Help understanding relationship of master cylinder to slave cylinder bore sizes

[Brian Diehl]

Please help me understand the relationship to pedal pressure of a master cylinder with smaller or larger bore size compared to the slave cylinder.  For example, I've found a pull type slave cylinder that is .88" in bore with 1.4" of stroke.  I have two master cylinder sizes to choose from: .75" and 1" both with 1.4" stroke.  So, which master cylinder would give the lightest pedal pressure?

[Sojourner]

.75" bore master cylinder but slave of .88" will be travel less than master's 1.4" stroke a little.

FWIW

Sojourn for Christ, Jerry

[skipn]

Brian,

    The easiest way I look at hydraulics is everything is a ratio master stroke bore vs slave stroke bore.

    Pressure is tied to the bore difference.  master 1 unit slave 2 units then to move slave stroke 1 unit requires 2x travel at the master with 1/2 the pressure.

    For clutch pedal depress pressure it is a fulcrom thus on the same side of the pivot master 1/2 length down the
   pivot produces twice the pressure on the master. master at the end of the pedal is the 1:1 point.


    Lousy engineering but just the way I figure things out.

     Skip

[Stan]

Brian: Another post came in while I was on the calculator, but here are the numbers you work with:
.88" diameter is .6079 sq. inches of area.
.75" diameter, is .4415 square inches of area which is 72.6% of .6079 so the travel will be 1.01" with 72.6% of pedal pressure to slave pressure.
1" diameter is .785 square inches of area which is 129% of .6079  so it is capable of 1,8" of slave travel with 29% More pedal pressure.

Somebody should be able to find a mistake in these numbers.

[Sean]

Lots of complex answers to a simple question...

The short answer:  The smaller the bore on the master cylinder, the lighter the pressure you will need to apply.  So in this case, the .75" will require less pressure.

The price you pay for this is that more travel is required to achieve the same result.

In this case, since the slave will have a larger bore, the slave piston will travel less distance than the distance traveled by the master (but applying more force).

All that being said, I question the wisdom of trying to design your own hydraulic system if you don't have even a basic understanding of hydraulics, and the ability to do these (very simple) calculations on your own.

Here are a couple of on-line resources to help you understand hydraulic systems -- you might want to go through at least the basics before tackling design of a hydraulic system.  (Since the master cylinder will, presumably, be attached to a pedal, you will also need to understand leverage and how to do those calculations as well.):
http://www.howstuffworks.com/hydraulic.htm
http://64.78.42.182/free-ed/MechTech/hydraulics01/default.asp

HTH.

-Sean
http://ourodyssey.blogspot.com

[Brian Diehl]

All that being said, I question the wisdom of trying to design your own hydraulic system if you don't have even a basic understanding of hydraulics, and the ability to do these (very simple) calculations on your own.

Well, I don't do hydraulics for a living.  So, surely others are much more capable than I.  I've always subscribed to the theory of asking others for help to try and prevent myself from making simple mistakes that could have been avoided by a simple question.  I'm starting with the things I already think I fully understand.  I'm hoping to validate my math and approach to the problem is correct.  I'm fully capable of a little bit of math and understand fulcrums and leverage.  Now that we have the question of my abilities out of the way we can move back to my original question. 

Thanks to everyone who provided me examples and numbers.  The information provided exactly matches what I had figured.

[skipn]

 Sorry, I am a little slow


     .75

   Did I pass the test?

  Skip

[kyle4501]

Well, let's try this as an answer,
The force applied to the master cylinder piston X the area = system psi

The system psi divided by the slave cylinder area = force of slave cylinder.

I would think it prudent to size the slave cylinder slightly larger than the master cylinder. You wouldn't want to over stroke it, would you? I suppose if the slave cylinder has stops it wouldn't hurt it tho.

You mentioned a 'pull' type cylinder with a .88 bore. What is the rod size? Usually you have to subtract the rod size from the bore size for calculating the pull of a cylinder. I don't think Sean was out of line in his remark, Rather I feel he was trying to let you know there are many pitfalls in hydraulic system design. But hey, what do we know?

For the record, I can cipher the math required for fulcrums & hydraulics, and I thought you might be in over your head too.

[Brian Diehl]

Sean or Kyle,

Since I don't know what I'm doing are you volunteering to design my system for me?  Maybe I missed something between the lines .... as surely we should never attempt anything unless we are experts at it.  Or, maybe I should never have posted the question in the first place?

[Nusa]

First question is how much stroke do you need at the slave? Full travel on a .75 master will only get you 1" of travel on the .88 slave instead of 1.4". If you need more, you need a master with a equal or larger volume than the slave (either .88 or larger diameter or longer stroke). Based on the choices you presented, I suspect you need the 1" master and a physics lesson about fluid hydraulics.

[skipn]

Brian,

 oh, Can I, can I, pick me, pick me,

Kyle I want to make sure I really understand what you posted.

  It's a matter of area? so if the master and slave that have 1 unit bore but
  the stroke is 1 unit on the master and 10 units on the slave than by starting
   the slave at the half way point I'll get a 5 to 1 area ratio with a 5 to 1 force reduction?

 Boy I am learning thanks
Skip

[Dallas]

I have to jump in here too and lend my support to Brians question.

I think it's a very valid question to be posted on a forum such as this. Not everyone has every skill needed to convert a bus and the learning curve is sometimes extremely steep.
With the hydraulics question, Brian could have worded it a little differently and gotten a whole different set of answers from those who know.
It's kind of like electricity, I know that for a given distance with a given voltage using a given gauge of wire that there will be a voltage drop of X. I don't have to know the math involved in coming up with the answer to X, all I have to do is either look it up in a book or ask someone who is more conversant in the use of each gauge of wire.
Telling someone that they shouldn't do something because they don't have the education needed is, in my opinion, totally wrong thinking.
How about the guy that wants to build his own cabinets? Should he go to school for a number of years to learn how to make dovetail joints that stay tight?
Or how about the person that has a degree in theoretical mathematics, should he not use a calculator when he goes to the grocery store to figure out the given number of artichokes he gets if they are 3 for $1 and he wants to spend $7.33 on them?

This board, isn't about what we can't do, it's a place to ask questions and find answers. We can leave the rest to SOB*


Just my couple of Drachmas,

Dallas




*SOB = Some Other Board

[luvrbus]

Brian,Jim Shepard the rvsaftey guy has been a engineer for 40 years and can do the math but he been working on this problem for 2 years so don't let people get you down someone has the answer

[prevost82]

Dallas ..you are right in your statement and the simple answer to Brians question is the larger one ... but I think what Sean was eluding to is that there is much more information req'd to solve this problem.

1- you need to now the force req'd to move the clutch lever (force)
2- how far does the lever have to move (stroke)

3-I would find out what a standard petal force is and it you want it lighter then change that parameter (gives you a starting point)
4-Design your attachment point for the Master cyl rod (distance from fulcrum on the clutch petal)
Number 4 is the most important part because it affects your petal force and the stroke req'd of the Master cyl. which relate to the movement and force of the slave cyl

Until you answer 1 to 4 you will be chasing your tail and may never get the right combination.

I could give you a hand Brian with it ...this is what I do for a living, designed hydraulic system for 35 yrs. I use highend 3d modeling CAD software so I can simulate movement and forces
Ron

[skipn]

Ron,
    Oh sure throw out this is what I do for a living.

    Brian, If Ron is willing to help that would be a very good thing for all.

    It will be interesting to see...
     4" at the bell housing (slave)
     ~2" at the master
      ~ 20-30 # at human pressure.
       ~80# at the bell housing

  Should be a fun time. An out of the box experience!

  Skip