jimlab

You're right, 1,000 psi is 1,000 psi, whether it's measured for a single square inch or a city block.

jimlab

If I have a vice with a 4" jaw and I put 1,000 psi of pressure on something placed between the jaws, am I putting any more force on the object if I applied the same 1,000 psi with an 8" jaw? No... 1,000 psi is 1,000 psi regardless of the amount of surface area involved.

1,001 psi would be more force.

Kento

The main reason for calipers with more than four opposed pistons is that the brake pads last longer. Six-piston calipers and the like were developed from endurance racing, where component life during the race is obviously of critical importance. While in theory it would provide more braking power, it's not that simple. It's not just the clamping force that provides the braking power; it's the friction created between the disc and pads. Simply clamping down with more force doesn't necessarily provide more friction. There's a hell of a lot that goes into pad formulas in relation to the caliper and disc design.

If simply adding more pistons to the caliper created more braking force, you'd see 12-piston calipers or whatnot.

rex u.k

Exactly what i was about to say. As regards to the stock master cylinder set-up there is definately enough pressure to lock up the front brakes when using a large ap front set-up and the nose dives greatly due to the overwhelming front brakes, i've just switched to larger rears/4 pots and dual master adjustable bias pedal assembly to balance things out a bit better.Will report back as soon as i can test it out.

maxcooper

Let me start this by saying that the idea of applying "1000 psi" to something in a vise is a strange way to think about clamping something, since you normally think of force rather than pressure when using a vise. But yes, it takes more force to apply 1000 psi of pressure over a larger area. Specifically, it takes 4000 lbs to apply 1000 psi over 4 square inches (assuming the 4" vise jaw is 4x1", smooth, and the whole surface is in contact with the work piece), and 8000 lbs to apply 1000 psi over 8 square inches (assuming the 8" vise jaw is 8x1", smooth, and the whole surface is in contact with the work piece). Psi is a pressure, not a force. But this example doesn't have much to do with brake hydraulics.

Pretend you have a hydraulic vise and a foot pump to operate it. Assume that if you push on the foot pump with 100 lbs of force, you get 1000 psi of line pressure (it has a piston with 1/10 square inches of area, or a 1 square inch piston with a 10:1 lever, etc.). You have two vises, one has a piston with 2 square inches of area, and the other has a piston with 4 square inches of area. If you push on the foot pump with 100 lbs of force, giving 1000 psi line pressure, the vise with 2 square inches of piston area will clamp with 2000 lbs of force. The vise with 4 square inches of piston area will clamp with 4000 lbs of force under the same conditions, but the foot pump will move further before you get to the final state of sitting there with 100 lbs on the pedal. Now install the foot pump in your car and put the vises in position to clamp your rotors and you have an automotive braking system.

Here's a howstuffworks link with some more hydraulic info:
http://science.howstuffworks.com/hydraulic1.htm

-Max

maxcooper

I found a cool page on howstuff works that explains how equal pressures are maintained in the front and rear brake circuits by the master clyinder without the need for a pressure differential valve to join the circuits (as I previously suspected):
http://auto.howstuffworks.com/master-brake1.htm

-Max

maxcooper

Bigger pads needed for endurance racing to get longer life.. need more pistons to apply the force to the pad evenly. I agree that it isn't JUST clamping force that determines brake effectiveness, but it should be clear that for a given system, more clamping force does indeed increase the amount of braking you get. That's precisely what happens when you push harder on the pedal.

Tar-Ox makes 12 piston calipers.

-Max

turbojeff

iTrader: (30)

So what Max is saying, piston area increases, line pressure stays the same = more braking force on that piston.

Doesn't matter if it is a 4" jaw or 8" jaw.

mad_7tist

it will now take twice as much force to twist out the object. (assuming no friction differences material deflection etc)

maxcooper

http://www.turdtwister.com/index.php

-Max

mad_7tist

do i want to know how or why you have that link???

Gadd

Rotor Turd

maxcooper

I couldn't figure out what you meant at first, though now I realize that you were talking about ripping something out of the vice. So I posted a link to the turd twister, of course. Be sure to clean up afterward: http://shitbegone.com/

-Max

Kento

Yes, pushing harder on the pedal obviously increases the amount of braking force, but once into the threshold pressures of very aggressive braking, increasing that force via more pistons in the brake caliper doesn't mean that you will automatically get more braking force. Have you ever seen a brake dyno graph (I mean an actual dyno that measures braking force)? Nearly all the graphs I've seen of various brake pads (for four-pot and six-pot calipers) show increased pressure beyond that point actually doesn't increase friction (which is what braking force depends on); in fact, in many cases it actually begins to taper off due to the heat generated. The only graphs that showed a marked increase in friction were carbon/carbon and ceramic composite brakes, and even then the effect was minimal. It's the friction generated by the pad/disc interface that determines the majority of the braking force at aggressive braking levels, not increased pressure.

The statement that using six-pot (or more) calipers will automatically gain you an increase in braking power over a well-designed four-piston caliper is a myth.

HDP

iTrader: (2)

Aren't we really applying the pressure to the fluid and not to the caliper itself? Wouldn't the fluid compress at the same rate reguardless if more was required to fill the larger caliper?

jimlab

Look at the hydraulic multiplication example. If you increase the size of the "slave" (larger) piston, it won't move as far with the same travel of the "master" (smaller) piston. It will take a longer stroke of the master piston or a larger diameter master piston to move a larger slave piston the same distance as the original, and exert the extra force you're talking about. Either of those options results in an increase in line pressure.

Using the example on that page, the 2" piston (3.14 sq. in.) moves the 6" (28.26 sq. in.) 1 inch when it travels 9, because the larger piston is 9 times the size of the smaller one.

If you increase the size of the slave piston to 7" (38.48 sq. in.), for example, the slave is now 12.25 times the size of the master. Pushing the master piston 9" now only moves the slave piston 0.73". So a 100 lb. downforce on the master piston results in... the same 900 lb. upforce on the slave piston, even though it's larger in diameter.

100 lb. * 12.25 = 1,225 lbs. * 0.73 = 894.25 lbs.

In order to increase the force on the slave piston, the master piston has to travel farther (and your brake pedal only travels so far), or the master piston must be increased in size (larger master cylinder). In the end, it's an increase in line pressure that increases clamping pressure... not just increasing the piston area.

rynberg

Nope, fluid is essentially uncompressible. Take a glass filled with water. Take a disk the same size as the glass. Try to compress the water. Ok, don't REALLY try it, because I don't want you to cut yourself...

rynberg

Jim, I think you're not seeing the forest through the trees here...the facts are that bias is changed by any of the following factors:

*rotor diameter
*brake pad friction coefficient
*caliper piston diameter

Any scientific reasoning you are attempting that disproves any of the above is flawed. I really wish David Zeckhausen from Zeckhausen racing would pop in and clear all this up.....

HDP

iTrader: (2)

I understand water/fluid is uncompressible... I've seen piston rods from engines that have ingested water

maxcooper

Yes! The brake pedal will move further with a big brake upgrade installed.

No! Force exerted by the larger brake calipers is proportional with how hard you are pushing the brake pedal, not how far. The pedal will be long and soft with the big brakes installed, but fi you apply the same pressure that you did before (e.g. 50 pounds of force on the pedal), you will increase the amount of braking you get at the front.

No, that is not correct. Stroke is irrelevant when talking about force.

100 lbs of force on a 3.14 in^2 master piston = 100 / 3.14 = 31.85 psi
31.85 psi * 28.26 in^2 (6") slave piston = 900 pounds of force
31.85 psi * 38.48 in^2 (7") slave piston = 1226 pounds of force

There is no magic -- you are just trading stroke for force, much like a lever or cheater bar on a wrench. But in the case of brakes, once the pads hit the rotor, it's all about force -- you aren't actually moving the pistons anymore.

Changing piston size alone will increase braking force for a given line pressure. This is what is relevant to our discussion, since this is precisely what installing larger front brakes does. If you push the brake pedal just as hard(1) before and after the upgrade, you will get more braking torque with the upgraded brakes than you did with the stockers, and you will get exactly the same braking torque in the rear(2). You will push the pedal further (it will have a longer travel for the level of pedal force you will be testing), which is annoying, but the stock mc has enough travel to operate all the common upgrade kits on the market before bottoming out. That said, I still recommend the 929 MC because it feels better to have a firm pedal, and you get a little more control since you have to push harder with the larger MC.

(1) Pedal force translates directly to line pressure for a given pedal and MC arrangement.
(2) The rear brakes have not changed, so you'll get the same torque for a given line pressure.

-Max

superior_force

little ccw canadian boy had no idea he was opening up a major technical discussion with the forum elders

jimlab

Let's stop right there... of course you'll get more braking torque; you moved the caliper farther away from the center of the wheel. You'd get more brake torque from the stock calipers if you did the same thing.

For example, Roger Mandeville's big brake setup re-used the stock calipers with a larger diameter rotor. What was the benefit? More mass in the rotor to shed heat more rapidly. Did it increase braking torque? Sure, but so what. You can reach lock-up a little sooner than you could before. Big deal. It doesn't decrease stopping distance any.

The stock calipers are not the limitation. As I've said several times now, they provide more than ample clamping force to lock up the rotors. The limitation is the size of the stock rotors. What did Mazda do with the '99-up FDs? Increased the diameter of the rotors.

Do the '99-spec rear calipers balance out a big brake kit? Sure, but if they have the same piston area as the '93-'95 rear calipers, how are they doing that? With a proportional increase in brake torque provided by moving them farther away from the wheel center. Fronts and rears now lock up at the same time, problem solved. No change in pad size. No change in piston diameter.

7_rocket

iTrader: (6)

I second that.. At least its useful info.

Larz

This is the best debate with usefull information I've seen on this forum in a long time.

maxcooper

I'll take this as an acknowledgement that larger diameter rotors do indeed change the brake bias for starters.

But calipers with more piston area mounted in the same position as the stock calipers would also increase the braking torque, since they will pinch harder when you push the pedal with the same force (which will give a fixed amount of rear braking torque no matter which fronts you have installed).

It does change the bias, though, which is how this whole discussion got started.

Agreed. But if you also want bigger pads, bigger (and/or more) pistons helps to spread the load more evenly across the pad. Big brake kits typically have larger pads, and larger pistons, than the stock calipers.

The 99 rears help balance out a big brake kit, but the rear brakes I have were designed specifically because some smart people worked it out and found that the 99 rears were not enough to give proper balance with the front kit that I have. So the 99 rears don't fully balance out any front upgrade, but since they are more effective than the stock brakes they are a step in the right direction and are probably close enough to balanced for most people's purposes. It stands to reason that if your upgrade fronts are more effective (larger rotors, more piston area) than the 99 fronts, upgrading to the 99 rears will still leave you with a front-biased brake setup.

-Max