Master Cylinder question

[brett@netacc.net (Brett Van Sprewenburg)]

Straight info on master cylinder bore vs pressure (paraphrased from 
an article in GRM):

The pressure generated at the master cylinder is equal to the amount 
of force from the
brake pedal output rod divided by the area of the master cylinder piston.

A cylinder diameter of 22mm, has an area of .59 square inches (radius 
squared * PI).
At 300 pounds of pedal force (not unreasonable out of the booster) 
(300 / 0.59, input
pressure evenly divided over the area, like I stated near the top of 
this email) will be
508 pounds per square inch of input rod pressure.

Increasing the master cylinder piston diameter will decrease the 
amount of pressure
generated in the fluid for a given input force.  Ie.  25mm MC, has an 
area of .75 square
inches.  At 300 pounds of pedal force (300 / .75) will be 400 PSI. 
Lower then the above.

Decreasing the MC size works this math in reverse obviously, pressure 
goes up as the
bore size decreases.  While this may seem desirable to decrease the 
MC bore, the braking
system as a whole will have some compliance, the system has to have 
enough additional
fluid on hand to fill all the extra volume caused by the flexing of 
components during the
compliance phase. Unfortunately, this is accomplished by increasing 
the diameter of the
MC - which we just learned reduces the pressure generated. 
Therefore, we have to make
sure the MC has a large enough diameter to meet the fluid volume 
requirements of the
system, but is small enough to generate the pressure required.


At some point soon, I'll scan the entire article and post it in the 
tech section of
scirocco.org.  It's quite enlightening.  Perhaps the people getting 
'better' braking out of
a 25mm MC might be better served with non-rubber brake lines, better 
brake piston
seals, more efficient calipers, better brake pads, or stickier tires. 
Food for thought.

==Brett




>>Chris,
>>While I know you probably have more experience with this stuff than 
>>me I will try to disagree with you for moment.
>>
>>A larger MC gives less pedal travel (than a smaller one) to actuate 
>>the brakes.
>>A larger booster gives more pressure (less required from you foot) 
>>to actuate the brakes.
>>Am I correct in this?
>>
>>Too large a MC can make it hard to modulate the brakes under racing 
>>or emergency conditions.
>>Too large a booster can take away some of the pedal feedback under 
>>these conditions.
>>
>>Either way you are removing the driver from the experience. (I hope 
>>that makes sense).
>>
>>In my opinion if you are having a hard time stopping a car 
>>(especially the light Mk1/Mk2) then a switch of pads is in order.
>>Mintex C-tech, Repco Metal masters, Ferodo ???, whatever wets yer whistle.
>>I realize It is a different story with a 3800lb S4 or 3200lb M3, 
>>but is because of their weight.
>>
>>If a person is driving around with stock or unknown pads and 
>>decides that in order to get better braking they need to go with a 
>>larger MC/Booser or larger calipers/rotors they are going about it 
>>the wrong way (not putting the time and effort to find out what is 
>>really the problem). A switch to better (or newer) pads and/or 
>>rotors (staying the same size) or even bleeding the brakes properly 
>>can yield overwhelming results if the previous setup was bad.
>>
>>BTW I am finially switching to the 16V booster and MC (thanks 
>>Mike!) with my full 16V setup (mk1) and am not looking forward to 
>>re-learning how to threshold brake and heel/toe with the new setup 
>>(was running the 16V brakes with 81 MC and Booster). I will be 
>>running 10.1"/8.9" Zimmerman X-drilled F/R and Front:Mintex c-tech, 
>>R:Mintex red-box.
>>In my opinion this setup will be MORE than enough for street or 
>>heavy track use (just ask people who have ridden with me how hard I 
>>am on brakes).
>>If it is not adequate for the track then I will get a second set of 
>>front rotors/pads for the track and run Hawk Blue pads, switching 
>>back and forth for street/track.
>>
>>Just my limited experience $.02 worth.
>>
>>Randy
>>www.myscirocco.org
>>
>>
>>
>>At 04:18 AM 03/13/2002 +0000, Chris DeLong wrote:
>>>although you may not "see" any great advantage to having a 25mm 
>>>master vs a 22mm master. The difference is very real and very 
>>>great.
>>>I work on these cars every day. I have seen jsut about any setup 
>>>out there. And I can say for certain that the 25mm master is a 
>>>great improvement.
>>>
>>>
>>>>
>>>>nope, but though I am not trying to make you change your mind, unless you
>>>>have 13" brakes on your car, I see little advantage in 25mm M/C versus a
>>>>22mm M/C on 10" brakes.
>>>>
>>>>beside the 22 can be had at 90$, save your money for other mods or better
>>>>brake rotors (crossdrilled or ATE Slotted)
>>>>
>>>>Patrick

 \/  '84 Scirocco (ITB racer 2B) | "Hot VW's, take two home. They're small"
\/\/ '88 Scirocco 16v (Show), '92 Passat 16v (Winter+) | - brett@netacc.net