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Brake tech - Upgraded master cylinders, calipers, bias, etc

Posted: Sat Mar 18, 2017 8:11 pm
by woody90gtz
I keep seeing threads asking about brake modifications. So here's a tech thread that we can pin for quick reference.

Some basic theory
Brake systems can be figured mathematically because they are hydraulic systems. Let's start from the beginning. You press on the brake pedal. The boost assisted force in lbs is multiplied by the pedal ratio and then divided by the area of the master cylinder piston. When the master piston is less than 1 sq in, it multiplies psi. This creates your line pressure. The line pressure is then multiplied by the area of the caliper piston(s) for the brake pressure on the pad.

Pad pressure is not everything, though. Obviously the pad will bite more or less depending on the coefficient of friction. The diameter of the rotor also changes the leverage applied down to the hub and then back out to the tire. A bigger rotor will create more brake torque for the same pad psi and cof. This creates some gray area when making calculations, but hydraulically these are good guidelines.

One big point to remember: brakes are always a compromise. Choose the best for your use, because there is no perfect answer.
Hydraulic advantage comes at the price of pedal travel. So bigger area calipers or smaller area master cylinders will require you to push the pedal closer to the floor.
Mechanical advantage comes at the price of inertia or dust. Bigger rotors will slow your 1/4mi and slightly reduce gas mileage. Grabbier pads will eat rotors and create dust.

Factory Specs
Stock 92 Beretta calipers: 2.241 single piston = 3.942 area
Rotor: 10.20
Master cylinder bore & pressure bias: 22mm & 1,400psi front, 500 rear
Front braking pressure: 5,518psi
Bias with drums: unknown

(I look at the Lumina as a good goal for bias, because it was a FWD engineered by GM)
Stock Lumina front calipers: 1.625 dual piston = 2.073 x2 = 4.146 area
Rear calipers: 1.376 single piston = 1.486 area
Master cylinder bore & pressure bias: 24mm & 1,400psi front, 950 rear
Braking pressure: 5,804psi front / 1,411 rear
Brake bias: 80% front / 20% rear

Possible upgrades:

91 Lumina master cylinder: I think the numbers prove that a rear disc conversion without a Lumina (or equivalent) master cylinder swap is a bad idea. You are throwing half of your rear brake torque out the window. So all these calculations (unless otherwise noted) are done assuming a Lumina master cylinder.

99 Neon rear calipers: 1.338 single piston = 1.405 area
Brake pressure w/ Beretta master: 702psi
Brake pressure w/ Lumina master: 1,334psi
Bias w/ Beretta front caliper & Beretta master: 89% front / 11% rear
Bias w/ Beretta front caliper & Lumina master: 81% front / 19% rear

99 N-body front calipers: 2.362 single piston = 4.380 area
Rotor: 10.945
Brake pressure: 6,132psi
Bias w/ Neon rear: 82% front / 18% rear

99 Camaro/Corvette front calipers: 1.750 dual piston = 2.404 x2 = 4.808 area
Rotor: 11.92
Brake pressure: 6,731psi
Bias w/ Neon rear: 83% front / 17% rear

Wilwood front calipers: 1.500 4 piston fixed = 1.766 x2 = 3.532 area
Rotor: 12.19
Brake pressure: 4,944psi
Bias w/ Neon rear: 79% front / 21% rear

That's all the popular combos I know of right now. I can run the math on other stuff though, just post. I will add some rotor info too.

Re: Brake tech - Upgraded master cylinders, calipers, bias, etc

Posted: Sat Mar 18, 2017 8:33 pm
by woody90gtz
Here is a spreadsheet I got from NastyZ. This is for manual brakes, so I cheated the master cylinder and input force numbers to get the output line pressures to a measured spec (1400/950). The rear weight bias numbers I found somewhere, but if someone can confirm that 58/42 is about right that would be good.

This is set up with specs for my car with stock 92+ front and Neon rear. The calc puts my car at 1.2G for brake capability.

You can modify the yellow cells with info from other combinations to see how they affect the brake capability.

Brakemath beretta.xls
(28.5 KiB) Downloaded 18 times

Just ran the max brake torque in in-lbs for several combinations through the spreadsheet:

Stock Beretta front: 18,779
Wilwood front: 20,624
N-body front: 22,520
F-body front: 27,345
Neon rear: 5,070