Disc Brake Systems
There are several criteria which are vitally important when choosing, designing and working with a disc brake system: (1) Keep deflection down; (2) The use of hard linings to avoid flex from sponginess; (3) The use of small diameter flex lines; (4) The use of steel brake lines whenever possible; (5) Volume requirements of the caliper; (6) Available pedal ratio; (7) Master Cylinder size and design.
Calipers: There are two types of calipers floating or non-floating. Calipers are generally made from three common casting metals: magnesium, aluminum and cast iron. Calipers are made of different materials the most common are aluminum and cast iron. The material used in the calipers becomes important to help eliminate deflection, deflection results in a spongy pedal. The modulus of elasticity is very important to eliminate the deflection (flexing) of the caliper. The higher the modulus of elasticity number, the greater resistance to flex. Magnesium has a modulus of 6.5 million, aluminum has a modulus of 10 million, cast iron has a modulus of 14.5 million and steel has a modulus of 30 million.
The floating
design was designed by the car manufacturers essentially to
make the caliper less expensive to produce. It successfully
applies the physics principle of "for every action caused an
opposite and equal reaction happens." With this in mind
they eliminated the piston(s) on one side of the caliper. This
floating caliper is not solidly mounted, but slides back and forth
on bushings or pins. When braking force is applied, the piston
push the brake pad on the primary side and the reaction is the rotor
being squeezed from the force of the pad primary side allowing the
horseshoe shaped caliper to slide on the bushings so the secondary
pads is used to squeeze the rotor. The caliper has to be very
rigid retain low deflection or the principle will be lost. Cast
iron and steel is used because of its' modulus number of 14.5 million
and 30 million respectfully. This also increases the "sprung
weight" and it retains the heat longer. The big advantage
to the full floating design (single piston) is if the rotor has
a slight run out (wobble), the floating feature will compensate
without creating any instability. The other advantage
is the single piston design is easier to bleed. The disadvantages
are it heavier, retains heat, requires approximately 100 pounds
of pressure more to "slide" the caliper and requires more
volume of brake fluid due to the diameter of the piston. Floating
designed calipers also come with 2 pistons on the same side.
Non-floating (fixed) calipers (i.e.
2, 4 or 6 piston) require a fixed mounting bracket. Most
race applications use this type of caliper, because they are generally
are made of aluminum which displaces the heat faster and requires
both less pressure and less volume to operate. . Calipers made of aluminum are
also not subject to corrosion and rust, like the Corvette calipers
in the 60’s. The fixed design allows all the
piston to be applied at the same pressure, because the pressure
is equalized when pressure is applied, thereby allowing the rotor
to be squeezed by opposing forces (piston on each side). Aluminum
will displace heat 1.5 to 3 times faster than the cast iron or steel
calipers. This is important when the rotors heat up to 1100
to 1200 degrees in a race car. Don't forget brake fluid
has a boiling point of 550 to 700 degrees F. We have come
a long way for the old 1965 Corvette design calipers, current non-floating
calipers are easily rebuilt and even have thermo barrier type pistons
that reducing the transfer of heat from the rotors.
OEM (floating) vs 4 piston (non-floating): Most of the brake kit currently being sold is the single piston OEM type caliper. In order for the caliper to squeeze the rotor it has to use a floating design, otherwise it would only apply pressure from one side to the rotor. Because of this design you loose approximately 100 psi. 4 piston caliper squeeze from both sides and are fixed (don't float), so they (4 piston) do not require as much pressure. The single piston caliper also requires more volume to work. The area of a 2-3/4" single piston caliper is 5.93 sq inches VS the area of two (2) pistons on a 4 piston design of 3.53 square inches. (multiply by 2 piston to get the area because the other 2 piston are being apply at the same time to squeeze the rotor, unlike the one piston design) 5.93 si VS 3.53 big difference. Does the volume effect the braking? Yes, it has a great effect on the master cylinder volume that is required for all 4 wheels. This will mean you will have to use a larger diameter master cylinder to meet the requirements of the calipers. The larger the master cylinder is the lower the pressure output.
Six Piston Calipers: 10 years ago when I first wrote this article there were no 6 piston calipers commonly used on street vehicles. You can thank computers for the fast advancement of c n c machined parts. The six (6) piston caliper is about the greatest thing since disc brakes itself. Generally there are four (4) smaller pistons and two (2) larger pistons. The two different size pistons allow the caliper to work at different levels without having as all the pistons operating at the same time. When you only need a little braking the 4 smaller pistons do the work, when you need to really stop the bigger pistons are activated. Great solution for street cars.
Copyright reserved by
Dean Oshiro. Reproduction without written approval is a violation
of Copyright Laws. Sept 1994-2006