Trivia question - clutch hydraulics

[JPSmit]

How many pounds of pressure does the system operate under?

 

[Bayless]

Don't know the answer but shouldn't be hard to estimate pretty close. Measure how many pounds of force it takes to push the pedal down and divide by the square inch area of the master cylinder. There you have the PSI of the master which will be the same throughout the system. Of course, the hardest part may be getting your bathroom scale into the foot well.

 

[aeronca65t]

Much less than brake PSI which can approach 1800 PSI under worst case.

I think it's safe to say less than 400 PSI.

 

[JPSmit]

Thanks Nial! now to get the bathroom scale out of the footwell - doh!

 

[Guest]

How many pounds of pressure does the system operate under?

Just above pressure plate and all frictional forces. :wink: (think about it...)

 

[Bayless]

Oops, I forgot about the pedal leverage. You have to multiple the pedal pounds by that ratio too. Might as well go ahead since you already have the scale stuck in the foot well anyway.

 

[Guest]

Oops, I forgot about the pedal leverage. You have to multiple the pedal pounds by that ratio too.

That what it's capable of, not the amount required to work. Think about the question, "how much is required to operate", not "how much can it produce". Once the PP starts moving, no more force is required nor can be added in the system (in a perfect world of course with no "bumps" in resistance). The line pressure remains constant at rest and once it starts to move, it only changes in between these two point if the velocity of the pedal is constantly increasing.

While it's not meant to be, it's actually a vague question with several answers but it really only has one. While we all know JP is asking about hydraulic fluid pressure (or not), there is also other "pressures" like you said. Either way the answer is still the same, just above whatever it takes to overcome oppositional forces (because unlike brakes, the load really doesn't vary much). Like Nial stated before, brakes are a higher operational pressure because the load is greater and it varies but most importantly, because the master doesn't bottom out against itself like a clutch m/c does. Because the pressure plate can move further than the clutch master cylinder, the line pressure stays the same once the pedal stops moving.


The clutch fluid pressure is always constant unless pedal velocity increases and is separate from any mechanical multiplication outside of the wet part of the system. You only have a variance once you get on the other sides of the pistons on either end, so any mechanical ratio of force multiplication doesn't apply if only asking about fluid pressure in a clutch system. If you had three gauges measuring fluid pressure: in the line, at the slave and at the master; all three would be equal at all times. The length of the pedal doesn't change line pressure required to overcome a 400# pressure plate, it just changes what is required of your foot.

The clutch master piston (and pedal) only encounter changing resistance at two points, o-to just before the plate moves and just after where it bottoms out. Once it starts to move or it bottoms out, the line pressure is still the same no matter how much harder you push on the pedal.

Given the vagueness of the question and all the possible "pressures" and points there are, not to mention the definition of "operate" (make the car move or clutch disengage?) there can only be one answer, several..... or none at all.

(note, fluid in bold just so it's not easily overlooked)

 

[Bayless]

I believe I said above that the pressure is the same throughout the entire system. But the leverage of the pedal absolutely has to be considered. If that were not true then there would be no need for that leverage. If you don't believe that then just disconnect the pedal and try to push the piston into the cylinder. I'm gonna bet you cant do it, any more than you can press the pressure plate springs by hand.

Also, what you are working against is those springs and springs are rated in units like pounds per inch. The farther you compress it the stronger it resists. So the farther you push the pedal, the more resistance, thus more pressure in the system. True though that it takes more force to compress it faster so increasing the speed would increase the pressure.

What we were talking about above thou is not how much absolute pressure there is in the system but a way (simple or not) to estimate that pressure without adding a gauge into the system. If you want to measure it from the force on the pedal the you really do have to consider the pedal leverage.

 

[Guest]

If you want to measure it from the force on the pedal the you really do have to consider the pedal leverage.

Right, but the title is clutch hydraulic, which (to me) implies wet pressure. I think I may be not explaining my thoughts clear enough. Sure, it increases, but it will never be greater than what the plate is measured at plus frictional forces. To "operate", it must be at full disengagement and the springs at the ful amount of allowed travel (not max, operational travel). The load is always the same no matter the point or state of anything (or everything). You can never put more force in than is required. You can put more pressure in a brake system than required but not a clutch. The pedal will never change the force the plate sees unless something is wrong, that's all I was saying. AS you stated earlier, it has a standard set resistance....but what "pressure" are we being asked about where...and in what vehicle? There isn't enough information.


It's a vague question so it really only has one vague answer, a force just enough to overcome resistance.

He's pullin' yer chain..(or not) :highly_amused: That's why I said think about the question, it's a trivia question with no answer. Notice he's been back since posting but never clarified the question. What does that tell you? ( I could be wrong but it wouldn't surprise me if JP told his wife, "Here Honey, hold my beer and watch this... "


Here's one for ya, "How big is a piece of glass?" layful:

 

[BWA]

The same length as a piece of string of course.......

 

[Guest]

The same length as a piece of string of course.......

Yeah, but what's its width? layful:

 

[jlaird]

Why they are both the same of course.

 

[JPSmit]

Right, but the title is clutch hydraulic, which (to me) implies wet pressure. I think I may be not explaining my thoughts clear enough. Sure, it increases, but it will never be greater than what the plate is measured at plus frictional forces. To "operate", it must be at full disengagement and the springs at the ful amount of allowed travel (not max, operational travel). The load is always the same no matter the point or state of anything (or everything). You can never put more force in than is required. You can put more pressure in a brake system than required but not a clutch. The pedal will never change the force the plate sees unless something is wrong, that's all I was saying. AS you stated earlier, it has a standard set resistance....but what "pressure" are we being asked about where...and in what vehicle? There isn't enough information.


It's a vague question so it really only has one vague answer, a force just enough to overcome resistance.

He's pullin' yer chain..(or not) :highly_amused: That's why I said think about the question, it's a trivia question with no answer. Notice he's been back since posting but never clarified the question. What does that tell you? ( I could be wrong but it wouldn't surprise me if JP told his wife, "Here Honey, hold my beer and watch this... "


Here's one for ya, "How big is a piece of glass?" layful:

Not pulling anyones chain I promise. As mentioned elsewhere, I am in the process of repairing/replacing the clutch hydraulics. Slave cylinder is on order - will be here this week. That said, in an offline conversation, Gerard mentioned the possibility of a pinhole. I din't think there is one - in the hose - but, when I took it apart, I noticed one end was kinked right where the hose meets the metal fitting, and, a couple years ago I was pulling into a cruise day parking spot and the end of the hose popped off the fitting. Up till now I fixed it by using hose clamps, but have been looking for a more permanent solution. So, I went to a hose place and got a new hose made up. At least part - the rest will come when the clutch is reassembled. Anyways, the hose they used is rated to 400lbs, which was the initial answer - and all I really needed to know. The rest of the conversation I sort of get, but, most goes over this empty head.

So, with the info gleaned I am installing the hose, but am also looking into a stainless option - with a bleeder screw at the top of the curve - hopefully a more permanent solution.

 

[Guest]

I have stainless on mine. You bought slave but no hose?

 

[JPSmit]

I have stainless on mine. You bought slave but no hose?

i bought a new slave and made up a new hose

 

[Guest]

It was a rhetorical question, why not buy the hose too?

 

[JPSmit]

It was a rhetorical question, why not buy the hose too?

Because I have been unhappy with the hose. It seems plastic, one end popped off in regular use, the end was kinked and almost split and it may have had a pinhole. Other than that its fine.

Seriously, before we slag the usual suspects, in appearance it is exactly the same as OEM - but, I have long believed that it was a compromise when they added the 1500. So, rather than go through it all again, I am experimenting with some alternatives.