Introduction to Braking Energy

We all know the statistics-we kill ourselves about 15 to 20 times each year by crashing our fire trucks. I’m not going to dwell on this statistic. I think it repeats itself every year like a broken record. The ultimate question is “Why”? What causes us to repeatedly drive off the road, flip our rigs, and eject ourselves into oblivion? There are many different reasons, and all of these reasons are preventable. The purpose of this series of articles is to address these issues and try to explain to drivers, in layman’s terms, why fire trucks crash.

Many fire departments require members to take some sort of emergency vehicle operator’s course (EVOC) before they can become drivers. These courses are great for new drivers who are trying to learn how to maneuver a rig in tight quarters. Now I don’t know about your department, but my department doesn’t respond to too many calls in a parking lot that require us to drive through a bunch of traffic cones. Our calls require emergency responses on real roads with real traffic along with lights, sirens, and radios blaring in the background. These factors combine to increase the driver’s excitement and often result in a deadening of the senses. Tunnel vision ensues, and the only thing that the apparatus operator can think of is getting to the incident scene fast. Throw in a mutual-aid company coming from another direction, and we all know what happens-the race is on.

So as you can see, while EVOC classes are a great start, they only touch the tip of the iceberg. Often, these basic classes fail to touch on one of the most important topics that an emergency apparatus operator must come to understand: the physics behind a moving vehicle. I liken this to trying to teach an emergency medical technician (EMT) class without teaching anatomy. The EMT student knows how to put on a bandage but doesn’t understand why. The goal of these articles is to make apparatus operators understand that no matter how good they think they are or how long they have been driving, at some point Mother Nature will take over and the vehicle will lose control.

Dynamics

Let’s start discussing the dynamics behind a moving fire truck. These concepts are based on the same concepts used by crash reconstructionists-the police officers who shut the road down for hours at a time trying to figure out why a motor vehicle crash occurred. The reason I say this is so that you don’t put the magazine down and walk away. Remember-if the cops can understand this, so can we!

The first concept an apparatus operator needs to understand is the concept of energy. Think back to high school physics and the often-heard term “kinetic energy.” Now before your eyes roll back in your head, forget the term kinetic energy and just think of a bucket full of water. The bucket is the fire truck and the water is the energy. The bigger the bucket (i.e., the bigger the fire truck), the more water it will hold (i.e., the more energy it has). Also, the faster the bucket is traveling (i.e., the faster you drive down the street), the more water (energy) it will hold.

So to bring your fire truck to a stop, you have to get rid of all the energy-or, in other words, dump all the water out of the bucket. If you want to just slow down, you have to get rid of some of the energy-or just dump some of the water out of the bucket. The question is: How do we get rid of this energy?

The most common way we get rid of energy and bring our rig to a stop is to simply apply the brakes. If you remember from high school physics, energy can be neither created or destroyed; it has to go somewhere. Vehicle brakes take the kinetic energy of a moving vehicle and convert it to heat. This heat energy is then dissipated into the atmosphere and the vehicle comes to a stop. The conversion of energy into heat is accomplished by rubbing the brake pads against the brake discs or drums. As this energy is “bled off,” the truck slows down and eventually comes to a stop. This is the ideal scenario and does not involve an emergency panic stop or vehicle-to-vehicle contact-i.e., a crash.

What happens when someone pulls out in front of you and you are forced to slam on the brakes and lock your tires up? When this happens, the brakes are no longer converting the energy into heat because the wheels are locked up and the brake pads aren’t rubbing against the discs or drums. The conversion of energy into heat is now accomplished by the friction of the locked tire sliding across the road surface. The friction between the tire and the road surface creates the heat, which “uses up” the energy and brings the vehicle to a stop.

The problem with skidding your tires on the road surface is that we lose all steering control of the vehicle. To understand this properly, we have to understand what a skid mark really is. While there are a lot of different theories on what makes a skid mark, one of the more common theories is that asphalt is made up of gravel, sand, and tar. When you lock your tires and your vehicle starts to skid, there is a tremendous amount of heat generated between the tire and the road surface. This heat liquefies the oils in the tar and causes them to float up to the surface. Essentially, your vehicle is sliding on a thin sheen of oil and you are now at the mercy of Mother Nature. No matter what direction you turn your wheels, the vehicle will skid in a straight line until you come to a stop or strike another object. As professional drivers, we must avoid skidding tires on the roadway so we are able to maintain steering control of the vehicle. For those wondering about antilock brake systems (ABS), we’ll discuss that later.

For non-ABS vehicles, drivers must recognize when the vehicle is about to enter a skid. Under this situation, the driver must “let off” on the brake pedal to prevent the vehicle from skidding and then reapply the brakes to restart the braking process. Many of us have heard this technique referred to as “threshold braking.” This method is difficult to master-especially for those of us who don’t drive heavy fire trucks on a regular basis-hence, the invention of antilock brakes.

Now let’s say that a vehicle pulls out in front of you so quickly that you do not have time to burn off your energy with the brakes or by skidding the tires. In this scenario, the vehicle’s energy will be burned off with “crush.”

Often I’ll respond to a crash scene and someone will say to me, “They couldn’t have been going that fast. They only left 15 feet of skid marks, but their vehicle is totaled.” Why is this? Think back to the bucket theory: Someone starts to skid the vehicle and the water in the bucket starts to “spill out.” The problem is that before he could use up all of the energy and dump all the water out of the bucket, he struck another car. The vehicle was still partially full of energy when it hit the other car and now this energy has to go somewhere so the vehicle can stop. Where does it go? The metal crushes. This is how a crash investigator can go to a junkyard the day after a crash, measure how badly the vehicle has been crushed, figure out how much energy it would take to crush the vehicle that much, and figure out a speed. It’s called “speed from crush.” Obviously, this is the worst way to use up your energy and bring the rig to a stop. I strongly recommend sticking with the brakes.

Apparatus operators must recognize the energy that is associated with driving a 40,000-pound fire truck at 50 miles per hour. Apparatus operators must know the ways to bleed off this energy to bring the rig to a complete stop in a safe and efficient manner. All drivers must realize that the faster you are going, the more time and distance it will take to bring your vehicle to a safe stop. In addition, we must be aware of overweight vehicles that create more kinetic energy than the brakes are designed to dissipate. Overweight vehicles create brake fade situations, which can lead to increased stopping distances or even complete brake failure. Speed and overweight vehicles can kill you. We’ll discuss this in the next article.

CHRIS DALY is a 25-year veteran of the fire service and a full-time police officer who specializes in the reconstruction of serious vehicle crashes and emergency vehicle crashes. He developed the “Drive to Survive” training program (www.drivetosurvive.org), which he has presented to more than 14,000 emergency responders across the country, and lectures nationally on preventing emergency vehicle crashes. Daly has a master’s degree in safety from Johns Hopkins University, is a contributer to Fire Engineering, and has presented at FDIC International for the past 10 years.

Introduction to Braking Energy

We all know the statistics-we kill ourselves about 15 to 20 times each year by crashing our fire trucks. I’m not going to dwell on this statistic. I think it repeats itself every year like a broken record. The ultimate question is “Why”? What causes us to repeatedly drive off the road, flip our rigs, and eject ourselves into oblivion? There are many different reasons, and all of these reasons are preventable. The purpose of this series of articles is to address these issues and try to explain to drivers, in layman’s terms, why fire trucks crash.

Many fire departments require members to take some sort of emergency vehicle operator’s course (EVOC) before they can become drivers. These courses are great for new drivers who are trying to learn how to maneuver a rig in tight quarters. Now I don’t know about your department, but my department doesn’t respond to too many calls in a parking lot that require us to drive through a bunch of traffic cones. Our calls require emergency responses on real roads with real traffic along with lights, sirens, and radios blaring in the background. These factors combine to increase the driver’s excitement and often result in a deadening of the senses. Tunnel vision ensues, and the only thing that the apparatus operator can think of is getting to the incident scene fast. Throw in a mutual-aid company coming from another direction, and we all know what happens-the race is on.

So as you can see, while EVOC classes are a great start, they only touch the tip of the iceberg. Often, these basic classes fail to touch on one of the most important topics that an emergency apparatus operator must come to understand: the physics behind a moving vehicle. I liken this to trying to teach an emergency medical technician (EMT) class without teaching anatomy. The EMT student knows how to put on a bandage but doesn’t understand why. The goal of these articles is to make apparatus operators understand that no matter how good they think they are or how long they have been driving, at some point Mother Nature will take over and the vehicle will lose control.

Dynamics

Let’s start discussing the dynamics behind a moving fire truck. These concepts are based on the same concepts used by crash reconstructionists-the police officers who shut the road down for hours at a time trying to figure out why a motor vehicle crash occurred. The reason I say this is so that you don’t put the magazine down and walk away. Remember-if the cops can understand this, so can we!

The first concept an apparatus operator needs to understand is the concept of energy. Think back to high school physics and the often-heard term “kinetic energy.” Now before your eyes roll back in your head, forget the term kinetic energy and just think of a bucket full of water. The bucket is the fire truck and the water is the energy. The bigger the bucket (i.e., the bigger the fire truck), the more water it will hold (i.e., the more energy it has). Also, the faster the bucket is traveling (i.e., the faster you drive down the street), the more water (energy) it will hold.

So to bring your fire truck to a stop, you have to get rid of all the energy-or, in other words, dump all the water out of the bucket. If you want to just slow down, you have to get rid of some of the energy-or just dump some of the water out of the bucket. The question is: How do we get rid of this energy?

The most common way we get rid of energy and bring our rig to a stop is to simply apply the brakes. If you remember from high school physics, energy can be neither created or destroyed; it has to go somewhere. Vehicle brakes take the kinetic energy of a moving vehicle and convert it to heat. This heat energy is then dissipated into the atmosphere and the vehicle comes to a stop. The conversion of energy into heat is accomplished by rubbing the brake pads against the brake discs or drums. As this energy is “bled off,” the truck slows down and eventually comes to a stop. This is the ideal scenario and does not involve an emergency panic stop or vehicle-to-vehicle contact-i.e., a crash.

What happens when someone pulls out in front of you and you are forced to slam on the brakes and lock your tires up? When this happens, the brakes are no longer converting the energy into heat because the wheels are locked up and the brake pads aren’t rubbing against the discs or drums. The conversion of energy into heat is now accomplished by the friction of the locked tire sliding across the road surface. The friction between the tire and the road surface creates the heat, which “uses up” the energy and brings the vehicle to a stop.

The problem with skidding your tires on the road surface is that we lose all steering control of the vehicle. To understand this properly, we have to understand what a skid mark really is. While there are a lot of different theories on what makes a skid mark, one of the more common theories is that asphalt is made up of gravel, sand, and tar. When you lock your tires and your vehicle starts to skid, there is a tremendous amount of heat generated between the tire and the road surface. This heat liquefies the oils in the tar and causes them to float up to the surface. Essentially, your vehicle is sliding on a thin sheen of oil and you are now at the mercy of Mother Nature. No matter what direction you turn your wheels, the vehicle will skid in a straight line until you come to a stop or strike another object. As professional drivers, we must avoid skidding tires on the roadway so we are able to maintain steering control of the vehicle. For those wondering about antilock brake systems (ABS), we’ll discuss that later.

For non-ABS vehicles, drivers must recognize when the vehicle is about to enter a skid. Under this situation, the driver must “let off” on the brake pedal to prevent the vehicle from skidding and then reapply the brakes to restart the braking process. Many of us have heard this technique referred to as “threshold braking.” This method is difficult to master-especially for those of us who don’t drive heavy fire trucks on a regular basis-hence, the invention of antilock brakes.

Now let’s say that a vehicle pulls out in front of you so quickly that you do not have time to burn off your energy with the brakes or by skidding the tires. In this scenario, the vehicle’s energy will be burned off with “crush.”

Often I’ll respond to a crash scene and someone will say to me, “They couldn’t have been going that fast. They only left 15 feet of skid marks, but their vehicle is totaled.” Why is this? Think back to the bucket theory: Someone starts to skid the vehicle and the water in the bucket starts to “spill out.” The problem is that before he could use up all of the energy and dump all the water out of the bucket, he struck another car. The vehicle was still partially full of energy when it hit the other car and now this energy has to go somewhere so the vehicle can stop. Where does it go? The metal crushes. This is how a crash investigator can go to a junkyard the day after a crash, measure how badly the vehicle has been crushed, figure out how much energy it would take to crush the vehicle that much, and figure out a speed. It’s called “speed from crush.” Obviously, this is the worst way to use up your energy and bring the rig to a stop. I strongly recommend sticking with the brakes.

Apparatus operators must recognize the energy that is associated with driving a 40,000-pound fire truck at 50 miles per hour. Apparatus operators must know the ways to bleed off this energy to bring the rig to a complete stop in a safe and efficient manner. All drivers must realize that the faster you are going, the more time and distance it will take to bring your vehicle to a safe stop. In addition, we must be aware of overweight vehicles that create more kinetic energy than the brakes are designed to dissipate. Overweight vehicles create brake fade situations, which can lead to increased stopping distances or even complete brake failure. Speed and overweight vehicles can kill you. We’ll discuss this in the next article.

CHRIS DALY is a 25-year veteran of the fire service and a full-time police officer who specializes in the reconstruction of serious vehicle crashes and emergency vehicle crashes. He developed the “Drive to Survive” training program (www.drivetosurvive.org), which he has presented to more than 14,000 emergency responders across the country, and lectures nationally on preventing emergency vehicle crashes. Daly has a master’s degree in safety from Johns Hopkins University, is a contributer to Fire Engineering, and has presented at FDIC International for the past 10 years.