By Jonny Lupsha, Wondrium Staff Writer
Automotive braking systems are a wonder of everyday engineering. Massive amounts of friction cause our cars to slow down and stop every time the brake pedal is pressed. Auto regulators want automatic emergency brakes on new heavy trucks.
The Department of Transportation announced recently that it will seek a mandate that auto manufacturers install automatic emergency braking systems on newly made heavy trucks. The National Highway Traffic Safety Administration plans to publish the rule in the Federal Register in April 2022.
The Insurance Institute for Highway Safety and Highway Loss Data Institute published figures that claim that in 2017, 2018, and 2019, more than 4,100 people per year died in the United States due to crashes involving large trucks. They also said that automatic emergency braking and forward collision warnings could prevent over 40% of these deaths.
In his video series Everyday Engineering: Understanding the Marvels of Daily Life, Dr. Stephen Ressler, Professor Emeritus from the United States Military Academy at West Point, explained how braking systems work.
Them’s the Brakes
According to Dr. Ressler, disk brakes and drum brakes are the two principal types of braking systems in vehicles. Disk brakes are generally considered superior and all modern cars use them in their front wheels.
“In a disk brake, a metal disk is rigidly fixed to the rotating axle,” he said. “A caliper, which fits around the disk, contains two hydraulically activated brake pads—one on each side. When you press the brake pedal, pressurized hydraulic fluid forces the brake pads inward against the disk.”
Next, the brake pads apply a pair of “compressive normal forces” to the disk, which generate friction that slows the rotation of the disk and the axle, slowing the vehicle down. However, very large friction forces are required, and all the kinetic energy is transformed into heat at the interface between the disk and the brake pads.
“This heat must be dissipated rapidly, to prevent a phenomenon called brake fade,” Dr. Ressler said. “This is the primary reason why disk brakes are inherently superior to drum brakes—because most of the disk is exposed to the air, and thus heat dissipates more readily.”
Drum brakes, on the other hand, involve a steel drum fixed to the rotating axle and a pair of brake shoes inside the drum that are fixed to the suspension system. However, the heat builds up inside the brake drum and dissipates more slowly.
Other Brake Technologies
Beside automatic emergency braking systems, another important innovation in automotive safety is the antilock braking system (ABS).
“As the name suggests, ABS is intended to prevent your wheels from locking up during hard braking,” Dr. Ressler said. “When a wheel is locked up and skidding, the tire is providing significantly less than its maximum possible frictional resistance. On snow or ice, the situation is much worse, because skidding generates heat at the interface between the contact patch and the road.”
That heat melts the snow or ice, making the situation even worse. This makes the car’s stopping distance increase drastically and it makes the car essentially uncontrollable. Dr. Ressler said it’s because there’s nothing to provide the additional lateral traction force that’s needed to turn the vehicle.
“ABS addresses these issues by using electronic sensors to detect when a wheel is about to lock up—and then releasing and reapplying the brakes in short pulses, many times per second,” he said. “This cyclic braking keeps the friction force primarily within [a reasonable] range, thus achieving significantly higher frictional resistance on each cycle, and restoring traction as soon as skidding begins.
“The result is a shorter stopping distance and better control, particularly on slick surfaces.”
Antilock braking has been mandatory on all cars in the United States since 2011. Automatic emergency braking on heavy trucks seems to be the next auto safety feature to follow.