Slow Your Roll

With the high number of large commercial trucks on U.S. roadways comes thousands of accidents each year and thousands of fatalities. When a tractor-trailer is involved in a crash, a claim or defense is often asserted that brake failure was the cause. But how likely is it that a big rig will experience complete brake failure? Not very.

Large commercial trucks’ air brake systems incorporate multiple failsafe and back-up systems that make it extremely unlikely a tractor-trailer will lose all braking power and crash. Although the air brake system may seem complex compared to a hydraulic brake system, it incorporates many safety advantages.

Brake Basics

The basic operating principles in the air braking system utilized in modern commercial trucks is nothing new. The automatic air brake system was invented by George Westinghouse in 1872 and was utilized in rail cars. The air brake system provided an easy way for multiple rail cars to be easily interconnected and controlled as a single system.

Similarly, today with truck tractors (tractors), trailers can be easily coupled together and the braking controlled as a single system by the driver. The air brake system incorporates several safety-related designs, including air reservoir storage, dual circuit design, spring brakes, and anti-lock braking systems.

In the air brake system, an engine-driven compressor generates the compressed air used to stop the vehicle, and that compressed air is stored in primary and secondary reservoirs. The Federal Motor Vehicle Safety Standards (FMVSS) require that the total air reservoir volumes be at least 12 times the combined volume of all the service brake chambers on the vehicle. In the event of an air compressor failure, this design requirement guarantees that there will be sufficient air supply in the air reservoir system for multiple full brake applications to safely bring the vehicle to a stop.

Additionally, the front and rear brake circuits are connected separately via the primary and secondary reservoirs. In the event of one circuit failure, the other circuit will remain operational. If a trailer is connected to the tractor, the greater of the primary and secondary circuit air pressures will be applied to the trailer brakes.

Furthermore, FMVSS requires air pressure gauges in the dash for both the primary and secondary systems with an audible warning if the reservoir systems are below 60 pounds per square inch (psi) and the vehicle’s ignition switch is in the “on” position.

Additional Safety Features

One key feature of the air brake system is the failsafe spring brakes, used primarily for parking. While an application of air pressure is required to operate the service brakes during normal driving conditions, the spring brakes are engaged by evacuating air from the spring brake chambers. In the event of any air leakage from the system during parking, the spring brakes will remain engaged, ensuring the vehicle does not move. The spring brakes also provide an important safety feature: In the event of an air system failure resulting in a loss of air pressure, the spring brakes will engage, stopping the vehicle.

Anti-lock brake systems (ABS) assist during wheel lock-ups and skidding, providing tractive contact with the road surface and improving brake performance. ABS systems typically reduce stopping distances on dry and slippery surfaces, however they may increase stopping distances on loose gravel and snow. Without an ABS system, braking in loose gravel and snow can cause accumulation in front of the wheel during braking, creating a wedge and reducing the stopping distance.

The ABS system monitors wheel speeds at four or six wheels and the electronic control unit (ECU) detects whether any of the wheels are rotating significantly slower than the others, indicating a wheel skid during brake application. The ECU will send a signal to the corresponding brake valve and modulate the brake pressure to a particular wheel numerous times a second in an effort to eliminate the skidding. An optional traction control system can be incorporated into ABS by modulating the brake at a wheel in which wheel spin is detected. ABS systems have been mandatory in the United States on tractors since 1996 and straight trucks since 1997, despite no requirement on passenger cars.

Advanced, optional features are also available on an ABS system. Electronic stability control systems can respond to potential rollover, jackknife, and loss of control conditions. By monitoring vehicle speed, steering angle input from the driver, lateral acceleration, yaw rate, and brake demand from the driver, the system can detect loss of control. If a reduction in engine throttle is needed, the stability control system can remove any throttle input from the driver and apply the brake at any of the four corners in an attempt to straighten out the vehicle and allow the driver to regain control.

Similarly, roll stability programs can reduce rollover events in trucks with higher centers of gravity. If the system determines that the vehicle is entering into a turn too fast, the roll stability program can remove the throttle input from the operator and automatically apply all of the brakes in an effort to reduce the vehicle speed.

Event data recording by the engine control module (ECM) or heavy vehicle event data recorder (HVEDR) is a useful tool in evaluating brake performance after an incident. Although systems vary, most will record the basics such as vehicle speed, engine speed, throttle position, brake pedal position, or brake pressure up to five seconds before impact. The downloaded file will indicate how many seconds before impact the brakes were applied (if they ever were applied).

If the brakes were applied, a comparison of time, vehicle speed, and brake application leading up to the impact can help determine if the brakes were effective in reducing the vehicle’s speed. Additionally, the modules will indicate if any diagnostic trouble codes were stored before the collision. Add-on satellite or cellular-based telematics systems provide similar real-time information to a central control station. If the vehicle is equipped with telematics, the data is more readily available for review compared to an ECM or HVEDR, which typically require special equipment, software, and personnel to download.

Brake Check

For proper brake performance, inspection and maintenance are required. The Federal Motor Carrier Safety Administration (FMCSA) requires that all motor carriers must systematically inspect, repair, and maintain all motor vehicles subject to their control. Driver inspection reports are required to document any defect at the end of the day, and inspections are also required before every trip. Without proper inspection and vehicle maintenance, braking performance can be reduced.

Smaller, lighter passenger vehicles can stop in much shorter distances than heavier commercial trucks. In 2011 and 2013, improved stopping distances were mandated by FMVSS 108 to reduce the stopping distances of two axle tractors, three axle tractors under 70,000 gross vehicle weight rating (GVWR), and four axle tractors under 85,000 GVWR by 30 percent. The newer standard requires these fully loaded tractors to stop from 60 miles an hour (mph) in a distance of 250 feet.

Although significant improvements have been made in stopping distance requirements in the majority of the commercial trucks on the highway, typical passenger cars can stop in as little as 120 to 130 feet. These distances do not include human aspects: The time it takes for the person to perceive danger and apply the brake pedal, and the time between when the brake pedal is depressed, and the brakes engage.

It is common in a collision for a driver to claim the failure of the brake system was the cause. Although some maintenance and repair deficiencies may reduce the brake system’s performance, it is rare that a total brake failure would occur in an air brake system. A forensic examination of the vehicle and air brake system should be performed to determine the true cause of the loss.