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Master Cylinder question



>Although this makes sense at the drive piston in the master cylinder 
>you also have to take into account that the piston size of the brake 
>which remains the same. Now increasing the size of the drive piston 
>in the master cylinder lowers the pressure against it (by virtue of 
>the larger area) but more fluid volume is transferred to the smaller 
>constant sized brake piston. From fluids the increased volume 
>pumping into the cylinder increases the force against the brake 
>piston. More force here means more braking power.
>
>Any physicists out there?
>
>-Raffi
>

No, this is not correct.  Think about it.  The fluid is pushed out of 
the MC at a lower pressure
to the piston of the brake.  The fluid pressure is spread out over 
the exact same area on
the piston no matter which MC you happen to use...if the pressure out 
of the MC is lower
the pressure at the brake piston is lower == less braking force. 
Pretty much end of story
there.  *HOWEVER*  the caveat here is something that is not easily 
quantified, and you
almost hit upon it for the wrong reason. It is _possible_ that a 
larger MC can make up
for additional braking system compliance with greater volume (at 
lower pressures) leaving
more pressure available to actuate the braking cylinder piston. 
Compliance occurs as
the pressures begin to build in the system, all the various 
components in the system will
flex until all the clearances have been taken up.  During this time, 
seals, clearance parts,
and other flexible components will stretch and deform, effectively 
increasing the hydraulic
volume in the brake system.  As I mentioned already, a larger MC can 
help make up
for this because it can move more volume in order to take up this 
compliance which can
leave greater available pressure 'left-over' to actuate the braking piston.

A better braking system starts with the tightest possible tolerances 
and the least flexible
parts...other improvements can be made as I detailed previously. 
Seeing as how few
people actually do that, a larger MC might be able to help...but we 
should all stop
believing it's due to the higher pressures of a larger MC.  We might 
want to try and
firm up the rest of our braking system as much as possible by 
replacing worn brake
calipers (or leaking seals) and rubber lines before diving into another MC.

==Brett


>
>At 09:00 AM 3/13/2002 -0500, you wrote:
>
>>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

 \/  '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