What exactly does a Clutch Slave Cylinder do?

[corey]

If someone could please explain to me (and anyone else who may need to know sometime) what exactly the clutch slave cylinder does, it would be greatly appreciated.
Just trying to learn more about my bike is all.
Thanks in advance!
-Cor

[DucHead]

If someone could please explain to me (and anyone else who may need to know sometime) what exactly the clutch slave cylinder does, it would be greatly appreciated.
Just trying to learn more about my bike is all.
Thanks in advance!
-Cor

The clutch slave takes the compressed brake fluid and uses it to push on a piston which in turn pushes a rod through the engine cases to move the clutch pressure plate outward on the right side of the bike.  With the pressure plate moved outward (compressing the springs on the plate), the clutch pack discs* move independently, and the engine is disengaged from the transmission. 

When you release the lever, the springs on the pressure plate expand and push the pressure plate inward, thus making the clutch pack discs move as one and the engine is engaged to the transmission.


*If you look at the clutch pack, some discs have tines that insert into the hub only, while others have tines that insert into the basket.  When compressed, the pack causes the hub to turn the basket which is attached to the transmission.  When not compressed, the individual plates are free to rotate independently, and the rotation of the hub does not cause rotation of the basket.

[Buckethead]

I'm sure someone else is typing a better explanation while I'm cobbling this together, but here goes.

Just as you use the hydraulic effect (non-compressability of liquids) to magnify the force applied to your brake lever and squeeze your disc brakes, you can use a hydraulic system to actuate your clutch, that is, to separate the pressure plate from the spinning clutch pack.

If you've ever seen a clutch pack apart, and looked closely, you'd see that half the plates have "teeth" on the outside, and have the plates have "teeth" on the inside. These plates are alternated in a clutch pack. When the clutch lever is released, the clutch springs push the pressure plate closer to the center of the bike, mashing these plates together and transferring the force of the engine to the transmission.

The pressure plate is pushed AWAY from the engine by a steel rod. This steel push rod is moved back and forth by hydraulic pressure created by your clutch lever. As you squeeze the clutch lever, a small piston (almost identical to the one that actuates your front brakes) forces hydraulic fluid through the clutch line to a similar piston that buts up against the end of that push rod. Rather than forcing brake pads against a solid disc, this piston simply pushes your pressure plate against the force of your clutch springs. As you release the lever, the spring pressure forces the slave cylinder piston back to its original position.

Hope that helps.

[Buckethead]

I'm sure someone else is typing a better explanation while I'm cobbling this together, but here goes.

I make the beast with two backsIN' KNEW IT!

Nice explanation, P. 

[DucHead]

I make the beast with two backsIN' KNEW IT!

Nice explanation, P. 

We said the same thing! 

[Rameses]

Good explanations, both of you.   

[mitt]

Also, you might ask at the same time what the master does.  The master cylinder is up at the handle bars.  It takes X cubic mm of hydraulic fluid, then moves it based on the piston diameter and stroke of your clutch lever.  This volume of fluid is what goes down the clutch line to the slave.  The slave then has a different piston diameter, resulting in less linear movement but more force, thus the reduction in effort for the operator to disengage the clutch.

mitt

[corey]

Excellent explanations from all parties.
I really appreciate it guys. Good piece of info to have around here as well.

Next question!
What are the benefits (aside from obvious shiny bits factor) of an aftermarket slave cylinder?

[mitt]

Excellent explanations from all parties.
I really appreciate it guys. Good piece of info to have around here as well.

Next question!
What are the benefits (aside from obvious shiny bits factor) of an aftermarket slave cylinder?

Number 1 benefit - less likely to leak.  The Ducati ones will eventually leak.  Something about the number of seals they use versus aftermarket.

Number 2 - most aftermarket have a easier, smoother pull due to a bigger piston diameter.

[corey]

Great!
Added to eventual mod list.

[alexisonfire]

Follow up questions:

1. What is the stock slave cylinder piston diameter?  If it varies, what is it for an '02 M900 (dry clutch) 

2. I've read that an aftermarket slave cylinder typically reduces clutch lever pressure.  Is this aftermarket piston usually larger or smaller in diameter than the stock piece?  Please explain why.

Having done a little sketch work on my notepad:

To me it appears that a larger slave cylinder piston diameter will decrease the force required at the master cylinder (lever pull).  However, if the clutch lever stroke remains the same length (master cylinder displacement), the larger slave cylinder piston displacement (stroke) will decrease (the pressure plate will not be pushed as far out when the lever is pulled).  Bottom line: the lever pull distance controls the volume displaced.  To accomodate for this, the slave cylinder piston will have to begin it's stroke at a position further inward. 

I'm betting the aftermarket piston is a larger diameter and begins it's travel several mm further in than the stock one.

[Ddan]

Keerect on the reduced travel, when I swapped in my Evo slave, I had to adjust my lever to be able to get neutral.

[mitt]

Yep, you nailed it.  Easier, but needs more stroke.

mitt

[DarkStaR]

With all that said, if one like the feel of the stock slave, but needs to replace or wants more reliability, what are the options?

...
Follow up questions:
1. What is the stock slave cylinder piston diameter? 
...

+1

[Ddan]

As far as I know, there are rebuild kits with better seals.