The Power to Stop

Tech Stuff


The Power to Stop

The difference between a good set of brakes and an inferior one may come down to fade resistance.

All of us who have explored the envelope of a high-performance car know how much confidence comes from a great braking system. But what exactly makes a brake system great?

Brake feel is important to a demanding driver. A car that slows in a fashion that is closely related to how hard and how far the driver is pressing the brake pedal allows the driver to control braking with great precision. Although braking systems have become more linear and predictable over the years, there are still noticeable differences among cars in brake feel.

Another key factor is a short stopping distance—the length of road needed to slow down the car. That’s why we measure 70-mph-to-zero performance on all vehicles we test. But every car has brakes strong enough for the anti-lock system to hold its tires on the verge of lockup for at least one stop. So when a vehicle’s brakes are cold, the stopping distance is more dependent on the traction of the tires than the power of the brakes.

That leaves fade resistance as the biggest differentiator among modern braking systems, and so it’s the focus of this test. And although just about every vehicle has one good stop in it, only the best vehicles can maintain their braking performance during hot laps on a racetrack or down your favorite mountain road. A pedal that goes mushy after the fifth hairpin bend will terrify even the most experienced driver.

Brake fade happens when the system becomes so overheated that the brakes can no longer slow the car efficiently. The driver feels fade at the brake pedal, which must be pushed farther and harder to achieve the same deceleration as when the brakes were cool. If the fade worsens, even pressing the pedal with every muscle in your body wouldn’t produce a hard stop.

Braking systems work by transforming a car’s motion into heat. When a driver steps on a brake pedal, the kinetic energy of the moving car is turned into friction between the brake pads and rotors (at 60 mph, there’s enough energy in a 3500-pound car to keep a 100-watt bulb lit for an hour and a half).

Although all the brake components at the wheels—calipers, pads, rotors—get toasty, most of the heat ends up in the rotors. The rotors then dissipate the heat to the atmosphere through convection cooling (vents in the rotors help the cooling process). But it takes time for the brakes to cool, and if you keep working them hard, they tend to get steadily hotter. If they get hot enough, the pads exceed their maximum operating temperature, causing them to lose friction against the rotors. To overcome this loss of friction, the driver must press harder on the pedal. And if the calipers get scorchingly hot, the brake fluid starts to boil, which introduces vapor bubbles into the hydraulic circuit. When this happens, the brake pedal fails, collapsing to the floor because the pedal pressure is simply compressing the bubbles instead of squeezing the pads against the rotors.

To find out how different brakes compare in their ability to endure repeated stops, we decided to measure the brake fade of 11 vehicles, ranging from heavy SUVs to sports cars. We split the vehicles into three categories: mainstream sedans, luxury SUVs, and performance cars. In each category, we chose representative cars from automakers based in Germany, Japan, and the U.S. to see if regional design philosophies affect brake-fade resistance. For example, do cars from Germany, where high-speed autobahn driving is a given, really have more powerful brakes?

The mainstream sedans were four-cylinder versions of the Chevrolet Malibu, the Honda Accord, and the Volkswagen Passat. The SUVs consisted of the Cadillac SRX, the Infiniti FX50S, and the Porsche Cayenne S. For the performance cars, we went with the BMW 335i, the Chevy Corvette Z51, the Nissan NISMO Z—a 350Z fortified with the NISMO sport package—and two Porsche 911 Carrera S models, one fitted with Porsche Ceramic Composite Brakes (PCCB). These brakes use lighter ceramic rotors instead of the normal cast iron and cost about 10 grand extra.

Everyday driving doesn’t result in brake fade, so we had to devise a severe test to create some. We employed a series of five-stop cycles. Each stop was from 100 mph, and the first one consisted of applying the brakes hard enough to slow the car at a 0.50-g rate (roughly half of maximum braking power). We used our Racelogic VBOX III GPS-based data logger to verify that we were stopping at the 0.50-g rate. During the 0.50-g stops, we employed a Futek sensor to measure brake-pedal force and a string potentiometer to measure pedal travel. Next, we accelerated to 100 mph again and performed a panic stop by pounding the brake pedal hard enough to engage the anti-lock system. We repeated the panic stop three more times for a total of four to thoroughly work the brakes. Those five stops completed one test cycle, and we repeated this cycle until we determined—by an increase in pedal force, pedal travel, or stopping distance—that the brakes were displaying severe fade.

A critical component of the test was the amount of time allotted between each stop to allow the brakes to cool and recover. We wanted to keep that time segment consistent for all vehicles—about 90 seconds (including acceleration time)—but found that gap was too easy on the performance cars. For that category, we reduced the time between stops to 20 seconds.

After two days of testing at Chrysler’s Whitman, Arizona, test track, we had performed more than 300 stops, obtained one very sore right foot, and gathered enough data to clog a supercomputer. Despite our efforts, the brakes on all the vehicles recovered after being given time to cool.

Here’s what we learned.


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