Since getting back from my morning appointment, I had plenty of time to read
all the comments submitted from various people (None of which appear to
agree with what I was told by someone who has over 50 years experience with
engines and auto mechanics).
While I see the point that various people are making, I think everyone missed
the real point.
The whole system is built around the premise that the fluid acts like a
variable solid rod and is used to push something out the other end. Hence,
the answer to the riddle is "You have to exert enough pressure to overcome the
resistance of whatever is being pushed on from the slave cylinder."
And basically, you really don't want all that much pressure because more
pressure required to move whatever it is you are pushing on requires more
pressure on the pedal to initiate that action. You size the slave cylinder
according to how much pressure you want to exert to actuate the device
that is being pushed upon.
Finally, while everyone's statements are basically correct, the statement I got
from my engine builder also is. The "System" pressure does increase
if you use a smaller slave cylinder (caused by more pressure on the pedal). It
has to, otherwise the system would not do it's job. Absolute
statements made with limited details can obviously provide much room for
controversy.
It has been a refreshing discussion indeed!
Cheers,
Joe (C)
Jack Brooks wrote:
>
> 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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