Actually this doesn't need much theoretical discussion, any book on
braking systems or basic hydraulics, or physics will explain completely.
In a nutshell, Jack is right. The relative area of the pistons determines
the force and travel. In a system with a master cylinder of the same area
as the slave, one inch of master travel equals one inch of slave movement,
one pound of master force applied equals one pound of slave force. If the
master is two times the area of the slave, then clearly the volume of
fluid displaced by any movement will be twice as much, so the slave will
travel twice as far and exert half the force. The converse is also true.
Going to a smaller slave cylinder increases the travel (more fluid to
displace the piston) and decreases the force applied so you have to press
harder to overcome the clutch springs.
Of course most braking systems also include levers which multiply either
force or travel in the same manner--the master cylinder attaches partway
down the brake pedal so the pedal travels farther than the master cylinder
piston with a consequent multiplication of force, and the slave cylinder
acts on a lever that is shorter than the throwout bearing yoke, so a short
distance of slave travel translates into larger movement of the clutch.
-----Original Message-----
From: Jack Brooks [mailto:brooks@belcotech.com]
Sent: Wednesday, June 20, 2001 9:51 AM
To: Joe Curry
Cc: Randall; Jim Gambony; Cwn74@aol.com; tedtsimx@q1.net;
MJSUKEY@cs.com; fot@autox.team.net
Subject: RE: GT-6/Spitfire brakes
Joe,
>I don't know exactly what you just said but I think I soundly
>disagree. If you assume the master cylinder is exerting a constant
>pressure, the size
>of the slave will determine the line pressure and the resulting
>pressure output of the slave.
No, the pressure is a function of the pressure you exert with your foot on
the
master cylinder; push harder, more pressure; push lighter, less pressure.
It's a closed system. The size of the slave only affects the force
exerted on
the brakes and the also the volume of fluid the master needs to provide.
>If you don't believe it, go to your garden hose and turn it on full
>blast with nothing on the end. Then try it with a nozzle attached
>to the end with
>the thing turned on full blast. Note the difference in the pressure
>of the water coming out. It works the same way with clutch hydraulics
(and
>electric current flow as well). Restrict the flow and pressure
increases.
Nope, different case. You are not dealing with a flow/restriction
situation.
You have a closed system. The slave does not restrict the flow, it is at
the
end of the line. You are not changing the resistance to the flow of the
fluid, you are changing the area the pressure uses to apply a force.
Joe, The following is fact:
Given the same pressure acting on it, a larger piston will generate more
force. It will use more fluid to do it (the downside), but it will
generate a
great force.
>Of course (as Jim G. points out) the pressure required to press the
>pedal increases with smaller slave cylinders
YES, to achieve the same braking effect, the smaller slave cylinder needs
a
greater line pressure to provide the same force on the brake pad. You
need to
provide a greater force (with your foot) on a smaller master piston to
create
this greater line pressure in order to provide the required force at the
slave
cylinder.
As I have said all along, at a given line pressure, the smaller slave
cylinder
provides less force, so the system pressure must be higher, for the
smaller
slave cylinder, to provide the same force as a bigger slave cylinder.
Going
to a smaller master cylinder, but needing the same force at the brake to
stop
the car, will require greater pedal pressure. The only way to make the
smaller master cylinder need less pedal pressure would be to increase the
size
of the slave cylinder, because a large slave cylinder will provide more
force
than a smaller one a given line pressure. The downside to a smaller
master
and larger slave, however is that your master cylinder may not have enough
volume to provide all the fluid it will take to move the bigger slave
cylinder.
>so in reality, you
>cna't divorce any
>part of the system from the rest in designing hydraulics. But the
>original question was about the size of brake cylinders and I think that
most
>people will agree that if the original master cylinder is still in
>place, going to the original wheel cylinders is the proper thing to do.
I didn't enter the thread to discuss what wheel cylinder to use on a Spit.
I
don't know which ones to use. I saw incorrect information being
disseminated
and chose to correct it before someone used it.
>P.S. I'll have to continue this discussion later today. I'm late
>for an appointment!
I don't know what else I can add,
Jack
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