Brake bias: Handy calculator to help you setup your car and tune your changes | SwedeSpeed

What brakes to use is probably the touchiest topic on this forum surrounded by a lot of subjectivity and sponsored users pushing their wares. Instead of more of the same, Here’s a simple calculator to help you make an informed decision based on data.

From the tool:

The primary goal of this calculator is to be used as a comparative tool. By starting with your current design and making changes you can evaluate those changes.

What is the ideal bias for my car?
There is no one size fits all answer to this question. As much rear brake as you vehicle will allow before the rear end becomes unstable is a simple way to look at it. However that does not take into account suspension settings for weight shift, road surfaces- which effect adhesion, tire compounds and sizes and of course; driver preference.

Here’s the tl;dr

• My stock 2017 Polestar car has a front brake bias of roughly 73% from a purely caliper, pad and rotor point of view; YMMV depending on whether you have updated parts
• When changing parts, the following items changes brake bias in order of most effect to least effect on brake bias: Caliper Piston area > Brake pad coefficient of friction > brake rotor diameter
• Mind your temperatures when choosing brake pads. Temperature can drastically change a pad’s friction coefficient.
• Caliper references
• Pad friction references

Some background:
With the advent of electronic brake distribution, what happens between your brake pedal pressure and the fluid pressure going to your individual brake calipers is mostly a black box. To simplify things, brake fluid pressure will be staying constant for these calculations.
The mathematical model used in this calculator is mechanical torque calculation on an already moving system.

Variables for this calculation are:
applied brake pressure as a function of brake caliper pistons diameter and brake fluid pressure*
pad-rotor coefficient of kinetic friction
mean radius of brake pad force application on brake rotor as a function of brake rotor and brake pad annulus (aka pad radial height)
*floating calipers, when it comes to applied brake pressure, act like 2 piston brakes with identical piston diameter.

So here’s what I found my car’s stock brakes look like:
371mm rotor
28mm, 34mm and 36mm caliper pistons.
57mm pad annulus
~0.44 pad average coefficient of friction (Ferodo HP1000)*

302mm rotor
42mm caliper piston
48mm pad annulus
~0.46 pad average coefficient of friction (Brembo HP2000)*

*this is a rough estimate based on Brembo’s literature combined with DOT codes stamped on the pads themselves (FF for HP1000 and GG for HP2000)
https://www.brembo.com/en/Varie/Brembo_SportAutoPadsCatalogue.pdf
http://faculty.ccbcmd.edu/~smacadof/DOTPadCodes.htm

This comes out to roughly 73% front bias.

With a baseline established, let’s start changing up the values to see what kind of bias effect you can expect when you swap parts. Here’s what happened when I added playground tuning’s rear kit:

365mm rotor
24mm, 28mm, 30mm caliper pistons
0.36µ rear(Street pad)
50mm pad annulus

Assuming no changes to the front, this gives a default street setup with 63.5% front bias.

For the next experiment, let’s also try pushing bias as far front as possible on a high performance street-able setup without getting race pads.
0.52µ Front – Hawk HP Plus @ 300C
~ 0.38µ rear – EBC Yellow stuff @ 300C
This puts us back at 67% front bias.

Here’s my personal setup.
0.48µ average (Ferodo DSUNO)
0.42µ average (Ferodo DS2500)
This comes out to a hair under 62% average front bias.
In practice, this is not a street setup; I’m expecting temps will climb to ~550C which shifts bias back to ~66%.
0.50µ front
0.37µ rear