On 30 Jan 2006 20:07:28 -0800, "Doug" wrote:

again, stop and think about it for a second.
we have 100 psi. that is 100 pounds per square inch.

if we double the square inches, we double the force.

another way to think about it.
assume we have a single piston, and 1000 psi of force being applied to
from teh piston, and further assume that the brake pads are 10 square
inches.
with a single caliper, there are 2 pads, so that 1000 psi gets spread
over 20 square inches, for 50 PSI on the pads. with 2 pistons, you
would have 100 psi, and double the friction.

When they make "Floating" calipers with the pistons on one side,
they use one piston with 10 square inches area, or two pistons with 5
square inches of effective working area each. The clamping effect
goes through the caliper body, and the caliper moves laterally on the
slide pins or rails to apply clamping pressure to the pads on both
sides of the rotor.

When they make a four piston fixed caliper, they put four pistons
with 2.5 square inch working area each, two on each side. They have
the same effective area (10 square inches total), so the clamping
forces applied to the brake pads are exactly the same.

The big advantage to a fixed caliper design is that the caliper
doesn't have to move laterally to apply and release, it can be bolted
down firmly. There are no large caliper slides (that have to transmit
braking forces) to get rusty and stick, and cause dragging shoes and
other brake problems.

But the four-piston designs cost more to make initially, so when car
makers try to save a buck they don't get used. And the Big Three in
Detroit wonder why they're going bankrupt...

-- Bruce --

--
Bruce L. Bergman, Woodland Hills (Los Angeles) CA - Desktop
Electrician for Westend Electric - CA726700
5737 Kanan Rd. #359, Agoura CA 91301 (818) 889-9545
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