This Is Why Trucks Have Air Brakes

This Is Why Trucks Have Air Brakes

I’ve always wondered why trucks have air brakes. I’ve also been curious about air brakes. What are they anyway?

So a quick trip to the local truckers’ hang out revealed a few surprising things about air brakes.

Air brakes are simply a more advanced braking system that trucks need to stop effectively. These are installed on most trucks instead of mechanical brakes to ensure that there is no physical activation failure that most mechanical systems are faulted with. This is a more complicated system than your average car’s brake system.

Trucks are extreme machines that weigh ten, twenty times more than your regular sedan. Some would argue that they weigh a hundred times more than your average car. In essence, it will, but only when it is in motion. It’s not that hard to believe seeing how some trucks actually carry a dozen or more cars in its storage area cross-country.

Let’s Do A Little Physics Lesson Here

Semi-Truck With Air Brakes
Semi-Truck With Air Brakes


To understand the concept of needing air brakes on trucks better, let’s have a walk down memory lane. Let’s try and remember what we learned in physics about momentum. 

Here’s the basic formula: 

p = mv

Here’s what that means: 

p stands for momentum

m stands for mass in kilograms

v is velocity in meters per second

So, you basically multiply mass and velocity and you get the result, or momentum. Got it so far? Ok, so let’s say your average semi-truck weighs 30,000 kgs. This is your truck’s dry weight or when it’s not laden down with cargo.

Now say it’s traveling the highway at a speed of 60 miles per hour. When converted to velocity, that means your truck is traveling at a speed of 26 meters. 

Now multiply 30,000 kgs to 26 m/s and you get an astounding value of 780,000 kgs moving at a fast clip. You need serious stopping power for that!

How Much Brake Force Do You Need?

Calculating the brake force needed, the external factors affecting the truck and its momentum is going to require an entirely new article devoted to just that. So to make it simple, you just need to effectively counter that momentum with the appropriate amount of force. 

Your deceleration rate, degree of inclination, the pull of gravity, momentum, driver ability, these are all taken into consideration. The air brakes are there as an added insurance to stop your truck from completely running over obstacles presented on the road. 

So, How Do Air Brakes Work?

Air brakes work in the same manner as an internal combustion engine’s piston does in its cylinder. When combustion takes place, the piston is pushed out of the cylinder helping turn the engine over before going back to its original position. This works in conjunction with other pistons aligned with it.

In the case of air brakes, compressed air takes the place of combustion. The same type of action occurs except it activates the brakes instead of turning the engine over. What turns over in its place is the slack adjuster.

A More Detailed Explanation Of How Air Brakes Work

  1. The driver hits the brakes through the brake pedal (treadle valve).
  2. Compressed air enters the brake chamber (cylinder) pushing the piston out.
  3. The piston is attached to the push rod and makes one revolution. This turns the slack adjuster.
  4. The push rod is attached to the S-cam which then pushes the brake pads that effectively stops the entire truck.

There are also two lines or systems involved during the operation of air brakes. 

Emergency Line
Emergency Line

The Supply Line (emergency line) 

The Supply line or emergency is the most important component within the entire air brake system. This is a large system with red fittings or is generally red in color. The emergency line is responsible for providing the air pressure to the reservoir tank and pushing the pistons out.

Service Line

The service line, on the other hand, is the more often used system. This is the smaller system within the entire brake system and is usually blue or has blue fittings to identify it. 

During normal operation, pressing the treadle valve (brake pedal) pressurizes the service line. This then activates the valve in the trailer connected to the air reservoir and emergency line. When the driver lifts his foot, the pressure decreases causing the valve to block any more air coming from the reservoir and the emergency line.

This is akin to an amplifier or a servo. That means: a slight amount of pressure from the foot on the brake pad is magnified by a thousand-fold to produce the desired braking result.

Emergency Measures For When The Emergency Line Is Compromised

As a security measure, if there is a drop in pressure within the emergency line, the spring brakes come on because there is no more air pressure to hold them back. This drop in pressure can be attributed to any of the following:

Valve activation

Disconnection of emergency line coupling

Emergency line breakage

Three Ways To Stop A Truck

There are three ways to stop a truck from moving. You can use the normal service brake system, you can use the parking brake system and last but not least, you can use the emergency brake system. All of these are part of the air brake system on a truck.

Service Brake System

The service brake system is what everyone is most familiar with. This is the brake system that you activate by just pressing on the foot pedal to bring your truck to a complete stop or at least decelerate if there are any obstacles in front of you. 

Parking Brake System

Parking Brake System
Parking Brake System

The parking brake system is the one you employ when you put the truck in park. This is engaged when the truck needs to be shut off for loading or unloading purposes. This system continues to operate with minimal power requirements. This totally locks the brake pads in place for ingress and egress purposes.

Emergency Brake System

The emergency brake system is the most important brake system as it uses both the service brake system and parking system to completely immobilize the truck in the rare event that the air brake system fails. This automatically kicks in if the system senses that there is anything wrong with the delivery of the compressed air through the hoses.

Most Recent Additions To Ensure Continued Air Brake Reliability

ABS Requirements

Trucks nowadays are also equipped with ABS to prevent the wheels from locking in place during hard braking or emergency situation. This prevents jack-knifing, excessive skidding and other brake-related issues while operating your truck on the public road.


Weather conditions usually prevent trucks from leaving the garage as too much cold can compromise the air brake system. This is added to the compressor to reduce the risk of compressed air lethargy which could seriously hamper your ability to stop.

High-Pressure Tensile Hoses

High-Pressure Hoses
High-Pressure Hoses

Air brake system failure can also originate from the standard hoses used in trucks. These can be replaced with high-performance, high-pressure tensile hoses. Aside from their inherent durability and resistance to cuts from external factors, they can also ensure a consistent supply of compressed air from the air compressor to the air brake chamber.

Final Considerations

Trucks are usually relegated to the slow lane when driving in busy areas. If you’re driving in front of one, always put into consideration the things you’ve learned here. Keep a safe distance between you and the truck behind you by at least three car lengths. 

Drive in an orderly manner and avoid unnecessary or abrupt braking when you’re in front of a truck. Better yet, get out of its lane. That truck outweighs you twenty to one and that’s not even putting into consideration its weight in motion. 

In the same manner, if you’re driving behind one, always put enough distance between you and the truck for events like sudden braking. Just heed the warning sign that says: “Keep Distance – Air Brakes” and everything will be fine.

Related Questions:

Can You Install Air Brakes On A Sedan?

It can be done but it would take too much space. Sedans are light vehicles that can effectively stop with conventional brake systems. It wouldn’t be practical to put air brakes on a small car unless the entire system can be miniaturized. Disc brakes are often enough to provide more stopping power.

What Causes A Truck To JackKnife?

A truck jackknifes when the trailer exceeds a 45-degree angle to the tractor. This usually happens when the tractor is empty and comes into contact with challenging road conditions like icy patches, oil slicks and/or mud. Hard braking when these road hazards are met usually results to jack-knifing.

How Long Does It Take To Completely Stop A Truck?

Truck In Slow Lane
Truck In Slow Lane

It takes 40% more time to stop a truck then it would a regular car. This is highly dependent on the total weight of the load carried, the amount of friction between the wheels and the road present road conditions and driver skill. We can’t stress enough how important driver skill is at this point.

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