Brain hurts. Will just pull the damn thing next time I'm under the car.
2006 CiC 6MT
ZHP, Cold Weather, Xenon
Sapphire Black / Black Leather / Black Cube
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The movement of the diaphragm spring inside the pressure plate is a bit complex but hopefully you can see that there is a direct correlation between the clamping force and the linear travel of the throwout bearing. The clamping force is variable from 0-100% just as the linear movement of the throwout bearing is variable from 0-100% Whatever the correlation is it is fixed. if 20% linear movement of the throwout bearing yield a 50% reduction in clamping force, that will always be the same.
Now realize that the amount of linear travel of the throwout bearing is fixed to the volume of fluid that is pushed into the slave cylinder.
Since the CDV has no ability to store fluid, any fluid pushed out of the master cylinder, must go into the slave cylinder.
It doesn't matter which direction you are analyzing:
volume in = volume out
Therefore:
THE MOVEMENT OF THE THROWOUT BEARING RELATES LINEARLY TO THE MOVEMENT OF THE CLUTCH PEDAL
There is no exception to this, unless of course, you have compressible air in the lines.
2006 CiC 6MT
ZHP, Cold Weather, Xenon
Sapphire Black / Black Leather / Black Cube
Of course volume in = volume out. But that doesn't mean the CDV can't slowdown the whole system. And that's exactly what it does. Everything eventually goes where it needs to go, but it takes a little (and I do mean little -- the delay is probably small fraction of a second) longer. The clutch return spring is still there, and the fluid does move with the pedal. But there is enough resistance to prevent hard launches and make it difficult to perform shifts which require precise timing.
Think of drawing fluid through a syringe with a needle attached vs. one without a needle. If you want to draw fluid at the same rate through both, you'll need to exert a lot more force on the former (and there's generally a small empty space, meaning that you need a pretty large pressure gradient to move the fluid that quickly). Why is this? It's because the resistance through the tiny needle opening is quite small. You can think of the CDV in a similar manner -- the CDV is more restrictive than a direct connection between the lines. The diaphragm spring and clutch return spring exert the same force regardless of whether or not there's a CDV. As such it is reasonable to conclude that the increased resistance delays everything. As for why the difference is noticeable in lower gears than in higher gears, that's because you don't really need full clamping force to prevent the clutch from slipping in higher gears.
At the end of the day, all of the empirical evidence disagrees with your conclusion. Almost every single person who has done this mod noticed a difference, whether or not they bled the clutch (I've personally never bothered bleeding the clutch both times... mostly because I noticed my clutch line had blue fluid in it even though I never actually bled my ATE superblue through it). So either everyone is wrong and you're more well versed in hydraulics than every BMW owner, or your understanding of the mechanisms are flawed. Take your pick.
Last edited by terraphantm; 04-24-2013 at 11:10 AM.
2006 CiC 6MT
ZHP, Cold Weather, Xenon
Sapphire Black / Black Leather / Black Cube
That equation would be used to calculate the movement of fluid from a high pressure area to a low pressure area.
If you had a hole in your line, P1 would be the pressure in the line and P2 would ~ 15psi (1atm) and the amount of fluid spraying out would depend on the size of the hole.
If your head gasket is leaking P1 would be the oil pressure ~ 60psi and P2 would be whatever the relief valve in the ET cap is ~ 20psi and again the flow of oil into the coolant would depend on the size of the hole.
Our clutch system is a bit different though. The pressure plate is providing the resistance to movement. You apply a force to the clutch pedal that increases the pressure in the hydraulic line. From Pascal's law: "increasing the pressure at any point in a confined fluid, there is an equal increase at every other point in the container" so according to that equation P1-P2 would always = 0 and there would never be any flow.
That doesn't mean the equation is incorrect, it's just not applicable to our system.
Think about this. If there is a bucket of water on the floor and I push it a distance with my foot obviously the water moved but could you calculate it with that equation? No.
2006 CiC 6MT
ZHP, Cold Weather, Xenon
Sapphire Black / Black Leather / Black Cube
I didn't forget about anything. If this was an assignment in a Physics class we would have to account for a whole bunch of stuff that I neglected because I believe they are insignificant compared to the larger forces in our system:
compressibility of the fluid
amount of gasses in the fluid and it's effect on density
compressibility of the gasses in the fluid
friction losses through the CDV
There's probably more but if you tink they are significant you should start with friction as it would be the largest.
The orifice in the CDV is .060 inches in diameter and .0165 inches in length.
If we call the friction loss through the CDV 'f' and the friction loss through the rest of the system 'F' then f+F / F would give the error factor of neglecting it.
2006 CiC 6MT
ZHP, Cold Weather, Xenon
Sapphire Black / Black Leather / Black Cube
I really enjoy reading all of this, and enjoy the debate we can have on our site. Unlike other sites, we don't lob grenades from the back of the fight or make snide comments to others of differing opinions. Im glad this thread is here. Lets continue this discussion the same way it started. Facts, observations, and principle of physics. I look forward to more info Wsmeyer, terra, and dane. I have a hard time shifting 1-2 compared to other cars, and want to find out why before i spend the time to remove something bmw thought should be there.
"No, Donny, these men are nihilists, there's nothing to be afraid of"
09 135i Msport 6mt
04 330i ZHP (sold)
David