Master Cylinders and Brake Bias
Author: Michael Hall (racepal.live)
Updated: 11/18/22
Terminology
Brakes strongly influences the behavior of the chassis, especially when turning. Understanding the science behind the brake master cylinders and brake bias will give you a substantial — and noticeable — advantage over your competitors, especially in fixed setup racing (where the brake master cylinders and brake bias are the only setup items to your avail). Start by understanding some basic terminology, so the rest of this article will make sense to you.
- Brake Bias
- The brake force applied to the wheels at the front axle, relative to the brake force applied to the wheels at the rear axle. Since a chassis has 100% braking available, a brake bias of 60% means that 60% of the braking force is going to the front axle, while the remaining 40% of the braking force is going to the rear axle.
- Loose
- "Loose" is slang, and is used to describe a chassis that tends to spin out. A "loose" chassis "steps out" at the rear end because it "spins" (or "over-rotates") when turning or braking. This is more formally known as "oversteer." A "loose" chassis can also be referred to as "free."
- Oversteer
- A condition where the chassis tends to turn too freely upon turning, especially while braking.
- Tight
- "Tight" is slang, and is used to describe a chassis that tends to resist turning. A "tight" chassis is also referred to as a "plow box" because it "plows" (or "pushes") towards the wall on turn exits. This is more formally known as "understeer." A "tight" chassis can also be referred to as "snug."
- Understeer
- A condition where the chassis tends to resist turning, especially while braking.
Master Cylinders
Master cylinders affect your brake bias with much greater influence than the “in car” or garage brake bias settings. Use the master cylinders to control your baseline brake bias at 50% for the start of a run; as well as how far down your pedals go before you cause a lock-up.
Operation
In operation, a brake master cylinder works just like the trigger on a squirt gun: It pumps fluid. In the case of a squirt gun, the fluid sprays out of a nozzle into open air towards your target (your cat, for example). A brake master cylinder, by contrast, forces brake fluid through the brake lines and hoses to each of the four wheels. It pressurizes the brake calipers to apply squeezing pressure from the brake pads to the brake rotors. The friction that develops slows or stops that wheel.
Size Selection
The Brake master cylinder size determines the amount of pressure (and therefore, breaking force) applied. Smaller sizes produce higher pressures because it gives the driver’s braking foot greater mechanical advantage over the calipers (at the receiving end of the brake fluid pressure/flow). This is analogous to the way lower transmission gears give the engine greater power over the drive tires; or the way a smaller steering pinion gear gives the wheel greater force over the steering. Smaller means more force.
Smaller means more force.
The size selection for the master cylinders have the following effects and purposes:
Fronts
| Front | |||||
| Rear |
Rears
| Rear | |||||
| Front |
Available Sizes
Currently, the NASCAR Cup Series’ Next Gen car offers eight (8) choices:
| Fraction | Decimal | Metric |
|---|---|---|
Garage Setup Procedures
- Set both master cylinders to a “middle” position; e.g., 3/4”.
- Adjust the brake bias to 50%.
- After building up speed, drive onto a straight, flat surface and slowly plant the brake pedal until all 4 wheels lock up.
- Refer to “Troubleshooting,” below, to make the needed adjustment; then repeat the previous step until the pedal travel to lock up meets the driver’s expectations and the lock up points (between the fronts and rears) are roughly equal. Replay camera views are very helpful with this.
- Test in traffic, watching carefully for plowing (too much front brake) or too much step-out (too much rear brake). The driver may use in-car brake bias to address this, but if the condition is persistent, a master cylinder adjustment is in order.
EXAMPLE 1:
On drive #1, the car locked up the fronts way too early and reached the “lock up” threshold with way too much pedal input. In “Troubleshooting,” this matches “Brakes lock up too late or not at all (pedal too far down)” so the solution was to reduce both master cylinder diameter sizes. Further, the rear brakes didn’t lock up soon enough after the fronts did, which matches “Rear brakes lock up too late or not at all” in “Troubleshooting,” so the rear master cylinder diameter size would be reduced even more.
EXAMPLE 2:
On drive #2, the car still requires way too much pedal input. In “Troubleshooting,” this matches “Brakes lock up too late or not at all (pedal too far down)” so the solution was to reduce both master cylinder diameter sizes. This time, the car tended to “plow” (tight) under heavy braking, which matches “the car tends to plow under heavy braking” in “Troubleshooting,” so the rear master cylinder diameter size would be reduced even more.
Brake Bias
The setup builder should always deliver a setup with the brake bias at 50% (smack in the middle of its adjustable range). This adjustment is intended for use by the driver, during a race, to address handling issues on the fly. It is NOT intended for the setup builder to set this value to anything other than 50% in the garage. If your brake bias requires garage adjustments, the setup builder has not setup the master cylinders correctly!
Operation
The brake bias is used to shift braking force forward (front axle) or backward (rear axle). This helps with the handling of the car because the driver can dynamically choose where the heat goes while braking, per the following chart:
| Bias | Heat | Tire Pressure | Handling (under braking) | ||
|---|---|---|---|---|---|
| Front | Hotter fronts | Pressures build in front | |||
| Rear | Hotter rears | Pressures build in back |
As you can see, rearward bias is very risky. This is why nearly every fixed setup comes pre-packaged with excessive front brake bias by way of both the in-car brake bias adjustment as well as the master cylinders. Fixed series races allow you to change the master cylinders. This is your secret weapon in fixed racing, and is part of the reason why there are specific drivers that seem to be much, much faster!
Race Adjustments
The “brake bias” adjustment is intended for use by the driver, during a race, to address handling issues on the fly. The setup builder should deliver a setup with the brake bias set at 50% and this should work fine early run.
At most tracks, every corner is different and optimal braking can be achieved by utilizing the in-car brake bias adjustment at every turn. Which values to use depend upon practice and experience. Most drivers underutilize this very powerful adjustment, while others develop a feel for it. The difference is always at least a few tenths and can sometimes be measured in seconds!
Refer to the “Bias” chart under “Brake Bias → Operation,” above, to achieve the desired handling effect with the matching bias adjustment shown in the table.
Summary
Acceptable braking balance can easily be tailored to each driver by way of the master cylinders, even in fixed series races. A perfect balance should be acquired with practice and testing, with the brake bias set at 50%. From there, the driver can dynamically adjust the brake bias in reaction to the ever-changing needs of the car and track. A carefully configured pair of master cylinders virtually changes fixed racing into open racing and will give you a competitive advantage most will never explore.
Troubleshooting
Brakes lock up too late or not at all (pedal too far down):
- Excessive brake master cylinder diameter front and rear (reduce both master cylinder diameters equally).
Brakes lock up too early or too easily (pedal too high at lockup point):
- Insufficient brake master cylinder diameter front and rear (increase both master cylinder diameters equally).
Front brakes lock up too late or not at all:
- Excessive front brake master cylinder diameter (reduce front master cylinder diameter).
Rear brakes lock up too late or not at all:
- Excessive rear brake master cylinder diameter (reduce rear master cylinder diameter).
Front brakes lock up prematurely, or the car tends to plow under heavy braking:
- Insufficient front brake master cylinder diameter (increase front master cylinder diameter).
- Excessive rear brake master cylinder diameter (reduce rear master cylinder diameter).
- Excessive front brake bias (reduce using in-car adjustment)
Rear brakes lock up prematurely, or the car tends to spin (over-rotation) under heavy braking:
- Insufficient rear brake master cylinder diameter (increase rear master cylinder diameter).
- Excessive front brake master cylinder diameter (reduce front master cylinder diameter).
- Insufficient front brake bias (increase using in-car adjustment)