RE:
http://web.camaross.com/forums/showt...hreadid=109684

I didn't want to further take away from BobC's thread.

Elliswon ... I read and re-read your your posts. Good information.

And it makes perfect sense in an open hydraulic system. By open, I mean a pump, which moves fluid to displace a pistion with significant movement. The fluid flows into a hydraulic cylinder, with fluid pressure relieved on the other side of the piston seal, causing the piston to move outside of the housing.

But isn't a brake system a closed system? One where all we are doing is adding pressure to the system to create force? Force against the brake rotor? There is no significant movement of the caliper piston ... just the amount of brake pad that is ground away against the rotor during a stop. And we all know how much that is ... miniscual ...

Given those assumptions ....

The whole issue I have in "visualizing" what you wrote and how it pertains to a brake system boils down to fluid movement.

I just don't see how fluid is moving in a brake system. I mean, where is it flowing? All the air is supposed to be bled out ... it's a solid system. There is no place for it to go. Mashing the brake pedal decreases the overall brake system volume which increases brake system pressure. To gain a pressure increase, no fluid has to move, just the volume of the container (the entire brake system) has to decrease.

Do you see where it is I'm coming from?

 

fluid is not compressible

 

D---------OO

D = master cylinder
-- = brake lines
OO = caliper pistons


1) As you press on the brake pedel, the arm supporting the pedal acts as a lever and compresses a piston in the Brake Master Cylinder (typically forward) to push brake fluid forward. The master cylinder's brake fluid resevoir holds excess fluid for use (we'll need this later), and keeps the system full of fluid. The piston/resivior interface is where the system trasitions from atmospheric pressure to high pressure (using a second piston and passageway that doens't really apply here). Summary: when you push the brake pedel, fluid is forced into the brake lines.

2) fluid is not compressible for 99% of the time... at our low pressures it's even more than that. Brake systems pressurize to about 3000psi (~200 atmospheres). Any more than that and you'll need some serious sealing equipement... I'm not sure what the Fbody systems are rated to, but I doubt it's much above 3000psi (if at all).

So where does this high-pressure fluid go? Down the brake lines.

3)highpressure fluid in the lines forces fluid near the caliper housing to enter the caliper

So how is this directed on the pistons? They're the only thing that moves.

4) This in turn forces the pistons to slide outward until the rotor/pad put an equal amount of pressure on the piston's outside edge to balance the forces it's experiencing.


5) after releasing the brake pedal... the piston retracts and pressure in the system releases fluid from the lines back into the Master Cylinder's resevoir. This only happens until the pad-rotor pressure is equal to the master cylinider's atmospheric pressure (~15psi)....

So yes, fluid does move, but not by much. The majority of the work on the system is due to the pressure properties of fluid: the pressure in a fluid is equal on all sides.

If we take a long lever arm (like the brake pedal arm) and a long arc length (brake pedal travel) with a small force (your foot/leg)... and transform it into a HIGH force on a small piston (in the master cylinder) and small distance travel (the small piston travel in the MC that actually contacts the fluid)... we can use our legs to create HUGE pressures on the brake fluid.

Since fluid is not compressible, some does move as pressure builds up, but the key is that the pressure on the fluid near the piston is the same as that near the caliper's piston. So how do we make the force on the brake rotor higher than what we pressed on the MC's piston? we use a larger surface area:

Pressure = Force / Area

This the definition of pressure... think psi or Pounds (force) per Sqaure Inch (area). Rearraning we see:

Force = Pressure * Area

So since the pressure in the fluid is constant (and a result of MC piston area and force applied to it) we can compare how much force calipers can apply to rotors simply by comparing their piston areas. (Obviously some designs will be more efficient on pad wear/contact so this isn't the final word... but it's close)



finally:
WHERE DOES VOLUME COME INTO PLAY THEN?!

Well, remember the displacement of small fluid during the caliper piston travel? that movement decreases pressure in the lines. The longer the piston travels, the more fluid you have to pump into the system until pressure builds up (and is converted to force on the rotors). Larger piston areas WILL put more force on the rotor (all things being equal), but to do so requires more fluid to be pumped into the system. This doesn't really affect you during a single stop on new pads, but as pads wear down and the pistons move outward, the fluid returing to the MC is less and less (since the pistons don't push back as much) so you need to make sure you have enough fluid reserve to ensure system integrety (no air) around the MC piston with all 4 caliper pads fully used up, or close to it. That total piston travel volume is what a larger MC compensates for.


If you compare the LT1 (70? mm) single piston and the LS1 dual pistons (44 mm, and of course the smaller C5's 40 mm dual pistons) you'll see the fluid requirements for the LT1 piston is greater (the bore it leaves behind in the caliper housing is larger than the volume of the 2 bores the LS1 leaves as it's pads wear).

The 4-piston Brembo/Porsche design is the same (it runs 38mm and 40mm pistons on each side) since there are only 2 pistons per side... being horizonally opposed to eachother cuts the piston travel in half (vs a floating caliper like the LS1/C5/LT1), effectively making the force calculations comparible by using only 1 side's pistons.

If you do the math, you'll see that due to the LT1's larger single piston volume... you do not need to increase the MC requirements when upgrading to LS1, C5, or even Porsche brakes... not for fluid volume anyway.

 

Thanks Steve ... we posted about teh same time ...
I'm really trying to understand this ...

Look at this example ...

If a garden hose is pressurized with water and the end has a closed nozzle on it, what would happen if you compressed or mashed a small section of that hose?

This is like the brake system where the garden hose is the brake line, the pressure is from mashing the pedal and the end nozzle is at the brake caliper piston.

And for arguement's sake, the hose is strong enough to withstand whatever pressure is applied. Bursting and expanding is not part of the equation, like having SS brake lines.

If the hose is compressed, would the internal pressure not rise because the same amount of fluid is now packed into a smaller volume? If not, where would the water go?

And I realize that at some point the hose pressure will overcome the head pressure from the water source. But, again, for arguement's sake, the pressure in the hose never overcomes the head pressure of the manifold. This would be like mashing the brake pedal so hard it breaches the seal(s) in the MC bore or the caliper bore.

 

Mitch,

Your analogy of squishing the water hose is only similar to a brake system in that the rubber brake lines will expand when you push on the brake pedal. Other than that, the system should be incompressible and that means that your brake pedal travel is transmitted directly to caliper piston travel. The brake fluid is incompressible so when you push on the brake pedal and the master cylinder pistons then slide in their bores, the caliper pistons are forced to move to displace the same volume as the master cylinder pistons they are connected to. Once the pads contact the rotors the system is locked up and your car starts slowing down.

Steve,

Excellent post above.

For those others who just want to better understand how a brake system works I recommend starting with something like http://www.howstuffworks.com and looking up automotive brake systems.