Tractor/Trailer Air Brake Systems

Today's challenging world of expressways, mountain roads, heavier loads, and greater horsepower, places a heavy burden on today's brake systems. Truck air brake systems have gotten much more complicated during the past 20 years to keep up with these increased braking demands. Trucks traveling faster, can stop quicker today.

Many innovations contribute to this braking improvement, and the purpose of this topic is to explain all of those brake components which make up today's air brake systems. Once you understand the brake components and what they do, then you can easily fault isolate braking problems. As you can see from the list below, today's air brakes cover a lot of information. There are no prerequisites for this topic, so lets get started and figure out how this entire braking science all works together.

Human Reaction Time

Stopping of your rig is delayed for two reasons. The first delay factor is the amount of time it takes your brain to realize that you need to make a panic stop, and the additional time it takes for your body to move your foot to the brake pedal and start pushing on the brake. It is reasonable to consider 1-1/2 seconds to get your foot on the brake for an unexpected panic stop. At 60-MPH, you travel 135-feet, nearly 2-1/2 times the length of your rig, before you can even get your foot on the brake pedal. Refer to the Oklahoma City High School experiment at for their conclusions about brake application reaction times. Although their test was to determine what effect music volume level had upon brake delays, they proved convincingly that the average person requires 1.55-seconds to apply the brakes for an unexpected situation. If the above website link fails, we will mirror a portion of their web page experiment data which can be viewed here.

The second delay factor is the time that it actually takes your brake system to start generating the stopping friction. This includes the time it takes the air pressure from your brake pedal to start charging the brake chambers, and the amount of time it takes for your slack adjusters to take the slack out of the brake chamber linkage, and to start generating brake friction. This whole process can easily exceed 1/2 second, so now your rig has traveled 2 seconds or 180 feet at 60-MPH (nearly 4 rig lengths) before the first braking action actually takes place.

What happens if a car or truck suddenly pulls out 150 feet in front of you while you are going 60-MPH? You will probably hit that car or truck before your brakes ever get a chance to engage. Just some food for thought. The whole purpose of this section is to get you to realize just how big of a factor speed plays when dealing with an unexpected and sudden braking situation.

We hope you have benefited from this topic and will see speed in a different way than you used to.

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