What's Stopping You?

Tech tips for brake upgrades

Created by Steve Temple
When you modify your vehicle with custom wheels and performance tires, you may need to upgrade your brakes at the same time. Stock brakes work fine with the factory rims and rubber, but as soon as you go up in size, the leverage on the wheel increases, and the vehicle's center of gravity changes as well, both of which impact braking effectiveness. Also, if the tires are stickier, your standard brakes may not be able to take advantage of the increased adhesion. Conversely, if you upgrade the brakes without improving the rubber that meets the road, you may not see a big difference in stopping ability. That's because brakes aren't really what stop your vehicle—tires do. What brakes do is slow the rotation of the wheels and tires.
6-Piston Caliper

That simple fact leads to another important point: You can't evaluate the effectiveness of a given system (stock or aftermarket) based solely on braking distance measured on a single stop from highway speeds. That number could change significantly simply by changing tires.
Not only that, stopping distance is only a small part of the entire spectrum of performance required of a brake system. There's a whole lot more to improving brake performance. Other elements to consider are control, balance and resistance to fade, among others. Just as improvements in handling require a complex interaction of several suspension components, so too does upgrading a brake system involve several different aspects. Simply adding bigger rotors with more pistons isn't necessarily the answer.
Recently a number of brake manufacturers have begun to offer race-derived, four-piston and even six-piston calipers for a variety of street vehicles. Increasing the number of pistons can improve a brake system, but perhaps not in the way you might think. What's interesting is that even though these units have more pistons, in some cases they may actually have the same—or even slightly less—total piston area than a two-piston system.
1-Piece Fixed Caliper

Why use a four- or six-piston caliper, then? Although they usually provide a shorter stopping distance (with the right tires), their primary virtue is allowing for larger brake pads in the same wheel package, which provide longer wear and higher heat-absorption capacity.
To understand how a greater number of pistons with a smaller overall area can stop a vehicle more quickly, we need to look at other important factors that affect braking torque: rotor diameter and caliper stiffness. Caliper flex is a critical factor because if the body of the caliper isn't rigid enough, the effective clamping force will be reduced. A stiffer caliper body allows for smaller piston size while realizing a net increase in actual clamping force. Many stock calipers have a two-part, sliding design, while racing-style fixed calipers usually have a more rigid, unitized design.
Stiffness

Although caliper body stiffness is important, the size of the rotors has even more impact. A greater effective disc diameter increases the brake torque simply because there's more leverage exerted by the pads when they clamp on the increased radius. A side benefit of a smaller piston area used in conjunction with larger rotors is that there is less pedal travel and the pedal feels much firmer and is easier to modulate.
As with piston area, a larger radius isn't the only issue to consider when upgrading rotors. Size matters, but so does thermal capacity. Why is that so important? Brakes work by converting kinetic energy to thermal energy (heat), and transferring it by conduction, convection and radiation into the air stream. As mentioned above, stopping distance alone is not the best measure of a brake system's effectiveness, since the tires and suspension can significantly affect this number. More important to brake performance are factors such as consistency, repeatability, durability, balance, and resistance to fade. Improved thermal capacity—the ability to absorb and transfer heat—has a direct affect on all of these factors.
Installed

That's why high-performance brakes have vents designed to draw air into the rotor and accelerate it outward from the wheel. You could compare the brake rotor's vanes to those on your car's water pump—the more efficiently they move fluid (or hot air), the better the heat dissipation. To improve airflow even more, some brake manufacturers use a floating, two-piece rotor and hat design with additional air passages designed to prevent coning or "belling" of the disc. The faces of the rotors are often slotted or drilled to allow out-gassing of the pads when they get hot. These slots also help to clear debris and provide a better pad bite.
A few other key factors are the front/rear bias, differential bore sizes, brake-line pressure and the composition of the pads. Those subjects will have to wait for another article. In the meantime, the bottom line is, if you plan to modify your vehicle's wheels and tires, consider upgrading the brakes as well.
Resources

Baer Brakes @ Baer Brakes, Brembo Brakes @ Brembo - Brake systems, Master Power Brakes @ Disc Brake & Brake Conversion Kits, Suspension Kits: Master Power Brakes., Stainless Steel Brake @ SSBC :: SSBC Performance Brake Systems, StopTech @ StopTech : Balanced Brake Upgrades, Wilwood Engineering @ Welcome to Wilwood Engineering - Specialty OEM Applications, Racing, and Street Performance Disc Brakes