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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