brake bias, hydraulic forces, master cylinders and prop valve questions

[jpnovak]

I have been reading up on some braking systems. Bill Verburg's Pelican site is a wealth of information w.r.t the caliper specs.

http://www.pelicanparts.com/MotorCity/wmv/brakes.htm

In verifing the calculated hydraulic bias I have uncovered a few questions. I am doing this for my own exercise of understanding, not to question Bill's data tables. I am looking for some information that is not in the archives. This is not meant to be a brake conversion thread.

First, It is easy to find the hydraulic bias. Simply calculate the piston area and then a ratio of Front area over rear for a given caliper set. It seems that the early systems range from about 1.6 to 1.7 for the early 911 and 930. The 3.2 Carrera drops down to 1.3 and uses a proportioning valve to limit line pressure to the rear and thus have more forward bias. Bill lists an "effective" bias ratio when used with a prop. valve. The bias for a 930 and 3.2 Carrera jumps to 2.97 and 2.41, respectively. Does this new valve induced bias enhance braking? Would an early 911 with a bias of 1.6 benefit from more front bias by use of a proportioning valve? I wonder since you could fit nearly the same ammount of front rubber to take advantage of this front bias. You would also have reduced rear bias which might make the rear end less wiggly under threshold braking using small rubber. Probably no effect since the tires limit the braking force through traction and grip. How does one find the ammount of line pressure reduction by the proportioning valve to calculate a new bias?

Second, How does one determine the size of MC required to push a specific piston diameter. Everyone here is aware of the general emperical data that says you need a 23mm MC when going to a big brake setup. I know the ratio of pedal length on the lever and its easy to find how much hyraulic line pressure is created but how does that correlate to pad force? Is this a function of pad area? Do you really need that much fluid flow and piston area to get the same clamping force? Let's say for example that you could find a caliper that would clear a 320x32mm rotor but had small pistons. Would you be able to generate enough pad force and rotor torque with a suitable small MC?

OK let the lecture begin... I am ready to learn.

[Bill Verburg]

Would an early 911 with a bias of 1.6 benefit from more front bias by use of a proportioning valve?

here are a few brief words on the subject, my time is limited right now

To a certain extent yes, but there are so many variables that one pat answer cannot be given. Thats why race cars adjust f/r bias w/ dual masters(mix 'n match to suit) and balance bars. Generally a little more front up to say 1.75&#177 is OK, bigger tires in front will definitely help take advantage of the extra front bias. But again there are a lot of other factors to consider, height being a big one. Suspension setup being another. Personal tatse is also an issue I very much like more rear at lower speeds and moderate braking going to the above #s or more depending

All factory p/v's reduce line pressure by 46% above the knee, the knees are changed according to need from factory #s are 33 to 60 bar

The slave to master ratio(d) will determine the feel of your brakes, whether boosted or not.
~16.5 < d < ~24

The lower end is more sporting w/ a higher, harder, more easily modulated pedal, the upper end is softer , spongier harder to modulate and will often cause premature rear wheel lock up. Again there are other factors that need to be considered as well.

d also determines line pressure along w/ mechanical advantage in the linkage and boost levels.

brake torque is a function of line pressure and rotor radius

pad area just spreads the thermal loads out a bit more reducing the point stresses. it has no bearing on brake torque.

The later cars can have far more front for a couple of reasons, their suspensions are set up w/ more negative scrub radius than older cars and so are more self correcting under braking and of course ABS