> power assisted brakes were introduced, people
> > routinely stopped vehicles by stepping hard on the brake pedal. >
> That is true, Ray, but also in those days we had front and rear drums, for
> the most part.
> Discs required a lot more pressure than drums, hence the power assist brakes > became
> a necessity rather than just a luxury convenience.
I had 4 wheel disc brakes on my '74 Fiat 124 Spyder, was the best stopping vehicle I had owned at the time and it neither had nor required power assisted brakes.
Well, you're both right - discs *do* require more line pressure than drums, but a Fiat Spyder is a fairly light car so it doesn't matter all that much. I used to have a Porsche 914 with manual discs, but in general, manual discs are pretty rare and unlikely to be found on anything but small, light sports cars.
Of course, there are exceptions to everything anyone says on here. Everybody seems to scan for the exception rather than to consider the truth of the principle.
Weight plays a big part in this. Obviously bicycles and motorcycles may also have disc brakes, and they can be stopped easily by a standard human being because they weigh so little. As the car gets heavier, it becomes harder and harder to stop an unassisted disc brake system. The design of the braking system is another factor... the makeup of the pad, the surface area of the pad, the size of the caliper pistons ( thus mechanical advantage designed into the system) all enter in.
Drum brakes have a pseudoservo action which draws one of the shoes onto the drum surface and gives some mechanical advantage. They may also have a much larger surface area than disc brake pads. Even so, power drum brakes were used in a lot of American cars before discs took over.
Disc brakes, per se, require more pressure than drum brakes, in general. Their advantages, IN GENERAL, are that they do not fade as badly when heated, and they perform better in the wet. Unsprung weight arguments can be made, but you can engineer lightweight drum systems that could compare with many disc caliper systems. Porsche used them for a long time.
For the larger and heavier American cars of the time (my reference), you had to have power disc brake assists to offset the weak and lazy asses of most Americans.
Not to mention such esoterics as the boiling point and vapor pressure of the fluid, the universal gravitational constant ... ;-)
Dependent upon application, ie whether a twin leading or leading/trailing design and the direction of rotation.
Are you alluding to swept area or static contact surface?
I was always partial to the finned aluminum GM drums.
Implicit perhaps, I saw no explicit reference.
No, reducing the ratio of master cylinder piston to aggregate wheel cylinder cross sectional or working surface areas (being careful to consider volume amounts and pedal travel) would accomplish the mechanical advantage provded by power assist.
But I'm just screwing with you, we're in fundamental agreement.
I know, and I dont mind that.. I get tired of all the "yeah, but" and "what if" bullshit. There are always factors which can be swayed, moved, massaged.
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