In a message dated 97-05-08, Michael D. Porter wrote:
> I missed the first part of this, but if I read this correctly, I agree.
> Smaller cylinders will apply less braking force than larger cylinders,
> but if the force required to lock shoes against drum can be met by both
> cylinder sizes, the smaller will lock up earlier for a given pedal
> application. The master cylinder will pump a given volume of fluid; the
> smaller cylinder will extend further than the larger cylinder with the
> same amount of delivered fluid, even though there is less force applied.
> For identical master cylinders, that means earlier and further wheel
> cylinder piston movement with the smaller cylinder, simply because
> displacement is a function of bore size and fluid delivered.
In a message dated 97-05-08, he also wrote:
> Yes, I agree that at the point that all friction material is in firm
> contact with braking surfaces is the point at which pressure builds.
> However (and this is a big however), further extension of the master
> cylinder _past_ the point of firm contact of all surfaces displaces more
> fluid. That amount may be trifling, but, regarding the relative sizes of
> the wheel cylinders, is still measurable in its effect. Smaller
> cylinders extend further than larger cylinders, provided that the limits
> of force have not yet been exceeded. If they are exceeded, the shoe
> doesn't move, because insufficient force is available to _make_ it move.
> If there is still sufficient force to make it move, it will move at a
> greater rate of travel than of a larger cylinder, which was the original
> issue.
Mike:
Interesting comments, and caused me to do a lot of head scratching. I knew I
didn't agree with you, but it took a while to figure out why.
After all is said and done, stopping is the result of friction between the
shoes and the drums. Friction between any two objects is determined by the
equation: friction = coeficient of friction (mu, I believe) times force
applied. Since mu is a function of the materials involved and is a constant,
the only variable of concern is force applied. Force applied is a function of
the hydraulic pressure and the cross sectional area of the cylinder. The
larger the cross sectional area (pressure being the same throughout the
system), the greater the force, and consequently, the greater the braking
power. I still say the larger cylinder will have the greater braking power.
BTW, congratulations on the purchase of the GT-6! A neat auto!
Dan Masters, standing by to eat crow if neccessary!
Alcoa, TN
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