CDV Hydraulics Explained

[danewilson77]

Great discussion in here.

Let's ensure we all come into this thread with open minds, and all in the best interest of learning the system better.

Let's not resort to the dialogue that a thread like this would produce at other forums.

HTC DNA, Williamsburg, VA

[RITmusic2k]

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

I concur with everything here; this is the stuff nobody gets to argue about

However -

Therefore:

THE MOVEMENT OF THE THROWOUT BEARING RELATES LINEARLY TO THE MOVEMENT OF THE CLUTCH PEDAL

Here's where I suspect there's something else at play. I concur that the above would be true if the pedal itself were hard-connected to the rest of the system. But from what I've observed it really seems like the pedal was springing up as quickly as my foot allows it to, though faster than the diaphragm spring would be pushing it, based on the delay in full engagement.

This would only be possible if the pedal wasn't hard-coupled to the system, and there would have to be a secondary "pedal return spring", and therefore should be easy to verify by inspecting the mechanism in the footwell. If that's not the case, I'll have to conclude that my foot was actually coming off the pedal sooner as the pedal rose more slowly than it would have unrestricted, but I just didn't notice. I almost want to reinstall my CDV to do some A/B testing and really clarify my observations...

[wsmeyer]

I concur with everything here; this is the stuff nobody gets to argue about

However -

Here's where I suspect there's something else at play. I concur that the above would be true if the pedal itself were hard-connected to the rest of the system. But from what I've observed it really seems like the pedal was springing up as quickly as my foot allows it to, though faster than the diaphragm spring would be pushing it, based on the delay in full engagement.

This would only be possible if the pedal wasn't hard-coupled to the system, and there would have to be a secondary "pedal return spring", and therefore should be easy to verify by inspecting the mechanism in the footwell. If that's not the case, I'll have to conclude that my foot was actually coming off the pedal sooner as the pedal rose more slowly than it would have unrestricted, but I just didn't notice. I almost want to reinstall my CDV to do some A/B testing and really clarify my observations...

Excellent!

You are now one step away from where the light bulb went off in my head.

The pedal is physically attached to the clutch master cylinder, it looks like this:



And there is a spring attached to it, it's part #8 in this diagram:

http://www.realoem.com/bmw/showparts...86&hg=35&fg=05

It's 13mm in diameter and looks like this:

http://www.ecstuning.com/ES63485/

There are also springs inside the master cylinder. They aren't there to bring the pedal back up though, they are there to ensure that the piston in the master cylinder is retracted back past the inlet passages after the pressure has been released. If these weren't there, as the clutch disc wore out the pedal would come to a stop farther and farther towards the floor. More about that can be read here:

http://www.tegger.com/hondafaq/maste.../howworks.html

Remember, when I first realized all of this was when I was bleeding my clutch line. If you hold the pedal to the floor and have someone open the bleed valve, the pressure of the pressure plate spring releases and pushes fluid out of the system. The fluid stops when all of the pressure is released. At that point, when you close the valve and take your foot off the pedal, it's no longer the pressure plate pushing the pedal back up, it's those small springs, which on my car took about 10 seconds.

[LivesNearCostco]

Question for WSMeyer... since I didn't understand all the details of how this thread get started. Are you saying that the CDV doesn't do what the BMW engineers intended, or it doesn't do what most of us enthusiasts and fanatics think it does, specifically that removing it doesn't provide the commonly ascribed benefit? I'm assuming that BMW engineers are good enough that the CDV does what they think it does, and removing it must do something, even if BMW might have mis-judged what drivers wanted or we drivers might not really know what removing it does.

My car came with it pre-removed so I don't know what it's like to have one. Sometimes my car is balky going from 1st to 2nd but my previous indie (whose sometimes drove his wife's ZHP 6MT) said it was due to worn 2nd gear synchros.

[Avetiso]

Question for WSMeyer... since I didn't understand all the details of how this thread get started. Are you saying that the CDV doesn't do what the BMW engineers intended, or it doesn't do what most of us enthusiasts and fanatics think it does, specifically that removing it doesn't provide the commonly ascribed benefit? I'm assuming that BMW engineers are good enough that the CDV does what they think it does, and removing it must do something, even if BMW might have mis-judged what drivers wanted or we drivers might not really know what removing it does.

My car came with it pre-removed so I don't know what it's like to have one. Sometimes my car is balky going from 1st to 2nd but my previous indie (whose sometimes drove his wife's ZHP 6MT) said it was due to worn 2nd gear synchros.

Mine is balky from first to second as well. 64k miles, and I think my synchro is in good shape, but it is a rough feeling sometimes when shifting.

[wsmeyer]

Question for WSMeyer... since I didn't understand all the details of how this thread get started. Are you saying that the CDV doesn't do what the BMW engineers intended, or it doesn't do what most of us enthusiasts and fanatics think it does, specifically that removing it doesn't provide the commonly ascribed benefit? I'm assuming that BMW engineers are good enough that the CDV does what they think it does, and removing it must do something, even if BMW might have mis-judged what drivers wanted or we drivers might not really know what removing it does.

I'm certain that the CDV performs exactly as the BMW engineers designed it to. It's designed to reduce driver fatigue by limiting the power of the release stroke. Of course in some situations this may result in negative side effects, but look at the myriad of perceived benefits of removing the CDV;

Smoother shifting / no jerking / lurching
If the CDV were delaying the clutch disengagement too much you'd end up with the opposite, the slipping clutch would let the engine rev and sound like a worn out clutch. Ever heard anyone say "My clutch slipping turned out to just be my CDV..."


More predictable / non varying engagement point
As I've tried to explain above, with or without the CDV the relationship between the pedal position and the pressure plate is fixed and the engagement point will always be exactly the same. A varying engagement point is a very common problem, it's not the CDV though, it's caused by the pressurizing and equalizing of an air bubble in the line.


Easier low speed driving (parking)
I don't even know what to make of this one but I'd guess they are trying to describe this one ^^.

All of this is just more anecdotal evidence that the effects of the CDV are misunderstood because BMW has been putting these in almost all of there manual transmission cars for roughly 15 years. If in fact the effects of it were negative for 100% of the people than why is it still there? At the very least why haven't they fine tuned it so that fewer people perceive the effects as negative? Have you ever heard anyone say:

"I like the way the car shifted better with the CDV"
"I liked it better without but the wife found it more difficult to shift so I put it back in"

I've done a of of searching and I have not found anyone claiming anything other than "much better" or "no change". How is that possible?

And another thing, the people removing the CDV are all enthusiasts, the group that drives there cars the hardest, yet you rarely hear anyone breaking or prematurely wearing out their transmissions. If it's there to protect the transmission, why don't you ever hear about someone damaging theirs?

My car came with it pre-removed so I don't know what it's like to have one. Sometimes my car is balky going from 1st to 2nd but my previous indie (whose sometimes drove his wife's ZHP 6MT) said it was due to worn 2nd gear synchros.

Second gear synchros almost always wear out first. This is because if you look at the ratios of manual transmissions, the biggest jump in ratio is almost always from 1st to 2nd and therefore the 2nd gear synchro does the most work.

If they are all balky, just 2nd seems worse, try bleeding your clutch line. If you have an air bubble in the line, when you press the pedal down the air pressure compresses the air bubble first, then it expands as it equalizes and engages the clutch. This means there will be a slight delay in pushing the pedal down and clutch engagement, because of the massive difference in mass of the air and pressure plate, the faster you push the pedal down the more the air bubble will be compressed and the greater the delay will be.

[wsmeyer]

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.

What is the universal test to see if your clutch is slipping?

The virtually universal answer is to shift 2 gears higher than normal and floor it, the car will accelerate as the torque output of the engine increases and then if the engine revs "run away" you know your clutch is worn out and slipping.

Why shift to a higher gear? Because the lower the gear the fewer mph the car needs to accelerate per engine rpm of slippage before the clutch locks again. When a clutch first starts slipping due to wear this will happen so fast that it won't even be noticeable in the lower gears.


To further explain this:

The maximum amount of force that can be transferred through the clutch disc / pressure plate interface is fixed and depends on whether or not it is slipping.

The MAXIMUM would be:
FL (force, clutch locked) = Coefficient of static friction x force pressure plate
FS (force, clutch slipping) = Coefficient of kinetic friction x force pressure plate

For all materials known to man:
Both the Coefficient of static friction and the Coefficient of kinetic friction are greater than 0 and less than 1
Coefficient of static friction > Coefficient of kinetic friction


It doesn't make any difference what gear you are in, if the output of the engine is greater than FL the clutch begins to slip and the maximum force transferred becomes FS. Since FS is always less than FL the engine revs spike, but the car will continue to accelerate because some force is still being transferred. For a cars clutch disc, the coefficient of kinetic friction will be pretty high, probably .9 or higher. Meaning more than 90% of the force of the engine is still being transferred through the slipping clutch.

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.

I stated in the very first paragraph of this thread that I was not looking to disprove anyone's observations, I was merely trying to explain why many of the observations cannot be attributed to the CDV and that they must be the result of something else.

[kayger12]

If the CDV were delaying the clutch disengagement too much you'd end up with the opposite, the slipping clutch would let the engine rev and sound like a worn out clutch. Ever heard anyone say "My clutch slipping turned out to just be my CDV..."

I experienced exactly this with quick shifting with the cdv installed, and have never experienced since the cdv was removed.

Sent from my SCH-I535 using Tapatalk 4 Beta

[wsmeyer]

I experienced exactly this with quick shifting with the cdv installed, and have never experienced since the cdv was removed.

I probably shouldn't have put that one in there as I can see how it would cause confusion.

I now believe I know WHY removing the CDV eliminated the clutch slip for you but it wasn't the CDV that was causing the problem per se. That's probably even more confusing but I will explain more later when I have more time.

[LivesNearCostco]

Will, thanks. I could see the CDV being designed to limit power of the release stroke.

I agree removing CDV would not affect actual clutch engagement point but couldn't it affect the perceived engagement point by slowing down how quickly the slave cylinder can release the pressure plate?

I don't see it affecting low-speed driving unless one is constantly working the clutch, which can happen in parallel parking, or starting on a hill.

I have bled my clutch line twice in 3 years and don't recall it making any difference in 1st-2nd shifting, but IIRC there weren't many air bubbles coming out. also having bled it with slave in place, it's possible I didn't get all the bubbles out. Plus my transmission has 173,000 miles.

I'm certain that the CDV performs exactly as the BMW engineers designed it to. It's designed to reduce driver fatigue by limiting the power of the release stroke. Of course in some situations this may result in negative side effects, but look at the myriad of perceived benefits of removing the CDV;

More predictable / non varying engagement point
As I've tried to explain above, with or without the CDV the relationship between the pedal position and the pressure plate is fixed and the engagement point will always be exactly the same. A varying engagement point is a very common problem, it's not the CDV though, it's caused by the pressurizing and equalizing of an air bubble in the line.

Easier low speed driving (parking)
I don't even know what to make of this one but I'd guess they are trying to describe this one ^^.

Second gear synchros almost always wear out first. This is because if you look at the ratios of manual transmissions, the biggest jump in ratio is almost always from 1st to 2nd and therefore the 2nd gear synchro does the most work.

If they are all balky, just 2nd seems worse, try bleeding your clutch line. If you have an air bubble in the line, when you press the pedal down the air pressure compresses the air bubble first, then it expands as it equalizes and engages the clutch. This means there will be a slight delay in pushing the pedal down and clutch engagement, because of the massive difference in mass of the air and pressure plate, the faster you push the pedal down the more the air bubble will be compressed and the greater the delay will be.