View Full Version : master cylinder sizes?

01-03-2006, 12:14 PM
I know theres some math out there to define what size mc is best for a specific brake set up given the number of pistons per caliper and piston size etc etc. but does anyone know the math behind it? or have a program or link to something?

also in dual mc setups, the balance bar, what exactally does that achieve?

01-03-2006, 12:21 PM
On the dual mc set the bar acts as proportion valve to the back brakes so you can adjust it. If I were to fit a brake set to car stick with production model all the way ie vette discs + master or any other model this is the easiest route as you know A it will stop the car and B it a tested system, or if going after market stick with 1 company .

01-03-2006, 12:37 PM
well the reason Im asking is that on my mx6 i can adapt vr4 front calipers, so much bigger disc and 4 pistons as opposed to the stock 1. I have two options as far as bolt on masters, the stock one and the 929 master which is larger. just trying to find which would be the best fit for it. As well as understand the math behind it for future referances.

so with a balance bar you would have no more need or use for a prop valve to the rears then?

David Pozzi
01-03-2006, 10:39 PM
Here is something posted a while back by dnult on Team Camaro, it has quoted forumulas from Camaro Performers Magazine. When comparing two calipers you should only use the bore areas of one side of a four piston caliper when comparing to a single piston floating type caliper.

I was reading an old Camaro Performers Magazine the other day (Winter 2002) and discovered an article about braking. I found it interesting and wanted to share what it said. I'm currently fiddling with brakes on my 68 Camaro. This article at least put the problem into better prespective. As with all rules of thumb, these are to get you close. The optimal spec for your car may fall outside these specs. Here's what the article said...

1) The angle between the push-rod and pedal should never be less than 90* or a soft pedal under hard braking will result. This will be more of a problem on small bore master cylinders that stroke a fair amount.

2) The Caliper to Master Cylinder piston surface area ratio should be between 12% and 19%. (Not sure if this is true for both Manual and Boosted brakes). Take the surface area of the MS piston divided by the suface are of one front caliper (d^2 / 4 * 3.14159). If using four piston calipers, take the sum of one side for caliper area. In other words sum the surface area of two pistons. The opposing pistons will apply similar force once the pads contact the rotor and cancel out. My 1" bore MS and 2.84" single piston caliper result in a ratio of 12.4% It was interesting to note that a 1.125" MS resulted in a caliper ratio of 15.7% which is right in the ball-park, but I believe experienced users will say this is a firm pedal.

3) The pedal ratio should be between 4:1 and 6:1. To calculate, take the distance between pedal fulcrum and pushrod attachment point. Divide that lenght by the distance from fulcrum to peddle centerline. The article further sited a recommendation of 4:1 for power brakes and 5:1 for manual. My 68 is 2"/12.5" for a 6:1 pedal ratio in the manual brake setting. The boosted position is another 1 and 1/2" lower resulting in a 3.5:1 ratio (low by comprison to 4:1).

4) The suface areas of the front caliper piston (for 4 w disc) should be at least 25% larger than the rear piston surface area. They go so far as to recommend 40% larger and site a 93-97 camaro as being 55% larger. What they don't say is if there is a relationship between this ratio and the rotor size. In other words, if the rotor size is not 1:1, how does this rule of thumb change?

While I was on a kick with brake math, I calculated the pressure provided by a vacuum booster. Using a 9" booster at 17"Hg, I came up with 531 lbs of force applied by the booster. Now I am looking for details on how the booster works. Surely this pressure is reserved until the brakes are applied. 531 lbs is enough pressure to create 417 lbs of line pressure with a 1" MS. That's more than enough to overcome the pressure of the spring in the MS. Still trying to noodle that one.

Hopefully this is useful information. I now feel a bit better about my 1" bore MS in a manual application. The resevoir size is my only concern at this point. But the quest continues.


Here is a link to an online brake calculator:

Here is a good reference page with formulas and a handy piston bore/sq inch chart. http://www.outlawdiscbrakes.com/faq.html

David Pozzi
01-03-2006, 11:20 PM
Here is a previous answer I posted on Team Camaro, it outlines brake analysis proccedure.

Camaro pedal ratio for power brakes is 3.8 to 1, manual brake ratio is 6.25 to 1.

so, to calculate, foot pressure in lbs, X pedal ratio plus booster pressure in lbs = total force on master cyl piston in lbs.

Piston force lbs X piston sq inches = line pressure (PSI). (Sq inch formula: bore squared X .7853982= bore in sq inches)
Line pressure X caliper bore in sq inches = clamping lbs. (Use one side of a four piston caliper piston area or the whole area of a single piston caliper.)

caliper clamp lbs times pad friction coeficient = caliper drag. (pad size doesn't matter here)
caliper drag X rotor (effective) radius to pad center, (not rotor OD) = rotor torque in inch/lbs.
Rotor inch/lbs torque divided by tire radius in inches = lbs of braking force at tire tread.

If you want to do more estimate forward weight transfer and add that to the front wheel load.

Tire loaded weight X tire coeficient of friction = tire limit of adhesion, if brake lbs exceed this, the wheel locks and skids.

01-04-2006, 01:32 AM
another question, does anyonw know how far the pad actually has to travel before comming in contact with the rotor? like any average number to use?

David Pozzi
01-04-2006, 07:50 PM
That's something I'd like to actually measure some day. I'd guess it's .005" per pad, on a race caliper it's probably less, maybe as much as the rotor runout.

01-04-2006, 08:58 PM
Another good post that I have referenced before is this:


The reply by spott is informative.

01-04-2006, 08:58 PM
im guessing it woudlnt be that hard to measure, using a feeler gauge should do the trick. By the way great responses, they are just what i was looking for and it helped me a bunch.

Norm Peterson
01-20-2006, 02:13 PM
Any interest?


01-21-2006, 09:27 AM
Yes I have interest in that... did you make that or did you come across that somewhere?

Norm Peterson
01-23-2006, 09:58 AM
I'm about the third person to put some time into this one, but I think I've put enough into it at this point in terms of cleaning up the presentation, adding notes, prop valve inputs and a (so far partially) macro-driven solution, etc., to be able to call it as much mine as anybody else's. For any sort of distribution, I need to date the sheet, list the credits and finish up the 'HELP' page. And add a control button or two for the macros and update the name to 'BrakeForce12'.

Can you think of any useful feature that may not be there?