The vacuum-operated brake booster works today much as it did 50 years ago when muscle cars ruled the road. Drawing vacuum on the front of the diaphragm removes atmospheric pressure. The rear chamber is vented to the atmosphere and the pressure multiplies the force a driver applies with the brake pedal. This cutaway shows where the diaphragm placement in the brake booster is and how it works in relationship to the brake pedal actuator rod.
How the force is amplified from the driver’s input into braking force is referred to as brake system gain. This gain is done mechanically and through vacuum assistance. It all starts with the driver stepping on the brake pedal. Without extra exertion, an average-size person delivers about 70 pounds of force on the brake pedal pad. The brake pedal is really a mechanical lever, and the positioning of the pedal pad in relationship to the mounting point (where the pedal pivots) and the point where the pushrod is attached to the master cylinder is how the force of the driver’s action is multiplied.
How Upgrades Affect Vehicle Performance
There are several things to consider when planning a brake upgrade. These include gain, modulation, heat capacity, cooling rate, and weight.
Gain
Gain is a fancy term for multiplying the mechanical advantage. These gains can come from changing the pedal ratio, adding a brake booster, upgrading to a larger caliper piston size, or changing the size of the rotor. Larger discs allow for more brake torque because the brake pad will apply pressure at a larger radius, while larger caliper pistons (or more pistons) result in more area of applying a specific pressure.
One of the quickest and easiest ways to improve gain in the braking system is to change the brake pedal. The braking ratio can be changed with an increase or decrease of distance between the pedal’s hinge point and where the master cylinder piston connects to the pedal.
Modulation
Brake modulation, in the simplest terms, is the ability to slow down or stop without locking up the brakes. Peak stopping power is just before the brakes lock up. The ability to control that peak range precisely is the goal of a well-designed brake system. Brake system upgrades usually have a better pedal feel and firmness with an improved ability to control brake lockup.
Heat Capacity (Thermal Mass)
Heat is the enemy of brakes. When temperatures get high enough, the brake pads start fading and eventually lose the capacity to work properly. Heat is frequently absorbed by the brake system components, which can be dissipated at a rate depending on the mass and material. Thermal mass or thermal capacity are terms often used to describe the ability of the brakes to shed heat without reaching temperatures that would interfere with proper braking. Modern aftermarket brake systems use materials and component designs that allow for higher heat capacity.
Cooling Rate
Upgraded brake systems usually have better cooling through material and venting design. Slotted and drilled rotors, curved cooling vanes in rotors, materials with better heat dissipation, and a larger surface area all factor into manufacturing modern aftermarket brake systems. In addition, drum brakes are enclosed. All of the heat is trapped inside the drum. Disc brakes have rotors that are exposed to airflow and more efficient cooling.
Weight (Unsprung and Rotating)
Converting from drum brakes to disc brakes often results in a substantial weight reduction. Whenever there is a weight loss, two terms apply: sprung/unsprung weight and rotational weight.
Sprung weight is any portion of the car that is held up by the coil springs; the rest is unsprung weight. The largest part of a braking system (drums or discs and calipers) is unsprung weight. For performance, car builders try to minimize the unsprung weight because it hurts handling. This weight is supported by tires and shocks.
Rotational weight is any part that rotates when speed is accelerated or decelerated. It takes more horsepower to turn rotating weight and to decelerated rotating weight. Taking off the heavy drum brakes and installing lighter rotors helps minimize the amount of rotating mass. This is also an important factor when upgrading to aftermarket wheels. Increasing weight requires more horsepower to turn and more brake force to slow down.
Modern Braking Performance Versus High Performance
When it comes to selecting a disc brake conversion or improving to a higher-performance braking system, a few factors come into play. What are your plans? Will the car be used as a daily driver to enjoy driving back and forth to work and home, or will it be going to the track, doing some canyon carving, or hitting the autocross course regularly? As with almost anything else, the budget for it will likely play a large role in the decision.
If you are simply looking for an up-to-date brake system that is compatible with today’s city traffic, or the budget is tight but you still want safe and confident braking, then an OEM-style replacement brake conversion kit is probably all you need. However, if you are hitting the track, a mid-level or high-performance kit will probably serve your needs better.
Evaluating an aftermarket brake system boils down to a few critical elements.
Larger Disc Radius: A larger disc radius allows for more brake torque. The brake pads will apply more pressure over a larger radius by virtue of being farther from the center of the wheel. Selecting the largest rotor that fits safely within the wheel is helpful for good braking force.
Caliper Piston Area: Selecting a kit with larger pistons or more pistons (increasing the piston area) allows the system to increase the brake force due to larger piston area. If the line pressure remains consistent, the increase in piston area means the applied force will increase.
When converting from factory-original drum brakes to aftermarket disc brakes, the differences are very noticeable. The weight difference is visually obvious.
In addition, the drum brake shoes are contained in the drum and shielded away from cooling airflow. Any debris from the shoes are also contained in the drum. Disc brake rotors and pads are open to the environment and make better use of cooling and cleaning.
Increasing Line Pressure: Increasing line pressure by adding a power booster or improved pedal ratio helps increase braking force.
Material Selection: Selecting brake pad and rotor material to improve the coefficient of friction between the pad and rotor can increase the braking force. There is a tradeoff when generating more friction, which is more heat. Larger rotors will help shed heat, especially if the rotors are designed with cooling in mind.
Rotor Design: In addition to larger rotors, cooling vents that allow for airflow through the center of the rotor greatly improve the ability to efficiently provide cooling and prevent brake fade. Slotted and drilled rotors assist by allowing gasses to escape and remove particles that are created by brake pads and rotors during braking.
The caliper piston area varies from manufacturer to manufacturer and even different-sized calipers can be found within the same manufacturer’s line of products. This is a compact single-piston caliper from Wilwood Engineering. (Photo Courtesy Wilwood Engineering Inc.)
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