How to use this Brake Calculator

This calculator can help you calculate dimensions of brake system components and give you an idea of the effect of changes in the dimensions of those components, vehicle dimensions, aero dynamic down force, weight and weight distribution. The master cylinder diameters are calculated directly, while dimensions of other components can be found by substitution of numbers you already have, or you think could work. The results tell you whether you need to go up or down (numerically), in order to reach a particular goal (like a certain force on the pedal for maximum deceleration, tire compound and diameter, M/C diameter, rotor size, pedal ratio, etc.).

In this calculator, the M/C diameters always represent a (hypothetically perfect) situation in which front and rear wheels lock up simultaneously under full deceleration, after all weight transfer has taken place. Changes in any of the other variables will be compensated for by automatic adjustments in M/C diameters to maintain the system's properties that allow for threshold braking and thus the maximum possible rate of deceleration for your vehicle.

Entries can be made as Standard or Metric. Just make sure you select the right option. The numbers already entered are examples and can be overwritten or cleared with the button. Each button contains specific info on the preceding input or result field and will open in a new tab.

 

DRUM Brakes

This calculator is designed for cars with disc brakes front AND rear, but in case your car has rear drum brakes you can do the following:

1) For “Effective radius of rear brake rotors”, enter: (internal drum diameter)/2.

2) For “Coefficient of friction of rear brake pads”, look at the code printed on the metal part of the brake shoes. If this code is visible, you will notice two capital letters as part of the code. For example: EE, FE, or FF. This tells you something about the friction coefficient. Click here to see what these letters mean.

If there is no code visible, then enter 0.3 for standard friction material intended for normal street use, or 0.4 for brake shoes made for heavier cars, sports cars and motor sports use. Still, try to find new shoes with a code printed on the side, so you have a better idea of what you’re working with.

3) After having entered all other fields with accurate numbers, for the field “Total area of rear brake caliper pistons” find by trial and error the value that causes the front and rear master cylinder result fields to display (nearly) identical numbers. This sounds more complicated than it really is, and usually takes less than a minute.

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Now you know the dimensions of your rear brakes as if they were disc brakes. In other words, if you would have disc brakes with these dimensions, they would have the same brake torque capacity at maximum deceleration as the drum brakes you have now.

Leaving the rear drum brakes in place when improving the brake system of an older car or classic sports car doesn’t have to be the beginning of a track day filled with brake trouble. Friction material for brake shoes that can resist extreme heat is available and there are ways to ventilate the drums, so hot air that otherwise would be trapped inside the drum can be replaced continuously with cool ambient air.

It’s even possible to equip existing drums with cooling fins to increase the surface area exposed to outside air, or replace existing drums with new ones that already have those fins as part of their casting or machining process. These can be found as cast iron drums or aluminum drums with cast iron inserts.

 

Rotating Mass

Energy from the rotating mass of a car represents roughly 10% of the total kinetic energy that needs to be addressed by the brakes when they are called upon for help with slowing down a car.

This calculator doesn't take rotating mass into account because it's a factor that can vary widely from car to car and between vehicle styles.

Knowing this, we can at least aknowledge that the force on the brake pedal necessary for threshold braking will be a little higher then the number that's shown in the field for that parameter. Probably somewhere in the neighborhood of 10% higher.