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Old 07-30-2012, 11:15 PM   #1
rcw3586
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1993 SL2
1994 SL
Default Clutch Hydraulics Analysis

I and many others have had issues with fork travel, release point and bleeding of the clutch so I wanted to get to the bottom of this.

I sacrificed the factory hydraulics from my retired 94SL in the interest of science today.


There is a lot going on in there and at less than $100 it's a bargain.


First, the pedal piston is .63" dia. while the slave piston is .83" dia. giving a mechanical advantage of 1.7x and a motion ratio of 1 : .57.



For every inch the master cylinder moves the slave piston moves .57". But remember, the master cylinder piston is attached about 4" below the pedal pivot and the pedal itself is about 15" below the pivot. That's a mechanical advantage of about 4x and motion ratio of 4:1. If we multiply the two mechanical advantages together we get 6.8 with a motion ratio of about 1 : .15. This means that if everything is working correctly we get .15" of slave piston travel for every 1 inch of pedal motion or .9" for 6" of pedal travel. Therefore - if we get much less than .9" of motion, there is something wrong!



Both pistons are returned to their maximum extension by springs behind the pistons. This fills the hydraulic system with fluid to its maximum volume before the system is installed.

The master cylinder is installed first but the rod is not attached to the pedal. When we install the slave cylinder we partially compress the slave piston spring and force excess fluid back into the reservoir through the open resevoir port.



MASTER CYLINDER
When the master piston rod is connected to the pedal, the master cylinder spring acts to push the pedal to its max height. The master cylinder has an interesting spring/piston arrangement. The spring holds the piston in the fully extended position with a small seal on the opposite end disengaged from the reservoir opening. This allows fluid to flow freely from the reservoir filling the system when the pedal is not pushed. When the pedal is pushed, the small seal extension of the master cylinder piston closes first sealing off the opening to the reservoir. The hydraulic system is now totally sealed and all the fluid goes down to the slave cylinder via ther damper. As the master's piston is pushed in it compresses the spring which keeps the seal closed on the reservoir. It must move the slave piston the whole .9" if the pedal goes down 6".

If it doesn't, then something is compressing, the volume is increasing or fluid is escaping. The latter should be obvious. The only logical conclusion is that a compressible substance is in the hydraulic system - trapped air... or possibly a spring loaded piston in the damper.


SLAVE CYLINDER
The system is self adjusting. The starting point of the slave piston/rod is where it comes to rest against the relaxed fork. The strong pressure plate springs overcome the slave cylinder spring when the slave cylinder is installed. It pushes the slave piston back against its spring. This reduces the volume of the hydraulic circuit forcing excess fluid back up into the reservoir through the open reservior seal. The slave piston has just about 2.5" inches of potential travel allowing it to work with a wide range of clutch tolerances.
No matter where the initial relaxed position of the clutch fork is, so long as it falls within the working range of the slave piston, the hydraulic system can accommodate the full range of motion.
Adding length to the piston simply acts to reposition the slave piston's starting position, further in. There is really no harm in this so long as it is not extended so much that the piston is bottomed out when the clutch is fully engaged. If it is bottomed out, the clutch will be partially disengaged when the pedal is fully up and there will be mechanical contact between the back side of the slave piston and the slave cylinder with the spring fully compressed. This could potentially break the slave cylinder and allow the clutch to slip.
What adding extension to the slave piston does do, is reduce the volume of the hydraulic system. This could potentially force some air out of the system which would restore some of the lost slave travel.
The damper still has the potential of trapping air which is forced out of the slave. But any air that makes its way back to the master cylinder will certainly be expelled from the system as it rises to the reservoir opening.
Based on my analysis I'd think the best way to purge air out of the system would be to position the damper, with ports facing up and force the slave piston quickly all the way in to its stop, numerous times, allowing time for any bubbles to rise into the reservoir before releasing it on each cycle. This will require at least 3 hands and considerable strength if done without some mechanical assistance.

OTHER
Inside the master and slave cylinders is a white grease-like substance surrounding the springs. It must be compatable with brake fluid as they occupy the same space.

The tough plastic tubing can not be removed from the barbed fittings without cutting it off. Those barbed fittings are removable if not totally rusted in place. They compress a rubber seal and are held in place by roll pins, dirt and corrosion.

If the slave piston retainer comes off, it's no big deal. You are not going to loose any hydraulic fluid. The piston is captive. It only facilitates correct installation of the slave cylinder and piston rod. On the new assembly I have it can be reattached. It does not break on first actuation. It just releases its tabs on installation.

The "wire test" does not measure the actual travel of the slave piston. It measures the motion of the fork about an inch closer to the pivot point. Therefore the slave piston moves slightly more than the wire. But, it's not enough to get excited over. The measurement error is probably far greater than the difference.

DAMPER
The purpose of the damper is to isolate engine/crankshaft vibrations from the pedal. I had thought that the damper would contain a spring backed piston which could vary the volume of the hydraulic system depending on the pedal pressure. If the volume were to increase with pressure then some of the slave piston's travel would be lost to motion of the damper piston. But it does not. It contains a very stiff .05" thick, steel diaphram. the diaphram is sealed on the port side with an o-ring. The back side is supported over a .066" deep recess. Any high pressure vibrations would only momentarily deflect the diaphram to reduce pressure peaks, and it should have no effect on volume or travel. I have not calculated the pressures in the system but they are not terribly high. One would have to measure pedal force or fork force to make that determination. I'll leave that as an exercise for the student!
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Old 07-31-2012, 12:05 AM   #2
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Default Re: Clutch Hydraulics Analysis

Nice write-up! Now maybe this will help dissipate the hydraulic fog.

It is almost impossible to get all the air out of this system. The pulsation damper is a great bubble trap.

The lines come off if heated. Requires a hot air gun on a low (~250F) setting though.

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Old 07-31-2012, 10:06 AM   #3
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Default Re: Clutch Hydraulics Analysis

Quote:
Originally Posted by OldNuc View Post

The lines come off if heated. Requires a hot air gun on a low (~250F) setting though.
Perhaps I just wasn't patient enough. I pried and heated until the plastic was bubbling with no luck. But I was trying to get back to watching women's beach volleyball.

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Old 07-31-2012, 11:12 AM   #4
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Default Re: Clutch Hydraulics Analysis

great analysis & pics, thanks!

How did you remove the actuator rod from the master cylinder?

I started disassembling the unit a couple of weeks ago and got side-tracked. I have the plastic ring off (the one in the 4th picture) but did not see any obvious way to remove the rod other than yanking, and thought it was kind of late to start doing stuff like that - I've been known to break things when I start getting impatient

I'll have to pick up some kind of glue if I want to reassemble it, looks like

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Old 07-31-2012, 11:43 AM   #5
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Default Re: Clutch Hydraulics Analysis

Quote:
Originally Posted by rcw3586 View Post
Perhaps I just wasn't patient enough. I pried and heated until the plastic was bubbling with no luck. But I was trying to get back to watching women's beach volleyball.
The fitting has to get warm as well and 250F is about the minimum temp. You have to push the line off the fitting as pulling just makes it tighter.

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Old 07-31-2012, 11:50 AM   #6
rcw3586
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Default Re: Clutch Hydraulics Analysis

Quote:
Originally Posted by alordofchaos View Post
great analysis & pics, thanks!

How did you remove the actuator rod from the master cylinder?
I have the plastic ring off (the one in the 4th picture) but did not see any obvious way to remove the rod other than yanking, ...

I'll have to pick up some kind of glue if I want to reassemble it, looks like
As you probably did, I twisted the plastic ring off with a pair of vice grips. Surprisingly, it did not break.
I then removed the retaining ring (seen on the rod) by pushing it in then pulling it out sideways. Then the whole piston assembly comes out, rod and all.
The rod can be removed from the piston by prying between the hat-shaped metal cap and plastic piston. It's just pressed in. That doesn't buy you much though.
I doubt that you can remove the rod without taking out the retaining ring. The hat-shaped metal piece is probably too big.

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Old 07-31-2012, 12:14 PM   #7
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Default Re: Clutch Hydraulics Analysis

Thanks! That is exactly what I was after, removing the whole piston assembly. I "misspoke" earlier, asking about just the rod

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