[pure&simple]

I'm trying to build a mathematical model of the 968 braking system as a learning exercise to help me understand how all the components work. This model will calculate brake torque based on pedal pressure (assuming 100% efficiency of all braking components). It will compare and contrast various brake kits upgrades for the 968 (M030, Big Red fronts, and Big Red fronts with OEM fronts on the rear), accounting for the following factors:

• Pedal pressure
  
  
• Pedal leverage
  
  
• Brake booster boost
  
  
• Master cylinder piston area
  
  
• Bias valve pressure reduction
  
  
• Caliper piston area
  
  
• Caliper piston leverage
  
  
• Pad coefficient of friction
  
  

It will, of course, be posted here when complete, but first I have some questions that I need help with:

1. How do I determine the contribution made by the brake booster?
   
   
2. The OEM master cylinder has different piston sizes - circuit 1 is 15/16" and circuit 2 is 13/16". Circuit 1, with its larger piston area, goes to the rear brakes to ensure the rear gets less line pressure, correct?
   
   
3. The bias valve reduces rear brake line pressure above the "knee point" of 18, 33, 45, or 55 bar by a certain percentage. What is this percentage? I have found conflicting information - in one place, I read the 5 in 5/33 means 50%, in another place, 46%.
   
   
4. What is the height of the friction material only (not the backing plate) for the M030 and Big Red pads? This statistic is needed to approximate the caliper's leverage on the rotor.
   
   

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[RS Barn]

Andy,

The MO30 calipers,rotors and pads are smaller than Big Red/Black . I found the Big Reds on my turbo would lock very easily. I ended up using the 45bar bias valve with greaat results.

I was looking into a Hydroboost system for FH with Big Intake manifold and started some booster research myself

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[pure&simple]

Thanks, Pete. Makes sense about the hydroboost. That FH is a hotrod now!

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[pure&simple]

After a closer look at the image you posted, it appears that output of the brake booster is a function of booster size and the amount of engine vacuum.



Does anyone know:

1. The booster size on a 968?

2. How much engine vacuum is seen by the booster?

3. The formula describing the relationship between these two variables and the amount of boost created?

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[Cloud9...68]

Ambitious little project you have there. Has this been done for other cars? Seems like there is a high degree of uncertainty associated with several of the parameters, and I would have no idea how the disparate parameters could interact in a common model. Dealing with the units alone sounds like a very daunting task. But if there are already existing models for this kind of thing, adapting one to the 968 may not be so difficult. Best of luck with it, as I'm sure the end result willl be very useful to a lot of us.

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[RS Barn]

Andy,

What is strange about booster systems is vacuum change. At idle 15-18" is normal. As rpm goes up vacuum goes down. When you lift off throttle and brake vacuum goes up. There is a vacuum acumulator to balance changes. Doesn't seem to be best possible system.

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[pure&simple]

So if I used maybe 2/3 of idle vacuum as an average, I might still have a reasonably accurate model. I just need to locate the formula describing output of the booster. Evidently one exists because someone must have used it to creat the chart you posted. I'll bet one of the companies that sells replacement master cylinders would know.



Or maybe I'll leave out this variable... that would leave the model accurate on a percentage basis, although the actual line pressure seen by each caliper would be low.



The other variables are reasonably easy to quantify. Gonna make a call to my local Porsche dealer parts dept and see what I can turn up.

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[RS Barn]

Excellent article about Formula 1 brakes in Racecar Engineering may 2011.

Soft pedal seems to come from Flex and uneven piston pressure. This problem grows with pad wear.

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[flash]

once you have eliminated air leaks, late developing soft pedal is almost always a result of inadequate heat dissipation, usually from poor pad to rotor physics - this is often due to a poor choice in pad compound in conjunction with the rotor to which it is applied - the coefficient of heat dissipation has to match between them - this happens a lot with a pad that is too hard for the rotor, or a rotor that can't cool enough for the pad - the leftover heat has to go somewhere, and it ends up in the hydraulics

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[pure&simple]

Please help me finish the model!! I've got a few outstanding questions:



1. The OEM master cylinder has different piston sizes - circuit 1 is 15/16" and circuit 2 is 13/16". Circuit 1, with its larger piston area, goes to the rear brakes to ensure the rear gets less line pressure, correct?



2. There are a number of bias valves available (5/18, 5/33, 5/45, 5/55) which reduce rear brake line pressure above the "knee point" of 18, 33, 45, or 55 bar by a certain percentage. What is this percentage? I have found conflicting information - in one place, I read the 5 in 5/33 means 50%, in another place, 46%.



3. I am still searching for a formula describing how the brake booster multiples pedal pressure and will include that if possible.

>> Excellent article about Formula 1 brakes in Racecar Engineering may 2011.



Thanks. Think I'm gonna subscribe to that!

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[eurocarnut9xx]

pure & simple,



The larger diameter piston (23.81mm) provides the front calipers and the smaller diameter piston (20.64mm) provides the rears. This is evidenced in the 968 parts diagram (968 catalog, illustration number 604-05). Following line 9 from the small master cylinder piston, through connector 15, and into line 6, the remark for line 6 indicates /H. This abbreviates 'hinten' which is German for 'in rear'. The opposite is true for the line coming from the large piston of the master cylinder, following line 14, to connector 15, to line 5. Line 5 has the remark /V abbreviating 'vorne' which translates to 'in front'. Thus, for a given force applied to the master cylinder, a higher pressure is initially generated in the rear hydraulics. This continues until the proportioning valve begins to reduce the pressure, discussed next.



The reduction coefficient is applied to the generating pressure once that pressure reaches the ‘knee point’. The reduction coefficient for most all Porsches is 0.46 as indicated by the ‘5’ in the bias nomenclature ‘5/18’, 18bar then being the knee (968 workshop manual, pg 46-01). Thanks to Bill Verburg who has cataloged this well.



The vacuum booster uses the difference between the engine vacuum and atmospheric pressure to generate the assistance force. Normally, engine vacuum is felt on both sids of the diaphragm, netting no applied force. The force is developed as the differential pressure across the diaphragm created when the brake pedal is depressed, actuating the air valve which applies atmospheric pressure to the variable side. The 968 uses a 9” diaphragm (968 workshop manual, pg 46-01). For the sake of simplicity, a boost coefficient of 3.4 is provided to lessen the need for complex, variable vacuum calculations. I hope all of this is useful.



I too have been working on a braking system model, which is how I came across your post. I have a little bit of computational verification to complete and sample data to tabulate, but expect to post shortly. I would really like you to take a look at it and provide some feedback.



You wouldn’t happen to know the center of gravity height for a stock height 968, would you?



Kindest regards,

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[pure&simple]

eurocarnut9xx,



Great info! Thanks for posting! I will email you to trade spreadsheets. Some aspects of the calculation can be verified here (http://www.tceperformanceproducts.com/bias-calculator/) but only those aspects applicable to sizing the caliper pistons.



By boost coefficient of 3.4, you mean to say that pedal pressure is multiplied by 3.4 times by the vacuum booster before exerting force against the master cylinder, correct?



Cheers,

Andy

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[Cloud9...68]

I'm curious - what does the creation of a mathematical model of the car's braking system provide? In other words, what are the outputs (dependent variables)? Thanks.

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[pure&simple]

Not of much value unless you are trying to build/purchase a big brake kit and want to calculate how much it will throw off your brake bias. I have brake bias numbers for OEM, m030, Big Red, Big Red with OEM fronts on rears, etc. This info can help you pick a big brake kit and/or a bias valve.



I got into this to help determine caliper piston sizes for a custom Stoptech big brake kit that I am slowly trying to put together.

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[Cloud9...68]

Ah, I see. Makes sense.

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[eurocarnut9xx]

pure&simple, Everyone,



The braking system model I was working on is ready to share. It is an Excel spreadsheet that allows the user to ‘build’ a braking system and then make comparisons of the results.



Examples of what you can evaluate are:

-How much of your traction budget you’re using on the front and rear tires

-Which will lock up first? Front or rear?

-Stopping distance

-Rates of deceleration



It is still under development, so please provide feedback.

-What other reports are of interest?

-What other parameters should be variable or accounted for?

-Do you have any general suggestions?



A link to the calculator is provide below.



Thanks in advance!



https://skydrive.live.com/redir.aspx?cid...DC617D!182

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[eurocarnut9xx]

The file doesn't open in the browser so you'll have to download it first. To download it, click on the '(i)' on the far right of the file's row. Then on the right side of the page, click 'Download' to get the file.



Enjoy!

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[flash]

cool stuff - please post a link to that file in "Geek Central" too (if it won't let you post the file directly)

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[eurocarnut9xx]

flash,



It's posted in 'Geek Central' as requested.

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