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I believe that the Hz value corresponds to tested anti-resonant frequency for the damper, measured on a test jig. I don't know what the degree value is, any ideas? Molding the rubber and casting the weight are not precision operations, components come out with different anti-resonant frequencies. So that's why there's a damper specific measurement value.
I believe that Porsche measured a torque tube housing and cabin resonance at 70Hz for five speed cars and 55Hz for automatics. I believe (but don't know) that they then installed the dampers with relatively low values from those at hand to automatics and those with relatively high values to manual cars. Although this type of damper is a tuned narrow-band damper, it does dampen a range of frequencies around the measured value.
I believe that this tube housing and cabin resonance is very different from the driveshaft resonance. Drive shaft resonance is what has the potential to break components. The way they eliminated the drive shaft resonance was by specifying those carrier bearings. More carrier bearings => shorter segment lengths => higher resonant frequency. Also, larger shaft diameter => higher resonant frequency. The resonant frequency of the driveshaft as installed with bearings was made high enough that the engine can't excite it either at frequency equal to engine rpm or 4*engine rpm for below redline rpms.
I believe that the rubber spring wears on that tuned vibration damper from forward-backward movement as the car accelerates and brakes. The bearings weigh less and they also hang on to the driveshaft, so the rubber bearing isolators don't wear in the same way from forward-backward motion.
I was told that the rubbers on the damper are what actually absorb the frequency wave energy. This energy is converted to heat, which is presumably why the rubbers always look so bad, compared to the rubber on the bearing carriers.
Just to add more to our lack of knowledge, the GTS torque tubes have a completely different counterweight design and different rubber supports.
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
I was told that the rubbers on the damper are what actually absorb the frequency wave energy. This energy is converted to heat, which is presumably why the rubbers always look so bad, compared to the rubber on the bearing carriers.
Just to add more to our lack of knowledge, the GTS torque tubes have a completely different counterweight design and different rubber supports.
Let me help you with your "lack of knowledge" about the different vibration dampener designs, from Porsche:
When the Super Bearings were developed, they were designed and tested over a few years of development after studying what Porsche had done and why. Every facet of the Super Bearing design was gone over carefully to increase their performance and their function in the torque tube over the stock units they replaced. They have now proven themselves in dozens of 928 applications around the world with no problems for years.
We do not need to be in this business to support ourselves, and if our products weren't working, we wouldn't be selling them. That simple.
I am extremely interested to see what happens in this application since what is being done is off the scales compared to a stock application. But, I have a high level of confidence in my products.
Let me help you with your "lack of knowledge" about the different vibration dampener designs, from Porsche:
When the Super Bearings were developed, they were designed and tested over a few years of development after studying what Porsche had done and why. Every facet of the Super Bearing design was gone over carefully to increase their performance and their function in the torque tube over the stock units they replaced. They have now proven themselves in dozens of 928 applications around the world with no problems for years.
We do not need to be in this business to support ourselves, and if our products weren't working, we wouldn't be selling them. That simple.
I am extremely interested to see what happens in this application since what is being done is off the scales compared to a stock application. But, I have a high level of confidence in my products.
Let me help you with your "lack of knowledge" about the different vibration dampener designs, from Porsche:
When the Super Bearings were developed, they were designed and tested over a few years of development after studying what Porsche had done and why. Every facet of the Super Bearing design was gone over carefully to increase their performance and their function in the torque tube over the stock units they replaced. They have now proven themselves in dozens of 928 applications around the world with no problems for years.
We do not need to be in this business to support ourselves, and if our products weren't working, we wouldn't be selling them. That simple.
I am extremely interested to see what happens in this application since what is being done is off the scales compared to a stock application. But, I have a high level of confidence in my products.
Cheers.
Here's a technical question. Even if the mass of the super bearings matches the mass of the damper and stock bearings, why would the super bearings dampen the same frequency that the damper does? Doesn't the tune mass damper anti-resonant frequency depend on the spring rate of the rubber isolators/springs? Or are you going for general broad-band damping by simply increasing the mass?
From some of your statements about the TT it seems there is a lot you don't know about this topic.
The TT was originally designed with it being attached to the battery box which helped to quell the vibrations. When this configuration was changed, Porsche had to come up with another way to control them, hence the vibration dampener, a quick band aid fix to the problem from our point of view.
It's construction, design and choice of rubber elements is not optimum and the rubber elements go away pretty quickly in use. The heat source is not through the vibrations, but rather through the transmitted heat from the exhaust the TT sits over. We have removed these dampeners from low mileage TTs only to find the rubber donuts already crushed down, limiting their effectiveness for their given application. Replicating these holders will only have them fail in the same way, unless the rubber compound is changed to make them more robust, which then can change their effectiveness in their given task
I also find it interesting that as the 928 engine power increased, the vibration dampeners design did not change. Any change in the engine and its operating parameters will change the frequencies of the vibrations being produced. So why no change in the design until much later and then only for the automatics? This also plays into all the stroker motors currently being built for which the old style vibration dampeners were not designed. So does that mean transmissions will fail due to the uncontrolled vibrations coming from the new stroker motors?
Too much is being given to the vibration dampeners as if they are a magical item. They really aren't and their are better ways to control the vibrations in the TT now so owners have an option.
Still waiting for someone to explain why Porsche only used two bearing units in some of their automatic TTs and some versions came with three. Leaving a long length of rod in the TT without support is not a good thing.
I find that excerpt that Constantine showed more than odd. 90 GTs and 90 S4s used one kind of damper. Then, in 91 they use 2 different dampers. I have it in my mind that Porsche was trying to damp different frequencies on both the manuals (at 4.200 rpm) and the automatics (at 3.300 rpm). So, why in 90, did Porsche use the same damper? And then in 91, use different dampers? At first look, it seems to make no sense.
What needs to be pointed out here is that just adding a damper to a system, changes the proper frequency of the system. So, it is not just the damper material absorbing the frequency, but potentially also a shifting of the frequencies to a point where they no longer interfere with the use of the machine. This would be the case for instance when going from 2 to 3 dampers. This might explain why the automatics had 2 and the manuals 3 dampers. Who knows?
We know what happens now when dampers on the motors slowly give out. The bearings get pounded out. I wonder if the same thing does not happen to the transmissions? Both my cars have the super bearings in them now and one has the ATI super damper from GB in it. We will see, I guess, because in either case (super damper / super bearings) I am more or less buying under the assumption that any new rubber is better than old rubber.
The TT was originally designed with it being attached to the battery box which helped to quell the vibrations. When this configuration was changed, Porsche had to come up with another way to control them, hence the vibration dampener, a quick band aid fix to the problem from our point of view.
It's construction, design and choice of rubber elements is not optimum and the rubber elements go away pretty quickly in use. The heat source is not through the vibrations, but rather through the transmitted heat from the exhaust the TT sits over. We have removed these dampeners from low mileage TTs only to find the rubber donuts already crushed down, limiting their effectiveness for their given application. Replicating these holders will only have them fail in the same way, unless the rubber compound is changed to make them more robust, which then can change their effectiveness in their given task
I also find it interesting that as the 928 engine power increased, the vibration dampeners design did not change. Any change in the engine and its operating parameters will change the frequencies of the vibrations being produced. So why no change in the design until much later and then only for the automatics? This also plays into all the stroker motors currently being built for which the old style vibration dampeners were not designed. So does that mean transmissions will fail due to the uncontrolled vibrations coming from the new stroker motors?
The rubber springs on the vibration damper also absorb energy from forward-backward accelerations. The bearing isolators less so, because of the shaft support.
My thinking is that there's two entirely different issues in the torque tube. One is the potential resonances in the rotating driveline. Another is the resonances in the non-rotating components, including the engine block, torque tube housing, and transmission case.
Engineering logic says that resonances in the rotating components can be damaging and need to be controlled. This can be done by making the shaft stiffer and/or adding bearing supports (which make the free segments shorter and thus resonant frequency higher). Bearings cost money, so it's conceivable to me that Porsche made the design worse for automatics with two bearing supports and thicker shaft. This may have gotten the shaft critical speed below the max rpm at a lower cost than using three bearings. Of course, since the thicker shaft is not 28mm all the way and instead necks down, the shaft itself breaks a lot easier than 25mm shaft.
I find that excerpt that Constantine showed more than odd. 90 GTs and 90 S4s used one kind of damper. Then, in 91 they use 2 different dampers. I have it in my mind that Porsche was trying to damp different frequencies on both the manuals (at 4.200 rpm) and the automatics (at 3.300 rpm). So, why in 90, did Porsche use the same damper? And then in 91, use different dampers? At first look, it seems to make no sense.
What needs to be pointed out here is that just adding a damper to a system, changes the proper frequency of the system. So, it is not just the damper material absorbing the frequency, but potentially also a shifting of the frequencies to a point where they no longer interfere with the use of the machine. This would be the case for instance when going from 2 to 3 dampers. This might explain why the automatics had 2 and the manuals 3 dampers. Who knows?
We know what happens now when dampers on the motors slowly give out. The bearings get pounded out. I wonder if the same thing does not happen to the transmissions? Both my cars have the super bearings in them now and one has the ATI super damper from GB in it. We will see, I guess, because in either case (super damper / super bearings) I am more or less buying under the assumption that any new rubber is better than old rubber.
If I understand, your use of "dampener" above is for the bearing units when you said using two or three.
The OE bearing units are not dampeners and were not designed that way. There was only one dampener used by Porsche in a TT.
What is more surprising, as I said earlier, is the vibration dampener design did not change when the engine outputs changed.
And this is not considering the other changes being done to the drive line in frequency attenuation by owners who use non OE parts like motor mounts, transmission mounts, different suspension changes, TT drive shafts of different compositions, changes in wheel diameters which changes the amount of rubber sidewall height which has a direct effect on vibration absorption from the road that gets fed into the chassis, and even exhaust changes.
Every change owners/shops make to the car's infrastructure and subsystems will have an affect on the frequencies of the car. It gets really complicated very quickly.
So trying to place all the work of vibration attenuation on one single part, like the TT vibration dampener, is asking a lot.
If I understand, your use of "dampener" above is for the bearing units when you said using two or three.
The OE bearing units are not dampeners and were not designed that way. There was only one dampener used by Porsche in a TT.
What is more surprising, as I said earlier, is the vibration dampener design did not change when the engine outputs changed.
And this is not considering the other changes being done to the drive line in frequency attenuation by owners who use non OE parts like motor mounts, transmission mounts, different suspension changes, TT drive shafts of different compositions, changes in wheel diameters which changes the amount of rubber sidewall height which has a direct effect on vibration absorption from the road that gets fed into the chassis, and even exhaust changes.
Every change owners/shops make to the car's infrastructure and subsystems will have an affect on the frequencies of the car. It gets really complicated very quickly.
So trying to place all the work of vibration attenuation on one single part, like the TT vibration dampener, is asking a lot.
Some things matter more than others quantitatively, and there are two largely unrelated resonances. One is the resonance of the rotating components (which could break components). Another is the resonance of non-rotating components which usually doesn't have enough energy to break shafts etc. In my opinion, torque tube tuned mass vibration dampener addresses the second issue only. More power, longer stroke, etc. matters mostly for the first issue. I am not surprised by a different stroke or bore or gear ratios not making much of a difference to the second issue, although they may make some difference to the first issue (thus different torsional vibration dampers in different stroke models).
Don't lose the fact that the damper is only designed to help reduce resonant vibration in the tube itself. While the shaft may have its own issues, it is a minor component with very low mass relative to the spinning pieces at either end. Porsche made the shaft rather elastic, and it changes length as it twists and un-twists under load. Constantine's bearings do a lot of dampening for the shaft, but the bearing mass is way to low to do much for the tube itself.
Meanwhile, it's very likely that tube vibration is the cause of the original bearings walking forward and back in the tube. The bearing carriers will move towards null points if allowed to do so.
Greg, I remember some tube damage reports in the 944GTR project, where the mounts for the engine and gearbox were a lot stiffer. A vibration-related failure would likely produce a helical fracture in the tube itself, while a straight torsion-related strain (from chassis twisting for instance) might more easily cause a radial fracture at the exciting end. You have a ton of 944GTR program experience. Got any memories to share on this?
Anybody yet feel the need for CF tube, and maybe even a CF wrap on the shaft itself?
Don't lose the fact that the damper is only designed to help reduce resonant vibration in the tube itself. While the shaft may have its own issues, it is a minor component with very low mass relative to the spinning pieces at either end. Porsche made the shaft rather elastic, and it changes length as it twists and un-twists under load. Constantine's bearings do a lot of dampening for the shaft, but the bearing mass is way to low to do much for the tube itself.
Meanwhile, it's very likely that tube vibration is the cause of the original bearings walking forward and back in the tube. The bearing carriers will move towards null points if allowed to do so.
Greg, I remember some tube damage reports in the 944GTR project, where the mounts for the engine and gearbox were a lot stiffer. A vibration-related failure would likely produce a helical fracture in the tube itself, while a straight torsion-related strain (from chassis twisting for instance) might more easily cause a radial fracture at the exciting end. You have a ton of 944GTR program experience. Got any memories to share on this?
Anybody yet feel the need for CF tube, and maybe even a CF wrap on the shaft itself?
This is incorrect since the three Super Bearings approximate the weight of the one vibration dampener, done so by design. And the effect is now spread over a larger area of the TT.
The C7 Corvette has used CF in their TTs to control some of their problems with limited success.
If I understand, your use of "dampener" above is for the bearing units when you said using two or three.
The OE bearing units are not dampeners and were not designed that way. There was only one dampener used by Porsche in a TT.
Yes, exactly. I was thinking that the OEM bearings are essentially tuned dampers that have a certain amount of rubber to absorb something. That may not be the case.
Nonetheless, my remark about the number of bearings essentially changing the proper frequency of the assembly would still hold true.
One more note. Although the shaft can't resonate at the engine rpm, the firing frequency is within the rpm range. I think that a good torsional vibration damper in the nose of the crankshaft will benefit the driveshaft. My math that assumes about equal segment lengths and three bearings is below:
I think that the torsional vibration amplitudes at engine firing frequency are driven by peak cylinder pressure, holding bore and stroke constant? Not sure, but that would make sense. The good news is that the peak cylinder pressures are not that different between turbo and normally aspirated engines, it's just that the turbo engines maintain that high pressure for a much longer segment of the crank angle degrees. The same torsional vibration damping should work with turbo as with normally aspirated engine.
06-14-2019, 08:46 PM
