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      06-02-2012, 12:35 PM   #1
r53s65e90's Avatar

Drives: r53 s65e90
Join Date: Mar 2009
Location: San Jose, CA

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Brake balance calculations

I am looking for piston diameters for brakes in M3 applications. Rotor sizes are readily available. If you want to be super accurate the fitment templates have extra information, such as where the pad bites on the rotor e.t.c.

Brake torque T can be expressed as follows T=c*A*D. c= a constant for a given master cylinder and friction material (pads). A=total piston surface area on one side of the caliper. D=rotor diameter (or more accurately diameter of circle that the center of the clamping force on the disc, but rotor diameter should be enough for good estimates). A can be calculated given the piston diameters that seem to be nowhere available online. Only stop tech provides diameters.

Given the above simple formula brake bias front to back is calculated as follows: B=(Af*Df)/(Ab*Db) f=front b=back

Why these calculations are useful:

1) Stoptech says you can mix and match stock calipers with their calipers. This assumes that both the st60 and st40 front calipers do not alter (increase) brake torque. I don't want to assume in general if it is easy to calculate.

2) Brembo says nothing about mixing and matching so I want to figure out whether their caliper increases brake torque at the front so that it needs to be matched by a corresponding increase in the back. This is another simple calculation:

B=(Abr*Dbr)/(As*Ds) br=brembo s=stock (either back or front assuming same pad material)

If B>1 brembo calipers generate more torque = they will stop a wheels going 100 mph faster to the point where most braking force is applied to actually stopping the vehicle instead of decreasing the rotational energy of the wheel.

I searched before posting this as I am definitely not the only one to look for information like that. I will measure stock pistons next time I change into my track pads. Can anyone provide information about the stock and aftermarket pistons? (Vendors you should know this stuff. If you don't please could you measure some of the calipers you have lying around).

I will update the data below adding balance ratios as needed:

Piston diameters:
stock front= 60mm
stock back= 46mm
st40 355 front = p1: 44mm p2: 42mm
st40 355 back = p1: 34mm p2: 30mm
st60 380 front = p1: 36mm p2: 30mm p3:30mm
brembo 6 for 365 rotor =?
brembo 6 front for 380 rotor =?
brembo 4 back 380=?
brembo 4 back 350(?)=?
ap racing 6 front=?
ap racing 4 back=?
(... whatever else ...)

Please point out flaws in the write up above. Its all high school physics.

(All the above given same friction material front and back). Hence, stoptech can rightfully claim they provide a balanced upgrade no matter what you go with. All the balances above are within at least 2.5% of stock. Nice!

Last edited by r53s65e90; 06-10-2012 at 10:26 PM. Reason: update with actual stop tech piston sizes available on their website