>At 07:18 AM 5/11/99 -0600, Haynes, Mark wrote:
>>Greg- By my recollection (which may be faulty) of hydraulic theory, if you
>>take a specific traveling volume of fluid (hydraulic line), and apply the
>>pressure over a larger applied area (i.e. a larger piston diameter) you wind
>>up with less pressure per unit area, thus decreasing the braking effect
>>(actually the same pressure over a greater area). Anyone know for sure if
>>my theory of hydraulics is correct??
>>Mark Haynes
>>'65 Spitfire RMVR #162
>>'62 Sprite HAN6
>>
Sorry Mark
Your recollection is incorrect. If you have a hydraulic pressure source
(the master cylinder) applying a pressure of 1000 psi, then if you have
an actuating cylinder (brake caliper) with a piston surface area of 1 square
inch you will have an applied force of 1000 lbs. If that same actuating
cylinder
had a piston surface area of 2 square inches you would have an applied force
of 2000 lbs
In brake systems a small master cylinder diameter and a large brake actuator
piston diameter leads to a high braking force, unfortunately you can't have
your
cake and eat it too, because it also leads to small brake shoe movement since
you need ever increasing volume of brake fluid. Soooo .... you always have to
balance out the system so you get good braking with adequate movement.
On a small race car a system may need 150 to 200 lbs of force (from you) to
lock
up the brakes (this is desirable since you get good control with a large
range)
However, on a passenger car the 5 foot demure housewife driving a 3 ton SUV
needs
a little help so they add a brake vacuum booster. This device uses the intake
manifold vacuum (~14psi max) and pushes/pulls on a piston (rubber/metal
diaphragm)
of about 6 to 10 inches which in turn pushes on the master cylinder. This
leads to
a potential force of about 50 to 100 times the manifold vacuum (upwards of
1500 psi)
which is much higher than most people can physically push the pedal. And
she can
do this by lightly pushing on the brake pedal which operates a valve to
control the
amount of vacuum to the booster. And then you have the new ABS systems where
you indirectly control the brakes and the actual pressure is controlled by
an electric
motor/pump and a computer controlled valve system to modulate brake pressure
whenever it detects wheel lock (Damn, I hate those systems!! If I have
any choice
in the matter I will never buy another car with ABS brakes, except maybe
for my wife
to drive in the winter)
Hope this helps
Regards, Nils
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