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Master Cylinder question
Science is wonderful thing...
I got to find my copy of Holiday and Redneck and work this out!
-Raffi
That might be Halliday and Resnick,
At 11:20 PM 3/13/2002 -0500, you wrote:
>>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
>
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