Air Brakes and Truck SafetyS

So in light of recent near misses and other close calls, I decided that it might be a good idea to throw some words together on what I know about semi truck safety systems. I don't work for Bendix or anything, their promotional stuff and information is just the easiest to find. The primary goal of this blurb is to talk about what systems are out there and how they operate.

The TL;DR version is watch this video of trucks driving on snow and stuff:

Air Brake System Overview

I thought I'd throw together an overview how air brakes work so that everyone is on the same page. An air brake system on a semi truck operates differently than the hydraulic brake in your car. In your car, you apply force to the pedal which is multiplied by the mechanical advantage of the hydraulic system:

Air Brakes and Truck SafetyS

Air brakes don't work by multiplying the input force from the pedal so lets start with the air brake chamber.

Air Brakes and Truck SafetyS

The air brake chamber actuates the push rod that operates the brake linkages.

Air Brakes and Truck SafetyS

There are air disc brakes along with drum brakes but drums are most common and the chamber operation is the same.

Service brakes are all pressure to apply while park/emergency brakes are pressure to release. This is accomplished by having a dual chamber on the rear tractor axles as well as the trailer axles. In the diagram below the large coil spring is for the park brake spring (spring brake). When the park brake circuit is unpressurized, the coil spring actuates the push rod engaging the brakes. When the park brake circuit is pressurized, it releases the spring brake. The service brakes are applied when the adjacent chamber is pressurized operating the same pushrod.

Air Brakes and Truck SafetyS

Below is a typical tractor/trailer plumbing diagram. Air pressure for the system is generated by the truck's on board, engine driven air compressor. The park brake circuit in yellow, the service brakes are the green (primary), orange (secondary), and red (trailer) lines.

Air Brakes and Truck SafetyS

More info on the charging system in this video:

So now that we have a handle on how the chambers work and the air system, a quick word on the treadle valve. The treadle valve is the foot actuated valve that releases air from the air tanks either directly to the chambers or to a relay valve. The relay valves operate the same way as the treadle valve except that instead of your foot regulating the air, it's air pressure, kind of like a transistor. There is usually 1 at the rear axles and 1 in the trailer.

Safety Systems


ABS is the first safety system to be implemented and is also required for on highway air brake equipped vehicles. The general way that they operate, is by having an electronically controlled modulator bleed air pressure from the brake chamber at specific wheels when the system senses wheel slip to modulate brake torque. Other than this, the ABS system on an air brake truck operates very similarly to the ABS on your car, with the wheel speed sensors sending speed data to the ABS computer.

Air Brakes and Truck SafetyS

Stability Control

Stability control builds on the ABS system with the addition of several sensors (steering angle sensor, yaw sensor, in addition to wheel speed sensors) and several electronically controlled valves. Again, this is similar to stability control systems in cars in detecting the intent of the driver and the behavior of the vehicle. The difference is the addition of electronically controlled valves to apply air to the chambers and the modulators bleed air from the individual chambers to control the orientation of the vehicle. Skip ahead to 2:25 if the embedding didn't work although the entire video is pretty informative.

The stability control system will work with both the tractor and trailer brakes to help prevent a rollover and jackknifing. It will automatically brake the trailer and the appropriate tractor wheels to keep the tractor under control. It will control both understeer and oversteer.

I know we all complain about electronic nannies on our cars, I certainly do, but it's hard to deny that modern stability control systems on trucks are a huge improvement on safety.