996 Carrera 4S broken AWD
11-13-2010, 06:54 PM
#16
Drifting
I'm no expert on this, but I did work for a company for 13 years that produced silicone oils of various viscosities for various purposes. I understand the oil in the front diff is silicone - and if kept clean will never need replacement. I also understand that Porsche changed to the viscous coupling from the electronic multi-clutch units (like the 959 and 964) only after sophisticated machine tooling was invented in the 90s. So the concept should work something like a shear thickening fluid - when there is increasing slip as the front wheels spin at a different RPM than the drive shaft, the fluid gets thicker (more viscous like honey) and begins to transfer more of the torque between two rotating elements. Not sure how the cold affects all this - as it seems the colder it is, the higher the viscosity. But of course lab measurements are made at standard conditions - usually room temperature. Some food for thought...maybe someone can correct this theory or produce the actual design theory from Porsche.
DC4
DC4
The VC case is hermetically sealed so as the fluid heats due to the shearing of the clutch plates it cannot expand in volume so it "thickens".
11-14-2010, 11:21 AM
#17
Three Wheelin'
Sorry, I don't buy either argument, liquids get less viscous with increased temperature. And the idea of the gas bubble is in the VC is simply strange - can you point to an example? I took the time to look it up, and I'm correct - as the shearing of the fluid increases, the viscosity increases - the torque is increasingly transferred from disk to disk via the fluid. It has nothing to do with temperature. Pre stiffening the fluid also will not work, as it simply reverts back to the lower viscosity when the shear is removed (here is a quote from Wiki):
Traction control
Dilatant materials have certain industrial uses due to their shear thickening behavior. For example, some all wheel drive systems use a viscous coupling unit full of dilatant fluid to provide power transfer between front and rear wheels. On high traction road surfacing, the relative motion between primary and secondary drive wheels is the same, so the shear is low and little power is transferred. When the primary drive wheels start to slip, the shear increases, causing the fluid to thicken. As the fluid thickens, the torque transferred to the secondary drive wheels increases proportionally, until the maximum amount of power possible in the fully thickened state is transferred. See also: limited slip differential, some types of which operate on the same principle.
To the operator, this system is entirely passive, engaging all four wheels to drive when needed, and dropping back to two wheel drive once the need has passed. This system is generally used for on-road vehicles rather than off-road vehicles, since the maximum viscosity of the dilatant fluid limits the amount of torque that can be passed across the coupling.
Traction control
Dilatant materials have certain industrial uses due to their shear thickening behavior. For example, some all wheel drive systems use a viscous coupling unit full of dilatant fluid to provide power transfer between front and rear wheels. On high traction road surfacing, the relative motion between primary and secondary drive wheels is the same, so the shear is low and little power is transferred. When the primary drive wheels start to slip, the shear increases, causing the fluid to thicken. As the fluid thickens, the torque transferred to the secondary drive wheels increases proportionally, until the maximum amount of power possible in the fully thickened state is transferred. See also: limited slip differential, some types of which operate on the same principle.
To the operator, this system is entirely passive, engaging all four wheels to drive when needed, and dropping back to two wheel drive once the need has passed. This system is generally used for on-road vehicles rather than off-road vehicles, since the maximum viscosity of the dilatant fluid limits the amount of torque that can be passed across the coupling.
11-14-2010, 01:04 PM
#19
Drifting
"...fluids get less viscous with increased temperature..."
Yes, EXCEPT....
If you take an already "thick" fluid that has a HIGH volume expansion ratio with increasing temperature and "trap" it in a fixed volume hermetically sealed container and then begin heating it.
With the gas bubble you can delay the onset of increased coupling so there is little or no reaction to "ordinary" F/R drive line slippage, turning or even slightly different tire wear F/R. But once the delay is overcome the slope of the onset can be dramatic.
"...as the shearing of the fluid increases the viscosity increases.."
Yes, absolutely...!
The "shearing" HEATS the fluid, the fluid CAN NOT expand in volume, so the fluid's effective density, "viscosity", increases, becomes more "syruppy" and the coupling coefficient between the VC clutch plates increases.
Yes, EXCEPT....
If you take an already "thick" fluid that has a HIGH volume expansion ratio with increasing temperature and "trap" it in a fixed volume hermetically sealed container and then begin heating it.
With the gas bubble you can delay the onset of increased coupling so there is little or no reaction to "ordinary" F/R drive line slippage, turning or even slightly different tire wear F/R. But once the delay is overcome the slope of the onset can be dramatic.
"...as the shearing of the fluid increases the viscosity increases.."
Yes, absolutely...!
The "shearing" HEATS the fluid, the fluid CAN NOT expand in volume, so the fluid's effective density, "viscosity", increases, becomes more "syruppy" and the coupling coefficient between the VC clutch plates increases.
11-14-2010, 01:24 PM
#20
Drifting
Post #7 and #15, Reads authoritively, be your own judge.
http://www.can-amtalk.com/forums/ind...howtopic=13134
Of special interest, notice...
...they do have the virture of failing gracefully...
http://www.can-amtalk.com/forums/ind...howtopic=13134
Of special interest, notice...
...they do have the virture of failing gracefully...
11-16-2010, 10:41 AM
#22
Team Owner
God help the person that tries to change out their viscous coupling fluid. That stuff isn't meant to be changed like engine or differential oil. Hell, I'm not sure where you'd even buy the stuff.
11-16-2010, 10:48 AM
#23
Cruisin'
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Yes, the rear wheels was spinning, and not the front wheels. So I am going to change the front diff, and hopefully that will fix the problem.
11-16-2010, 01:04 PM
#24
Drifting
But even with the VC having failed "gracefully" there should still be enough coupling to turn the front wheels off the ground with no resistance.
Drive line/shaft coupling...??
11-16-2010, 01:13 PM
#25
Drifting
Twenty seconds....??
Leads one to understand why Porsche has gone the way of adopting Ford's (Escape) ECU controlled electromechanical coupling clutch.
11-16-2010, 01:20 PM
#26
Team Owner
Hmm, I don't think that means what you think it does. The test procedure simply requires the tech to check at the 20-second point and isn't indicative of when the front wheels are activated.
11-16-2010, 01:31 PM
#27
Cruisin'
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Were you (able to) watch both...? Due to the diff'l one front wheel might turn but not the opposite.
But even with the VC having failed "gracefully" there should still be enough coupling to turn the front wheels off the ground with no resistance.
Drive line/shaft coupling...??
But even with the VC having failed "gracefully" there should still be enough coupling to turn the front wheels off the ground with no resistance.
Drive line/shaft coupling...??
11-17-2010, 06:03 AM
#30
Burning Brakes
Do you think this Porsche C4 in the link is also broken?:
http://www.youtube.com/user/JohnnyGu...25/iV8pB6OQCTI
