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Discussion Starter · #1 ·
A lot is written about the clutch slave cylinder . Could someone please to me in total layman's terms what is the function of this piece of equipment so I can better understand my bike. Thank you
 

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From the backside of the tranny through to the center of the clutch is a steel rod about the thickness of a Sharpie inkstick...

One end rests against the pressure plate on the clutch, the other inside the Slave. When you pull the clutch lever it pushes in the rod (pin) thus pushing in your clutch....

Your end of the lesson.. :wave
 

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The slave cylinder replaces the older mechanical linkage to release the clutch. It has been decided by OEM's like BMW that it is easier to run a small hydraulic line Vs a cable and mechanical arms, pivot points, etc..

Does this explain it??
 

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Master and slave....What the master wants, the slave does...When you pull on the clutch lever on the handle bar, it is connected to a hydraulic master cylinder, which is a pump of sorts, it pumps pressure into the hydraulic line that runs from the handlebar to the clutch... at the receiving end of that hydraulic line is the clutch slave cylinder, it uses the pressure that you pump into the line with the clutch lever, and pushes a rod into the clutch pressure plate that engages or disengages the clutch. As was previously stated this hydraulic system replaces the old clutch cable, springs, levers, in the less technical and older bikes. As is most noted in discussions on this forum, a big problem that plagues us LT riders is leakage of the slave cylinder seal. You see, the hydraulic fluid creates pressure in the slave cylinder and that pressurized fluid pushes the rod that operates the clutch pressure plate. There is a seal that keeps the hydraulic fluid inside the cylinder yet allows the rod to move in and out of this cylinder. If this seal fails, and hydraulic fluid leaks past the seal and into the clutch area, it eventually contaminates the dry clutch with a lubricant which makes the clutch slippery and won't hold tight anymore. (yea, its ruined) The idea of drilling a weep hole is designed to allow any hydraulic fluid that leaks past a failing seal, to run down the outside of the clutch housing thereby eliminating contamination of the clutch, and saving a huge headache (and don't forget big $$) of replacement of the clutch...Man! I can be long winded!!! :rotf:
 

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Discussion Starter · #5 ·
So ,just as holding the clutch lever in on a cable system will generally wear out the cable faster, will holding in the lever needlessly on this system wear out the slave cylinder quicker ?
 

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tenrocky said:
So ,just as holding the clutch lever in on a cable system will generally wear out the cable faster, will holding in the lever needlessly on this system wear out the slave cylinder quicker ?
Holding a lever on a cable will not wear it out faster, nor will it wear out the slave faster. There is a small "throwout" bearing the rod rides in at the end of the slave piston. It is always spinning whether you have the clutch pulled in or released and it is always under some pressure. It just does not make any difference.

By the way I always stay in gear at the lights with the clutch pulled in - 71K and no issues so far.
 

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Discussion Starter · #7 ·
Thanks for helping me to understand things a little better. By the way, 54000 miles with no problems, just trying to learn all I can.
 

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John, do you know what the failure mode(s) is/are for the clutch slave (when fluid eventually leaks) ? I had mine off my '05 to drill the weep hole and looked at the thrust bearing as much as I could see without removing it from the bike, very small. Being a small diameter bearing the surface speed will never be very high, and the bearing looks to be sealed ?? So does the bearing eventually seize, causing the slave piston to rotate and wreck the piston seal ?
I pressed a little moly lube in around mine while I was in there, it may or may not help. I note the slave cylinder unit is by Magura, but obviously not available from them, only as an OEM part from BMW. Not rebuildable according to Clymers ?
 

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Here's another one,

Let's say that you've got the clutch operating rod with the groove for the felt piece.

If you put a new felt thingy in there ............... should you also put a couple drops of engine oil on it to help it slide inside the xmsn shaft ?

I'm doing the weep hole mod as we speak.
 

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jzeiler said:
By the way I always stay in gear at the lights with the clutch pulled in - 71K and no issues so far.
+1 This one good habit to develop, along with keeping an eye in the rear view mirrors.
I see many riders struggle to get in first gear when the light turns green, with impatient cagers right on their tail... Not the safest combo!
 

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Dennis,

It is a small bearing and when it does lock up it will spin the slave pistion. The diameter of the rod is tiny and you are right it does not spin fast but it is under a certain amount of pressure all the time. A little dab of lube can't hurt the bearing.

rhawk

I would leave it dry - you should not have any trouble sliding it into the shaft. Its purpose is anti -rattle not anti oil contamination.
 

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Thanks John, I suspected that may be the failure mode. Sounds like a PITA, but I might just make a point of going in there occassionaly and doing a regrease of the little bearing.
I assume that the clutch actuation rod has a hardened end on it ? I didn't examine it too closely when I did my weep hole. That being the case, I presume the clutch rod normally survives the slave failure.
 

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Yeah, The air heads used to weld them selves to the release disk. Not the case with the LT the tip goes through a hole and a shoulder rests on the release spring. I don't think the slave end get damaged when the piston starts spinning. At least I don't remember David Shealy mentioning that when his spun.
 

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Years ago hydraulic clutches, and brakes, replace cables or rods to connect the motion of the control.....a handle or pedal......to the motion of the component.....a clutch or brake.

Imagine two large syringes. Remove the needles and have the two ends connected by a long tube. The syringes and the tube are filled with water. If you pressed on the plunger of one syringe, the plunger on the connected syringe would move out. You're pressing on the "master" cylinder piston.....and the "slave" cylinder piston is moving. The cool thing about this system is that it doesn't care how long the tube is, within reason. So, if I want something that I do with my hand.......ie squeezing the clutch handle, to result in mechanical motion somewhere else on the bike, in this case a clutch assembly, I simply hook up a piston with a plunger to the handle of my clutch....the master cylinder......by a long hose filled with oil.....to a companion piston with a rod attached......the slave cylinder.

When the rod on the slave cylinder moves, it actuates the clutch mechanism.

To complete the system, you need a spring on the clutch to "push back" when the clutch is to be reengaged. This all works because the oil flows in the lines when pushed, but because the system is sealed and there are no air bubbles in the system, what happens at one end, is immediately happening at the other end.

Push the master piston rod....the slave piston rod moves. Let go of the handle moving the master piston rod, the spring on the slave piston assembly pushes everything back where it was.

Some time ago folks decided that such a system, long term, was more reliable and easier to maintain than a flexible cable, or a series of complex rod linkages.

JD
 

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They'll also use a larger diameter piston on the slave for less effort at the master. You will loose travel on the slave piston as compared to the master but you don't need much.
Self adjusting to boot.

The downside, they can leak :mad:
 

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jzeiler said:
Dennis,

It is a small bearing and when it does lock up it will spin the slave pistion. The diameter of the rod is tiny and you are right it does not spin fast -------------------------------.
It spins at whatever the engine RPM is, I consider this quite fast. Most standard electric motors only spin at 1,700-3,600 RPM, the LT engine idlesat around 1,750, and goes up to near 8,000.

That is fast for most any normal bearing.
 

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David, my reference to the bearing speed is based on its diameter. "Fast" is a relative term only. I'm well aware of the potential engine rpm's, but this bearing is extremely small in diameter, so itssurface speed (m/min, ft/min) will be quite low. If someone can quote me the bearing number I can do some calcs on surface speeds and potential (theoretical) L10 life for the bearing. Remember, there is a point very close to the centre of every circle where there is virtually no velocity but there's plenty at the outside diameter. rpm's will always be the same though.
 

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K100Dennis said:
David, my reference to the bearing speed is based on its diameter. "Fast" is a relative term only. I'm well aware of the potential engine rpm's, but this bearing is extremely small in diameter, so itssurface speed (m/min, ft/min) will be quite low. If someone can quote me the bearing number I can do some calcs on surface speeds and potential (theoretical) L10 life for the bearing. Remember, there is a point very close to the centre of every circle where there is virtually no velocity but there's plenty at the outside diameter. rpm's will always be the same though.
It is a ball thrust bearing, not a radial bearing, I do not think there is a replacement bearing available, as it it pressed into the piston. It is about 12-14 mm outside diameter, from memory, have not measured one. The ball race is likely around 10mm diameter.

I MAY still have the piston that failed, will have to look this weekend to see if it is still in my LT junk box.

That bearing does not have to get very "tight" to cause the piston to spin, which pretty rapidly destroys both the piston and cylinder.

Here is the picture of my "spun" piston/cylinder
.
 

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Thanks for the info David, looking at it in the SKF General Catalogue, if it's a thrust ball bearing of your approximate OD it is possible that it's a BA-5 or a BA-6. It may of course be some modified derivative, as these things sometimes are when the OEM does a deal with the supplier of the parts. However, that bearing won't be a sealed type bearing (not possible for this type), so there is probably some dust seal arrangement built into the Magura assembly and the bearing has some lube applied during assembly, who knows. That being the case, it may be possible to get some fresh lube in there if one was serious about preventing the failure. For the record, both those bearings mentioned have designated limiting speeds of 24000 and 28000 rpm respectively. Man, the seizure sure does a number on the piston and cylinder !
 

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K100Dennis said:
Thanks for the info David, looking at it in the SKF General Catalogue, if it's a thrust ball bearing of your approximate OD it is possible that it's a BA-5 or a BA-6. It may of course be some modified derivative, as these things sometimes are when the OEM does a deal with the supplier of the parts. However, that bearing won't be a sealed type bearing (not possible for this type), so there is probably some dust seal arrangement built into the Magura assembly and the bearing has some lube applied during assembly, who knows. That being the case, it may be possible to get some fresh lube in there if one was serious about preventing the failure. For the record, both those bearings mentioned have designated limiting speeds of 24000 and 28000 rpm respectively. Man, the seizure sure does a number on the piston and cylinder !
And, the bearing was not reallied "seized", as it still turned, but with a little roughness. It does not take much torque to spin a lubricated piston in a cylinder. The seal is probably the only real resistance to it turning. When the hard anodize coating is worn away by the turning, the seal is then destroyed as it moves over the ridges. The coating is only a couple thousandths thick, and the base aluminum is soft.

In my opinion, I think that a great many of the failed slave cylinders, if not most, are because of this. Few have ever looked at a failed one to see.
 
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