frequency, it will absorb energy. It would seem best to place it nearest to the corresponding end, where the torsional amplitude is greatest.
BTW, are the rotating components affected by the damper? There is certainly a set of natural frequencies for the driveshaft, alone and when connected to other components, in a complicated variety of torsional and "whipping" modes. I'm guessing "no", at least not in any direct way. Guessing.
B) "Bending oscillation is caused by unbalanced rotating parts of the drive train."
Perhaps something got lost in translation - this is not phrased like the first sentence, it doesn't quite say if the damper has some role to play in this non-torsional mode of vibration. This mode is more like considering that the drivetrain vibrates like the string of guitar. Hard to imagine the TT moving like a string of a guitar. Anyone know German and English well and has the German WSM? This being such a different mode than the torsional mode, how could one damping device (and location) address both? Totally puzzling.
C) "Although the crankshaft, flywheel, clutch and starter gear ring as well as the torque convertor are balanced at the plant prior to assembly, residual unbalance can produce vibration, which will be noticed on cars with manual transmissions at approx. 4200 rpm [70 Hz], and with automatic transmissions at 3300 rpm [55 Hz]."
So here they are going on to discuss the "guitar string" mode. And to be clear, we're talking about the excitation the unbalance conveys to the engine/TT/transaxle system - the latter vibration is what the damper is supposed to fix, since it is attached to that system (this view may be too simple). Anyway, excitation and resonance are different things. The most common vibration we experience - an unbalanced tire - is an exception, where the excitation and resonance occur in the same thing (or seems so - the wheel is the excitation, the wheel + suspension does the resonating).
The two given frequencies are very different. The imbalance is constant at all rpm; we're talking about the point where the imbalance excites a resonance. Do they correspond to our engine or transaxle natural frequencies measured in "A"? It seems more likely that they're referring to a change in the transaxle frequency, since that's a more drastic change in configuration. Guessing again.
D) "The cast iron vibration damper with rubber spring elements designed for the bending resonance makes the transaxle system insensitive to residual unbalance and permits the replacement of separate components, e.g. during repair, without “assembly balancing” (balancing of flywheel + clutch + starter gear ring together)."
Ah - now they're clearly referring to "bending resonance", the "guitar string" mode. I don't think they mean the damper is needed only for repair, but it's interesting that in this context, the imbalance introduced in a repair may be highly variable. It might be particularly untimely to remove the damper at the time of a repair.
The "assembly balancing" mentioned here and earlier caught my eye. Earlier, the torque convertor was mentioned, but not here.
E) For cars with manual transmission the vibration damper is located in the central tube between the 2nd and 3rd guide bearings. For cars with automatic transmission the vibration damper is installed in the central tube behind the 2nd [of two?] guide bearings.
So, the damper is located towards the transaxle end of the TT, suggesting that the target vibration is strongest at that end. Why the differences between manual and auto? Does one more badly need the third bearing? Does one need the damper as close to the end as possible, more than a third bearing?
More: What are these frequency and degree numbers on the yellow labels? As we did measured the natural frequency of the TT in "A" (in our imagination), perhaps Porsche put these dampers in a machine and tested them. The properties of rubber being very temperature dependent, they may have tried to do the tests at a specified temperature, and recorded the actual temperatures. It's also possible that the vibration tester might have displayed the phase difference between the rotations of the fixed and rotating parts that could be used to figure the actual damping; that would be a pretty fancy machine. Anyway, these frequencies are somewhat similar to the 70 Hz residual vibrations mentioned in the WSM for manuals. Not very close to the 55 Hz for autos, though....
Finally, some may know of my particular interest in this subject. What I've learned here is that the vibration frequencies Porsche was concerned about (3300 rpm, 55 Hz for autos) is well above the vibration I've been feeling since my TT adventure (2100 rpm, 35 Hz). I've also learned, maybe, that Dwayne was smart to record the clocking of his rear flex plate with the torque convertor - that this may well have been system balanced at the factory; I forgot to do this, so re-clocking might help (there are three possible positions, 50:50 chance of getting it right on the first try, 100% on the next). For this to make sense, I have to ignore a lot of contrary facts and lesser indications.
Vibration is a nasty subject. Will I ever learn to avoid it?
+1 on the SuperBearings from BlackSea R&D (Constantine).
i opted to do the install myself on the Coke Car. Pretty simple. Easy afternoon project (assuming TT or engine is already out). Previously I had a rebuilt TT sans damper, but 1 extra rear pinch clamp that was rattling around. I thought the rattle was a bearing or race that was dancing around because the TT vibrated like crazy above 5k rpm. Turns out all “stock” carriers and bearings were in place - it was just that extra clamp. As best I can tell, the vibration was due to the omittence of the damper. However, with the Super bearings, I do not notice any vibration (and still no damper). Oh... and I removed the extra clamp. Great product, great people behind it.
I am rebuilding my TT and I would throw the damper back in if new rubber was available. It has part 928 421 061 00 on it but that is not available and not even in PET. My damper has no sticker but something written in red marker.
If you look carefully at the rubber bushings you have there is a lot of complexity in that piece.
The rubber bushings that Porsche specified for your version of dampener will cost a lot more to replicate to work without modification, but will also fail as the original pieces. Porsche changed them to a simpler design but it won't work for the dampener you have.
Thinking something different for simplicity and ease of construction, but it will still be more than what you would to want to spend.
Wow, there is a lot of information here. I remember scanning through the erkka thread years ago but not taking the time to digest it. Now it's coming together. Lots of solutions here. Constantine's being top of the heap. And apparently true for 944/51/68 too.
I wonder why something similar and maybe even simpler couldn't be clamped to the outside of the tube and maybe even moved around some to find any kind of "sweet spot."
Yes, a great thought and one that we initially thought about too.
However there are lines on top of the torque tube that would make that difficult to do, besides limited space.
And the whole reason for the weight to be suspended in rubber holders was to counter act on the drive line vibrations, another design element that would be difficult to incorporate into an outside the tube dampener design.
It is funny about the current interest in the vibration dampener and the assigning of almost otherworldly endowments to it. It is a weight suspended in rubber holders used in the tube since the design of the original bearing units don't have any drive line vibration reduction engineered into them.
However work has already begun on refurbished units for those that believe in them. Hopefully the price point will make sense.
07-31-2018, 06:25 PM
