Need coffee, brain tired. Da--ed great big light bulb on horizon... However:
You are correct. If you are pushing with X force on both the larger and
smaller master cylinder, the smaller bore unit will have a higher hydraulic
pressure generated. This will definitely give you more braking force. That
force is given by the ratio of the square of the original MC radius to the
square of the new MC radius.
However nothing is for free; While you will get more force, you will also
have more pedal travel in proportion to the ratio of the two MC piston
cross-section areas (same as pressure ratio). If the difference in bore is
too much, you will bottom out your brake pedal before the system presses the
pads firmly.
This is all easy to show if we could draw graphics or insert formulae into
this list, however it's text only, so we persevere...
By the way, we have all been ignoring the action of the servo in all of
this. I must confess to a vast ignorance of the internal operations of that
unit. (sound of universal gasp, "Mark says he doesn't know something!" birds
fall from sky etc...) Can somebody tell me if the servo assist is in any way
affected by the travel of the pedal, or is it simply the foot pressure
opening a valve or passage? You know, assist force proportional to pedal
pressure at any point in the travel. That sounds right, but I don't know for
sure.
Mark Hooper
72 TR6
-----Original Message-----
From: Rave Racer [mailto:Ravewar@home.com]
Sent: Wednesday, June 20, 2001 11:45 PM
To: Mark Hooper; Triumphs Mailing List (E-mail)
Subject: Re: brake pressure versus MC/slave cylinder ratio
I have a question to insert in this thread here. I have a GT6 and
the Master is shot. My options are to find a place to rebuild it or to
replace it. I have toyed with the option of using a MC from a spitfire as
they are more common and relatively the same. The MC from a spit though,
seems to be slightly smaller in diameter. If I were to use the one from the
spit, according to your info here, you say that I will be able to apply more
pressure force at the pads with the smaller diameter MC. Or do I have it
backwards and it will take more pressure to exert the same amount of force
at the pads. Or am I wrong altogether?
> Actually the same pressure over a smaller area would result in a smaller
net
> force applied to the pads.
>
> Brake hydraulic pressure is independent of slave cylinder size. Pressure
is
> given by the ratio of the force applied to the master cylider piston and
its
> diameter. (50 lbs appied to a 1/2 inch square surface = 100 lbs/inch
> square). The force applied to the brakes is that pressure multiplied by
the
> area of the slave cylinder. The ratio of the master to slave cylinders
gives
> the amount of travel required to actuate the system. Thus if the area of
the
> slave is 3 times the master, then the slave will travel 1/3 the amount the
> master travels. However this ratio will have no impact on the pressing
force
> once the slave is tight against the pads and disk. That force, as
previously
> mentioned, is entirely dictated by the pressure from the master multiplied
> by the slave piston area.
>
> The reason that one cannot have enormous forces applied to the disks is
that
> the pedal travel becomes unreasonable. With a tiny master cylinder piston
> and a huge slave cylinder one could press the disks flat. However one
would
> arrive at a ridiculous situation of a multi-foot pedal travel giving a
> micrometer movement of the pads. Bearing movement would soak that up
> immediately.
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