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Performance clutch + master/slave cylinder riddle

aristg

aristg

El Greco Capitano
300+ Club
Joined
Sep 19, 2013
Location
Greece
Ιt seems that we have a difficult situation here. Let me explain in detail:

I have a M90 on a B21ET which worked perfectly with the dog dish flywheel with the master - slave cylinder setup from a Volvo 740: FAG S5394G (master) + FAG 6472G (slave), engaging and disengaging at a normal position and also a reasonable foot pressure.

Then I changed to a steel flywheel by Bakaxel.se, a 765 clutch and a 6 puck sprung clutch disk also by bakaxel.se. I shimmed the clutch fork with 2 shims and used a long neck pivot ball. The result was that the clutch fork almost maxed out the slave cylinder which was good, because I could use the whole pin travel of the slave cylinder.

The clutch fork can be pressed towards the slave pump by 1,5mm .. it is that maxed out !

20230709_120420.jpg

However there is a problem when the engine is running: The gears can not shift smoothly, they need too much force to shift. This means that the clutch is not disengaging fully. Also the clutch engages and disengages/engages too low (in the way it does), close to the firewall. Ok, i know everything is new, however there should be some more clutch fork travel to work as it should, and with a higher engaging pedal.

The reason for this problem is that the 765 needs some more clutch fingers movement, more than the M90 stock config, to disengage. Jonas from Bakaxel said it needs 9mm to open up. No idea how much the old one needed, but definitely was less.

So the first task is to measure how much is the additional clutch fork travel needed to function properly. I am thinking to make an adjustable pin just for testing purposes by some kind of rod to find out how much more movement it needs to disengage. The slave cylinder will be maxed out totally plus the clutch will not engage fully, but I am just looking for the extra travel to disengage.

However this is half of the problem: how can I achieve this "unknown" extra movement ?

The slave cylinder's bore is 22,10mm and by considering its bore and movement I calculated the master cylinder's bore that is about 19mm, which is what is probably the spec of the 740's MC. It also makes sense because having a smaller bore MC than the slave, gives a lighter clutch pedal.

Also measured the travel of the MC which is 25mm and the clutch fork's movement (which actually is the same with the slave cylinder's movement) at 17.5mm. There is an additional 21mm of travel in the slave cylinder until its piston comes flush to the face of the housing. To increase the travel of the clutch fork there are three solutions:

1) Find a master cylinder with more stroke than 25mm @ 19mm bore. This could be a 264 MC which as Dave Barton says on his site has a stroke of 33mm @ 19mm. It is a unicorn to find though and I am not sure that even if I found one used I could trust it with such heavy clutch. This however would result in a same effort pushing pedal as now.
2) Find a MC with a larger bore (eg tilton). Would make the clutch heavier
3) Find a slave cylinder with smaller bore. Would make the clutch heavier

So here is where I am stuck. Any suggestions/opinions ?
 
The stroke of the slave is defined by the amount of fluid that is pushed into it by the master cylinder. So I'm not sure how your option 1 can work. If the suggested slave has the same bore as your current one, then its stroke will be the same.
Option 2 and 3 make sense.
It might be worth taking some accurate measurements for the bore of the master before deciding.
 
crogthomas said:
The stroke of the slave is defined by the amount of fluid that is pushed into it by the master cylinder. So I'm not sure how your option 1 can work. If the suggested slave has the same bore as your current one, then its stroke will be the same.
Option 2 and 3 make sense.
It might be worth taking some accurate measurements for the bore of the master before deciding.
Click to expand...
You are right. I am just saying that my current MC has a travel of 25mm while the 264's has 33mm for the same bore (19mm). So in theory it would provide 32% more travel at the same slave cylinder, since it will push 32% more fluid to it, at the same pressure, just because the master cylinder will make a bigger movement. Also if i am thinking it correctly the pedal will come higher !
The slave cylinder has an additional 21mm to travel so it can accept more fluid as you said.

However the problem is which master cylinder to chose that fits the 240 !
 
We have a similar concern with the clutch in our 244 w/GS6-37DZ transmission, although maybe not as bad. We're also using a 19mm/22mm master/slave cylinder combination. We found some improvement by locating the clutch fork pivot so that the fork is perpendicular to the input shaft when halfway through its travel. At this point we are ok with it, but if the pedal was another inch off the floor, I'd be happier.

Try that and see if you have a better pedal.
 
Ah, this sounds familiar. I couldn't get my M90+765+steel flywheel+organic disc to disengage at all first. Got it to work with a 20,6mm slave from an early na 945 and a new braided hose from the master to the slave. Still not entirely happy with it.
 
Thanks for the input guys, I am trying to achieve everything at once which looks a bit difficult:

  1. Make the clutch driveable, ie disengage: I made a custom slave cylinder pin which i could change its length, and i found out that if i make it 84,20mm long the clutch disengages properly. The stock pin i have is 80,70mm so considering the 1,5mm i showed on the pic in my first post (the longer pin bottoms the slave cylinder which actually makes no difference in the pin travel - just for testing config) the calculation is: 84,20 - 80,70 - 1,50 = 2mm. So actually the clutch fork travel has to be increased by 2mm for the clutch to disengage. I tried with a shorter pin of 83mm instead of 84,20mm but it failed to disengage properly.
    So if i find a MC which bore remains the same (19,05mm) but the pedal stroke increases to 27,90mm (so actually 3mm more than what i got now) the clutch will disengage properly.

  2. Make the clutch pedal lighter: In order to do that i need to find a MC with a smaller bore compared to the one i have now but increase the pedal travel even more:
    - If i choose a 17,78mm bore (0.70") then i need a travel of 32mm .. so 7mm more than i got now
    - If i choose a 15,875mm bore (0,625") then i need a travel of 40,18mm .. so 15,18mm more than i got now.
According to Dave Barton who has seen it in person, the stock 264 MC has a travel of 33mm on the 240's pedal assembly (actually is the same). So 8mm more than i have now. I need however to investigate my config a bit more and take some measurements (without the MC attached), because my current setup almost uses the whole pedal travel from the firewall to the top of the pedal assembly that does not allow the pedal to come much higher. I need to figure out how they did it on the 264.

However i noticed something interesting on my current setup. The FAG MC has a metallic bump stop on it, which actually stops the MC piston from going deeper. Maybe the 264 didn't have such a thing ? Check this pic i got from my car:

IMG_1850.jpg

As for the engagement point of the pedal i have still not figured out how to change it. In my opinion these options above will keep exactly the same point as i got now, but it will disengage the clutch.

Also another thing that concerns me is that Wilwood MCs have the connection towards the slave cylinder facing upwards. How the hell do you bleed them without air remaining in the line?? (the bend i have marked with a red circle)

260-15091-lg.jpg

PS.. Indeed these need a bunch of math to calculate if anyone is interested i could explain :eek:
 
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shoestring said:
We have a similar concern with the clutch in our 244 w/GS6-37DZ transmission, although maybe not as bad. We're also using a 19mm/22mm master/slave cylinder combination. We found some improvement by locating the clutch fork pivot so that the fork is perpendicular to the input shaft when halfway through its travel. At this point we are ok with it, but if the pedal was another inch off the floor, I'd be happier.

Try that and see if you have a better pedal.
Click to expand...
I should have done that before installing the gearbox ... next time i get it out i will do it for sure. I am not sure if there is a way to remove a shim from under the pivot ball without removing the gearbox first.
 
The travel of the pedal is limited isn't it? With a stop at the top and the floor at the bottom?
So whichever master cylinder is fitted, the total travel will be the same. Unless the pedal itself is changed/modified.
 
crogthomas said:
The travel of the pedal is limited isn't it? With a stop at the top and the floor at the bottom?
So whichever master cylinder is fitted, the total travel will be the same. Unless the pedal itself is changed/modified.
Click to expand...
Yes indeed, however there is a paradox. With this pedal assembly the 264 had a MC travel of 33mm (at least), where this one i have has 25mm. So i need to take some measurements to check the situation after i remove the current MC. This is why i am looking at other options. Otherwise the bigger bore MC or smaller bore slave is the only way to go.
 
shoestring said:
I think it can be done with M90.
Click to expand...
Well was thinking of the same at some point but i think that the clutch fork has to be removed. And then how to aim for the release bearing again with no view in there ? Plus nothing has to fall in the bellhousing. I don't know, maybe someone can tell us !
 
You decrease leverage when you increase travel at the throwout bearing compared to the pedal pad. You only get an easier pedal when you go the other way and lose travel at the bearing.
 
mikep said:
You decrease leverage when you increase travel at the throwout bearing compared to the pedal pad. You only get an easier pedal when you go the other way and lose travel at the bearing.
Click to expand...
I am trying to increase pedal travel at the same time time as the bearing's. I am looking for a master cylinder that will have more travel by at least 3mm compared to the the 25mm i got now, which will translate into 2mm more at the slave. The other 2 choices (that i believe make pedal softer) increase even more the pedal travel but the travel of the pilot bearing remains the same, so actually happens what you said, "loosing travel" !

Fm/Am=Fs/As (force master/bore master = force slave/bore slave) hence Fm=(Am/As)*Fs. So if the slave bore is same as the master the forces needed are equal. However if say the bore of the master is half than the slave then the force used at the master cylinder doubles at the slave.

The fluid quantity moved in all my calculations is the same, this is why i am worried about the pedal remaining at the same engagement height.

Here is an interesting table by Wilwood: https://www.wilwood.com/PDF/Flyers/fl162.pdf (they take into account the pedal ratio as well)
 
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I am sorry, I somehow managed to delete Mikep's answer which was correct ! How did i do that ?? I thought i was correcting my answer. Anyway he wrote:

"Half diameter does not equal half area."
 
So answering to Mikep:

Let's see. For 1/2 bore master of the slave:

Fm=(Am/As)*Fs= ((pi*(1/2*rs)^2)/(pi*rs^2))*Fs=Fs/4

So yes it becomes 4 times smaller not 2. You are correct, my mistake, forgot the square, its difficult to write math as text ..
 
I deleted it after trying to rewrite without seeming condescending, then forgot, since i’m cleaning out my girlfriend’s mother’s refrigerator.
If you can adjust the pedal height up and lengthen that pushrod without bottoming out the mc, would that help?
If not, look at small increases in mc diameter, and look at the usable stroke as well.
 
mikep said:
I deleted it after trying to rewrite without seeming condescending, then forgot, since i’m cleaning out my girlfriend’s mother’s refrigerator.
If you can adjust the pedal height up and lengthen that pushrod without bottoming out the mc, would that help?
If not, look at small increases in mc diameter, and look at the usable stroke as well.
Click to expand...
Ah, and i thought i made a mess .. :lol:

My current MC (if you check out the pic i posted above) has actually a stop on both ends so there is nothing it can be done for it, unless i take it apart and trim its bottom end stop, so that the rod goes further inside the cylinder. Either that or get another MC and try from scratch, which sounds a safer option. However, i could try both .. just out of curiosity !

I was wondering, does anyone know about the part number or some kind of identification of a 20,5mm slave cylinder ?
 
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