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      06-02-2012, 02:58 PM   #2

Drives: 2014 Shelby GT500
Join Date: Apr 2008
Location: Vancouver, BC

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Originally Posted by r53s65e90 View Post
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, but I couldn't find which ones are used for the M3 applications.

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=?
stock back=?
st40 355 front =?
st40 355 back =?
st60 380 front =?
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.
Stock piston sizes are cast into the calipers.

As you say, the Stoptech ones are available from ST. Brembo doesn't say anything. AP Racing diameters are on the UK site in the assembly drawings.

I've done all these calculations and the various calipers I've evaluated are all within about 10% of each other. Basically, the auto industry has what looks like an industry standard for front and rear piston sizes, at least within a narrow range of total areas. Most BBK's match the stock sizes closely, usually slightly less piston area. Rotor sizes and swept diameters are also pretty standard, although the OEM rotors are usually deeper from rim to inside edge of the friction face.

Keep in mind that when you change pads, all your careful calculations go out the window because the mu of the new pads could be anywhere from half to double the mu of the pads you took off.