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Why Do Semi Trucks Use Air Brakes? The Science Explained

Close up of a semi truck air brake system with hoses on the back of the semi truck body

As a regular sight on our nation’s roads and highways, semi trucks are a vital part of the stream of commerce. In fact, without them and the billions of pounds of cargo that they transport, the economy would come to a grinding halt. But because of their large size and incredible weight when fully loaded, semi trucks require special technologies to make them safe to drive and this includes air brakes.

Semi-trucks rely on air brakes to come to a stop because hydraulic brakes, like those found on passenger cars, have inadequate stopping power. In addition, air brakes are more reliable, provide greater protection against brake failure, and have vital safety features like built-in emergency braking.

In many ways, semi trucks are the engine that makes the economy of this country go. But their power and hauling capacity are such that special precautions must be taken to protect life and property along the roadways upon which they travel and chief among them are air brakes. So why do semi-trucks use air brakes? The science-based answers are below so read on.

Why Do Semi Trucks Use Air Brakes? (And Why Does It Matter?)

When considering the reasons why semi trucks use air brakes, it is important to first understand why the type of brakes used by semi trucks even matters at all. The first thing to consider is what type of motor vehicle we are talking about here and just how many of them are out there on the roads today. Here are a few noteworthy statistics that help provide the right perspective:

  • Over 72% of all the freight in the entire country has been moved at some point in its journey on commercial trucks
  • This equates to over 10 billion pounds of cargo in 2020 alone
  • Hundreds of billions of miles each year are logged by trucks on the nation’s roads and highways
  • There are over 37 million trucks used for commercial purposes and this includes nearly 4 million class 8 trucks (a category that includes semi trucks)

To say that semi trucks are vital to the nation’s economic health would be an understatement and these figures just go to show the incredible number of them traversing roadways across the country. Not only that, but an individual semi-truck can measure up to 70 feet in length (the tractor and the trailer) and weigh nearly 80,000 pounds total when fully loaded.

This is a huge amount of mass traveling at highway speeds or navigating crowded city streets. And this is why what type of brakes semi trucks use matters a great deal.

Two white semi-trucks driving on the interstate highway with sunset

The Science Behind Air Brakes and Why Semi Trucks Need Them

When it comes to braking systems on motor vehicles, there are two basic types: hydraulic and air (pneumatic). Hydraulic brake systems rely on pressurized brake fluid in brake lines to provide the necessary action that engages the brakes. This is the type of system found on passenger vehicles. In contrast, large commercial vehicles like semi trucks, have air brakes to stop their motion when needed.

Air Brakes Work Differently Than Hydraulic Brakes

There are several key differences between hydraulic and air brakes and recognizing what these are help to explain why semi trucks use the latter rather than the former:

  • Where hydraulic brake lines only pressurize when the brake pedal is pressed down, air brake lines are constantly under pressure
  • Any leak of brake fluid in a hydraulic system will result in a loss of braking power but in an air brake system, the air compressor can make up for a minor loss of air pressure and in a scenario involving a major air leak, the built-in failsafe measure will engage the emergency brakes
  • A hydraulic brake system has a fixed amount of the braking substrate (e.g., the brake fluid) and any loss cannot be replenished without servicing the vehicle; in contrast, an air brake system is designed to recharge the air pressure as needed
  • Running hydraulic brake lines from the tractor to the entire length of the entire trailer (up to 70 feet total) is not feasible, whereas air brake lines can easily and safely run that distance
  • Air brake lines can be quickly connected and disconnected using the quick-connect feature built into modern air brake hose fittings, meaning that the air brake lines from tractors can be disconnected from one trailer and connected to another in a matter of minutes

Given the obvious differences between semi trucks and passenger cars, it is easy to understand why they would utilize different types of braking systems. In order to gain an even deeper understanding of why semi trucks use air brakes, it is necessary to examine the major components of this type of braking system, starting with where all that air comes from.

Black semi-truck driving under a bridge with the air braking brakes system visible on the rear

Air Brakes Stop Semi Trucks Safely and Effectively

The concept of air brakes to stop large, heavy vehicles has actually been around for over a hundred years. In the late 1800s, George Westinghouse invented air brakes for use on trains and rail cars so the eventual application of this braking technology (as opposed to the hydraulic brakes found on passenger cars) to heavy commercial vehicles like semi trucks was a natural transition.

To truly understand the advantages and benefits of using air brakes on semi-trucks, it is necessary to examine the major components of this type of braking system. But before getting started it is helpful to look at air brakes as an independent, self-sustaining system within the complex workings of a semi-truck, complete with its own built-in safeguards. Here is an in-depth look at the primary parts of an air brake system and how they work:

  • Air compressor – this is the heart of an air brake system as this is where the necessary air pressure is generated; by federal law, an air compressor in a semi-truck must be able to increase the pressure in the reservoirs from 85 PSI (pounds per square inch) to 100 PSI with a specified time (to illustrate, for a medium-sized truck the time requirement would be roughly 25 seconds)
  • Governor – this device regulates the activity of the air compressor, activating it when the system’s air pressure falls below a certain threshold (the “cut-in” pressure) and deactivating it when the air pressure reaches a certain level (the “cut-out” pressure); cycling the air compressor in this manner ensures that the compressor itself is not overworked
  • Brake reservoirs – the brake reservoirs (holding tanks for compressed air) are responsible for supplying the air pressure necessary to engage a semi truck’s brakes; federal law requires that the total volume of compressed air in truck reservoirs be sufficient for 12 braking cycles (just in case the air compressor fails) and for 8 braking cycles in trailer reservoirs
  • Gauges and warning signals – semi trucks are equipped with a number of gauges and warning signals that allow various aspects of an air brake system to be monitored; for example, pressure gauges indicate the air pressure levels at various points in the system and warning signals can alert drivers when reservoir pressure dips below a certain point and the compressor has not activated
  • Brake pedal (foot valve) – in a semi-truck the brake pedal is a vital part of the air brake system in that it acts as a valve – pressing it down lightly releases 10-20 PSI of air pressure into the brake chambers resulting in a moderate braking action while pressing down harder results in 40 PSI being released and a hard, aggressive braking action will result
  • Brake chambers – mounted on each brake assembly (i.e., where the actual braking action occurs) are brake chambers that convert the air pressure they receive into the mechanical force that drives the friction material (e.g., brake shoes or brake pads) onto the braking surface, whether the semi-truck is equipped with drum brakes or disc brakes

The underlying premise of air brakes on semi trucks is that they provide powerful and reliable braking action (via air pressure) throughout the entire length of a semi-truck, from the steering axle on the trailer to the rearmost axle on the trailer.

Furthermore, the air pressure that makes the air braking system work is a replenishable resource that re-generates as the truck runs. And in the unlikely event that an air leak occurs or worse yet, the air compressor completely fails, safeguards like reserve air pressure in the reservoirs will allow the driver to bring the semi-truck to a full and safe stop.

Air brake compressor tanks visible on a semi-truck close up
Air brake compressor tanks (brake reservoirs)

Air Brakes Provide Semi Trucks With Emergency Braking

One important aspect of air brakes that cannot be overlooked is the activation of emergency braking when a malfunction occurs in the braking system. This vital safety feature does more than provide redundant braking ability in the event of a serious drop in air pressure – it can save lives and property. Here’s how it works:

  • In addition to the service brakes that are driver-activated by pressing down on the brake pedal, semi trucks feature a separate emergency brake system
  • The emergency brakes are also tied into the semi truck’s air brake system
  • But unlike the service brakes which are activated when the brake pedal is pressed down, the air pressure in the brake lines prevents the emergency brakes from deploying so long as the air brake system is functioning normally
  • Normal levels of air pressure in the brake lines hold back powerful springs that are connected to the emergency brakes – if the air pressure in the lines drops below a certain level (due to a leak or malfunction) the springs are released and the emergency brakes deploy to stop the vehicle
  • Thus, the same air pressure that powers the service brakes also keeps the emergency brakes primed for deployment in case of an emergency

This ingenious feature is built into the design of semi-truck air brake systems and is automatically activated when air pressure within the brake lines drops below a certain PSI threshold. It is an indispensable safeguard that provides yet another reason why semi trucks use air brakes.

Close up of a semi-truck parking brake release lever on the dashboard
Parking brake release on the dashboard of a semi-truck

Air Brakes on Semi Trucks Are Strictly Regulated

With the number of semi trucks on roads and highways today, important regulations are in place to safeguard public safety. The National Highway Traffic Safety Administration (NHTSA) is a division of the U.S. Department of Transportation and it is primarily responsible for overseeing highway safety regulations designed to keep our roadways safe.

Semi-trucks are regulated by the Federal Motor Vehicle Safety Standards (FMVSS) and a key provision relating to braking systems lays out specific requirements for air brakes (which are federal law):

  • Heavy truck tractors must come to a complete stop within 250 feet or less when loaded to their gross vehicle weight rating (GVWR) and traveling at a speed of 60 miles per hour
  • Heavy truck tractors must come to a complete stop within 235 feet or less when loaded to their lightly loaded vehicle weight (LLVW) from a speed of 60 miles per hour
  • For the largest and heaviest of commercial motor vehicles (heavy severe service tractors), the mandated stopping distance is 310 feet under the same testing conditions

When the current version of this regulation was enacted, the primary motivation of policymakers was to reduce stopping distances for commercial trucks by a whopping 30 percent compared to prior standards at a time when highways and city streets were getting more congested with traffic and semi-trucks were becoming the primary means of transporting freight.

It is worth noting that the federal regulations governing air brakes on semi trucks and other commercial vehicles can be found in Section 571.121 of the Code of Federal Regulations. This comprehensive piece of legislation not only contains a laundry list of air brake components that are subject to regulation but also provides detailed specifications for how they must perform.

Two semi-trucks driving fast on the Nevada highway interstate

The Future of Air Brakes on Semi Trucks

With public safety in mind, braking systems on semi trucks and other commercial vehicles continue to be tightly regulated as these vehicles become more powerful and capable of traveling greater distances and transporting more cargo.

As far as what the future holds, there are indications that the NHTSA is evaluating a new requirement that semi-trucks be equipped with smart technology that can engage the service brakes if onboard sensors determine that a rear-end collision is imminent without intervening action. This would certainly add yet another layer of protection for life and property on streets and highways across the country.

3D illustration of a semi-truck driving on a scenic route by the ocean water with advanced braking capabilities at dawn


Air brakes were originally conceived to make the job of stopping rail cars easier and more effective. Now they are being utilized on another vital instrument of commerce, the semi-truck. With the incredible volume of commercial freight that semi trucks transport from coast to coast every year, the value of air brakes, which make semi trucks safer to operate, simply cannot be overstated.

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North Shore Driving School Ltd.

Metalphoto of Cincinnati

American Trucking Associations

Economics and Industry Data


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Brennan Inc.

Why Are Air Brakes the Safest Option for Commercial Vehicles?

Federal Register

Federal Motor Vehicle Safety Standards No. 121; Air Brake Systems

United State Department of Transportation

Air Brake Systems

Code of Federal Regulations


Transport Topics

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Ernest Martynyuk

An automotive enthusiast who's been tinkering with vehicles since I was 15-years old. Repairing automotive electronics has been my main job for over a decade now and have a passion for everything technical regarding cars.

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