drive-to-survive

No matter how long they have been driving or how “good” they think they are, emergency responders must still respect the limits to the safe operation of an emergency apparatus or a personal vehicle. At Thursday morning’s class, “Drive to Survive” Firefighter/EMT Chris Daly (Goshen Fire Department, West Chester, PA) addressed advanced topics that are not normally covered in basic driver training programs, especially those that relate directly to vehicle dynamics, crash causation, and common driver errors.

No matter how long they have been driving or how “good” they think they are, emergency responders must still respect the limits to the safe operation of an emergency apparatus or a personal vehicle. At Thursday morning’s class, “Drive to Survive” Firefighter/EMT Chris Daly (Goshen Fire Department, West Chester, PA) addressed advanced topics that are not normally covered in basic driver training programs, especially those that relate directly to vehicle dynamics, crash causation, and common driver errors.

One area covered was tires. Proper tire inflation and maintenance are essential for safety since the area of a tire that makes with the pavement is about the size of a hand. Underinflation and overinflation reduce that area of contact, and increase the possibility of a tire blowout. Daly also covered, among other items, roadway friction coefficients, rollover thresholds, critical curve speeds, and the proper handling of a tire blowout.
Chris Daly is a 20-year veteran of the fire service and a full-time police officer who specializes in the reconstruction of serious vehicle crashes. He lectures nationally on fire apparatus crash prevention and has presented the “Drive to Survive” seminar to more than 7,000 emergency responders nationwide. This is the sixth year he has presented the seminar at FDIC.

DRIVE TO SURVIVE

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BY CHRIS DALY

We all know the statistics: We kill fire- fighters about 25 times each year by crashing our fire trucks. I’m not going to dwell on this statistic. 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 reasons, and all are preventable.

Many fire departments require members to take some sort of Emergency Vehicle Operators Course (EVOC) before they can become drivers. These courses are great for new drivers 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 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 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.


(1) Make sure your drivers know which vehicles are equipped with an antilock brake system (ABS) and which aren’t. Large, noticeable signs on the dashboard may be warranted in a fire department with a mixed fleet. The standard ABS light may be too small for a driver to notice. (Photos by author unless otherwise noted.)

So, although EVOC classes are a great start, they touch only 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 understand, the physics behind a moving vehicle. Failing to teach emergency drivers the dynamics of a moving vehicle is like trying to teach an EMT class without teaching anatomy. The EMT student will know how to put on a bandage but will not understand how the bandage controls the bleeding. Drivers must understand that no matter how good a driver they think they are or how long they have been driving, there is still a point at which the vehicle will lose control.

STOPPING YOUR VEHICLE SAFELY

Think back to high school physics and the often heard term “kinetic energy.” Easier yet, forget the term kinetic energy and just think of a moving 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-in other words, dump all of 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. A key point drivers must understand is how to get rid of this energy.

Energy can neither be created nor destroyed-it has to go somewhere. The most common way to get rid of energy and bring your rig to a stop is to apply the brakes. 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. Energy is converted into heat by the brake pads rubbing against the brake discs or drums. As this energy is “bled off,” the truck slows down and eventually comes to a stop.


2) In this case, the majority of the vehicle’s energy was dissipated through the crushing of the metal as it struck a fixed object. Remember, the brakes are always the preferable method for bleeding off your kinetic energy. (Photo by Justin Dimedio.)

But, what happens when someone pulls out in front of you and you are forced to slam on the brakes and lock your tires? Now, the brakes are no longer converting the energy into heat because the wheels are locked and the brake pads aren’t rubbing against the discs or drums. Instead, energy is converted into heat by the friction of the locked tire sliding across the road surface. This friction creates the heat that “uses up” the energy and brings the vehicle to a stop.

The problem with sliding tires on the road surface is that we lose all steering control of our vehicle. To understand this properly, we have to understand what a skid mark really is. Asphalt is made up of gravel, sand, and tar. When your tires lock and your vehicle starts to skid, a tremendous amount of heat is 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 which direction you turn your wheels, you will skid in a straight line until you come to a stop or strike another object. Drivers must avoid skidding tires on the roadway so they can maintain steering control of the vehicle.

This is a perfect time to touch on the topic of antilock brakes. First, how many of us know which apparatus sitting in the station are equipped with antilock brakes? It is imperative that all driver training programs address which rigs have antilock brake systems (ABS) and which don’t. I would even recommend clearly marking this fact on the dashboard of the vehicle in easy view of the driver. The reason for this is that the braking methods used for an ABS vehicle are different from those used to bring a non-ABS vehicle to a stop.

Many people believe that ABS brakes will bring you to a stop in a shorter distance. This statement is up for debate; however, the real advantage to ABS is steering control. ABS brakes prevent the tires from locking and causing the rig to slide on a thin sheen of oil. The ABS allows a driver to brake hard and still maintain steering control, hopefully steering around whatever obstacle may be in the way. Drivers of ABS-equipped vehicles must understand the concept of STOMP, STAY, and STEER. In other words, don’t pump or let off the brakes when you are coming to an emergency stop. STOMP on the brake pedal, STAY on the brake pedal, and STEER around the hazard. This technique is different from the old method of “threshold braking” taught for non-ABS vehicles.1

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 again to restart the braking process. This method is difficult to master, especially for those who don’t drive heavy fire trucks on a regular basis.

Training is important. Take your rig out to an open parking lot and practice. Allow the drivers to take the vehicle up to 20 mph and slam on the brakes. Many drivers stomp on the brake pedal of an ABS vehicle and feel the pedal start to “thump” beneath their foot. This shouldn’t scare or startle the driver; this is a normal occurrence that indicates the system is working. Drivers need to experience this phenomenon under a controlled situation instead of during an emergency stop on a highway. For vehicles not equipped with ABS, drivers should be given the opportunity to practice “threshold braking.” Some engineering staffs may frown on the “abuse” this type of training puts on brakes and tires, but they should consider the alternative-a wrecked truck. It’s far better to train drivers on proper braking techniques and then send the rig out for maintenance than to risk a crash because of an untrained driver.

Another way to use up energy is crush. Many times, I’ll respond to a crash scene and someone will say to me, “They couldn’t have been going that fast; they left only 15 feet of skid marks. But the vehicle is totaled.” Why is this? Think back to the bucket theory. A vehicle starts to skid and the water in the bucket starts to bleed off. The problem is, before they could use up all of the energy, the truck struck another car. The bucket was still half full of energy when it hit the other car; this energy now has to go somewhere so the vehicle can stop. Where does it go? The metal crushes. Obviously, this is the worst way to use up energy and bring the rig to a stop.

BRAKE FADE AND OVERWEIGHT VEHICLES

Ideally, when you purchase a fire truck, the engineers who built it will specify the correct amount of braking force needed to stop the truck based on the size and weight of the truck. As long as the truck stays within the weight parameters set by the engineers and you maintain a safe speed while driving, the brakes on the truck will provide enough braking force to bring the truck to a safe stop.

However, problems tend to arise after the rig is delivered and we decide to add just a few too many tools or hoses than the vehicle was designed to carry. By exceeding the maximum gross vehicle weight rating or by driving the vehicle too fast, fire departments are tempting disaster.

Fire apparatus operators must be aware of “brake fade.” To understand brake fade, you must have a basic understanding of how air brakes work. Following is a brief summary on air brake operations.2

1To apply the brakes, press your foot down on the “foot valve”-the brake pedal.

2Air travels from an air tank reservoir through the air lines and into a brake chamber. Air presses against a rubber diaphragm in the chamber, which in turn pushes a plate and push rod. The push rod is pushed out, which pushes a slack adjuster, which turns a camshaft, which twists the S-Cam, which forces the brake linings to make contact with the drum.3

3The amount of force these brake chambers can create depends on the size of the chamber and the air pressure being applied. The size of this brake chamber depends on the size and weight of your truck, as well as on which axle it is located. They come in sizes that range from 9, 12, 16, 20, 30 and 36 square inches and work along the same principles as a lifting air bag. Applying 100 pounds of air pressure to a size-20 brake chamber results in 2,000 pounds of force on the push rod.

4The distance the push rod has to travel to properly apply the brakes is known as the “stroke.” Properly adjusted brakes have enough “stroke” so that when the brakes are applied, the brake shoes are spread apart and come in full contact with the brake drum. The friction of the brake shoes’ rubbing against the brake drum creates the heat which “uses up” the vehicle’s kinetic energy and brings it to a stop.

5The problem arises when there is too much energy that needs to be converted into heat so that the vehicle can come to a stop. The amount of energy created by a moving fire apparatus depends on how much it weighs and how fast it is going. If you are going too fast or if the rig weighs too much, there may be more energy than the brakes were designed to “bleed off.” This can result in brake fade.

6In a brake fade situation, the excess heat created by the brake shoes’ rubbing against the brake drum causes the metal brake drum to expand. Each time the brake drum expands, the push rod has to travel farther so that the brake shoes can contact the drum. Eventually, the brake drum may expand to a point greater than the push rod can travel-in other words, the drum gets too big, and the brake shoes aren’t able to come in contact with it properly. This results in a loss of braking efficiency and possibly the complete loss of braking ability.

To avoid brake fade situations, first ensure that your vehicles are not overweight. When was the last time you had your vehicle properly weighed at a certified weigh station? Many departments assume that after all of the equipment is mounted and the hoses are properly packed, the vehicle is within safe operating parameters. I strongly recommend finding the nearest weigh station, a state or locally run station or a local trucking terminal or quarry. Ask the operators to weigh your rigs and make sure that they meet manufacturer’s specifications as well as pertinent state laws. Ensuring that your vehicles are not overweight is the first step toward helping to avoid a brake fade situation.

Next, make sure your brakes are properly adjusted. As brakes wear down and the size of the brake shoes decreases, the push rod requires more travel distance for the brakes to apply properly. It is imperative to ensure that there is enough reserve stroke so that the brake shoes are able to properly engage the brake drum and create the friction necessary to bring the vehicle to a stop. Even if your apparatus is new and is equipped with automatic slack adjusters, it is still a good idea to check the brake adjustment on a regular basis, just to be safe.

If one brake is not properly adjusted, it means that it isn’t doing its share of work to convert energy into heat. Instead, the other properly adjusted brakes must make up the difference and do more work. This can cause the working brakes to become overheated as they struggle to bleed off the excess energy that should be bled off by the maladjusted brake. It is akin to having to work extra hard when a slacker is on your shift.

This scenario is especially important in fire departments that cover territory that requires driving on a lot of hilly terrain. Traveling downhill for long distances leads to brake fade. That is the reason you often see runaway truck ramps on long stretches of downhill roads. If you don’t have anyone qualified in-house to check the brake adjustment, work out an agreement with a local garage or even your local police department. Many police departments have certified truck inspectors who would be more than glad to help.

The most important way to avoid brake fade is to ensure that your operators understand how excess speed can contribute to brake fade. As noted before, the faster a vehicle is traveling, the more energy it has to bleed off to come to a stop. If a vehicle is going too fast, there may be more energy to bleed off than the brakes are designed to handle. By maintaining a safe speed, drivers can help prevent a brake-fade situation.

We should also design our trucks to help prevent brake fade. Secondary braking devices, such as engine brakes, driveline retarders, or transmission retarders can absorb up to 80 percent of a vehicle’s energy. These secondary braking devices allow drivers to slow their vehicles without using the service brakes. This allows the brakes to stay cool for when they are needed most. Each type of secondary braking system has its pros and cons, but it is important that drivers be familiar with their operation and understand how they work.4 Air-cooled disc brakes are also important considerations when designing fire apparatus. Disc brakes resist brake fade and allow for better braking efficiency.

Brakes are one of the most important tools for vehicle safety. Drivers should understand how brakes work and the conditions that can lead to their failure. Fire departments should schedule regular maintenance checks of the brakes to ensure that they are properly adjusted and in good working order. I encourage apparatus operators to visit the Web site www.drivetosurvive.org and check out the “Brakes” page. There are other good Web sites that provide a good overview of brake operations and safety.

TOTAL STOPPING DISTANCE

A common question I am often asked is, “How many feet does it take to stop if I am going X miles an hour?” The answer is, “It depends.” The question that should be asked is, “What is the total stopping distance of my vehicle at ‘X’ miles an hour?” Total stopping distance is a very important concept, and fire apparatus operators must understand it.

Total stopping distance is the distance that takes into account all the steps that bring a speeding fire truck to a complete stop. Drivers must understand that this distance is more than just the distance necessary to slow down the vehicle and come to a stop. Total stopping distance also includes the time it takes you to see a hazard in the roadway, process this hazard in your brain, and then send the signals to your arms and legs that allow you to press the brake pedal and start the braking process. This is known as perception and reaction distance.

Coefficient of Friction


Before discussing the distance needed to bring a speeding fire truck to a stop, we must first discuss the roadway on which the driver is operating. Every roadway has a certain “stickiness,”-in technical terms, “coefficient of friction.” This coefficient is a value measured with specialized equipment that gives crash reconstructionists an idea of how sticky the road is. A dry, asphalt road that was recently paved can have a coefficient of friction as high as 0.9. A wet, worn-down road can have a coefficient of friction of 0.4 or less. This value is extremely important in determining how long it will take a driver to come to a complete stop while traveling at a particular speed. It takes a lot longer to stop on a wet or icy road than a dry road (Table 1).

Feet per Second

In addition to recognizing how road conditions affect stopping distances, apparatus operators should understand speed in terms of “feet per second” instead of “miles per hour.” 60 mph is equal to about 88 feet per second, so in one second your vehicle will traverse 88 feet. Stop and think about that. It takes the average person around 1.5 seconds to see, process, and react to a hazard. That means that at 60 mph, it takes you 132 feet just to realize there is a problem ahead and start pushing your foot down on the brake pedal. Once you start pressing your foot down on the brake pedal, you are initiating the braking process. Most drivers aren’t skilled or experienced enough to master the art of “threshold braking”; instead, most drivers tend to lock their wheels and start skidding (unless you have ABS brakes). Now, the vehicle is skidding, the driver has no steering control, and it takes 263 feet to skid to a stop on a typical dry asphalt roadway with a coefficient of friction of 0.7. Add this skid distance to the reaction distance, and you see that it takes approximately 395 feet to stop a fire truck while traveling 60 mph on dry roads. Imagine that! It’s more than an entire football field. On a wet day with a coefficient of friction of 0.4, this distance can be as much as 500 feet. Remember this fact when you are approaching a stale green light at a busy intersection. It is important to slow down and cover the brake, thus reducing your reaction time and total stopping distance should someone suddenly pull out in front of you. You must also remember to allow extra stopping distance when traveling on wet or icy roads.


This total stopping distance was calculated using a proven crash reconstruction formula called the “skid-to-stop formula.” This formula uses variables for speed, distance, coefficient of friction, and braking efficiency. Nowhere in the formula do we multiply for years of experience or how good you think you are. In other words, if you’ve been driving for 40 years and teach advanced EVOC, you’ll still need 395 feet to stop your truck, the same as for the brand new driver in the pumper behind you.

Braking Efficiency

Fire apparatus have a much lower braking efficiency than a standard automobile because of two major differences in their construction. The first is that most fire apparatus are equipped with air brakes, which take longer to activate than standard hydraulic brakes. When you press your foot on the brake pedal in your personal vehicle, the hydraulic system applies the brakes immediately. In a fire apparatus equipped with air brakes, a “lag time” is associated with the air brake system. This lag time is caused by the travel time necessary for the air to flow through the air lines into the brake chambers and in turn activate the brakes. This lag time can be as much as 12 second to one second on most air brake-equipped vehicles. That means that if you are traveling 60 miles an hour, you will have traveled approximately 88 feet before your brakes even begin to slow you down.


Once your brakes have activated, a second factor affects the stopping distance of a fire apparatus. This difference is a result of the composition of the tires, which are more “slippery” on the road surface than a standard automobile tire. The reason for this is that truck tires are designed for weight and wear. Truck tires are better able to handle heavy loads and travel longer distances before wearing out. In exchange for this increased durability, traction and braking ability are sacrificed. So what, you ask? Pretend you are traveling behind a small car at 60 mph. A child runs in front of the small car; the driver slams on the brakes and skids to a stop. It will take the car 171 feet to skid to a stop on a roadway with a coefficient of friction of 0.7. It will take your fire truck 342 feet to skid to a stop on the same roadway. What happens when the small car stops at the 171 foot mark and your fire truck is still skidding for another 171 feet? Your truck will slide into the back of the small car with a tremendous amount of energy and seriously injure the people inside. For this reason, apparatus drivers must remember to leave plenty of room between their truck and the vehicle in front of them.

Training Is a Must


(3) Drivers should adjust the steering wheel so they can see the speedometer.

We often drill on pulling hoselines, throwing ladders, and rappelling from buildings, but how often do we really get to do these things? On the other hand, even the most “routine” automatic alarm requires us to drive to the scene. But, when was the last time you drilled with your drivers? To emphasize the concepts we’ve discussed in this article, I would recommend the following drill. Purchase a measuring wheel, or borrow one from the trunk of the next police officer who stops at the station for lunch. Go out to a busy intersection you routinely drive through and measure a distance 350 feet from the intersection. As you stand 350 feet away from the intersection, look around. How well can you see the traffic on the cross street in front of you? Is there a gas station blocking your view of cross traffic? If so, how far away from the intersection must you be to have a clear view of the cross street in both directions? Does this distance allow you enough perception, reaction, and stopping distance to approach a stale green light at 50 mph? These are the types of drills that will help your drivers to understand the importance of slowing down, maintaining control, and operating their fire apparatus in a safe manner.

CURVES AND CRITICAL SPEEDS


(4) Lag time is the time it takes for air to travel through the air lines into the brake chambers and to activate the pushrods and brake pads. This leads to increased stopping distances in vehicles equipped with air brakes. (Note: The brake chamber is located in the rear portion of the photo.

I’m sure everyone has had this sort of experience: You are riding shotgun on the way to a call and you’ve thrown your helmet up onto the dashboard so that you can read the map book and find a hydrant before you get there. As the driver rounds a particular curve in your district, your helmet starts to slide across the dashboard and nearly goes out the window before you are able to grab it. That was close; you almost missed a job because you had no helmet to wear! What sneaky culprit almost stole your helmet? Inertia-and it’s more dangerous than you think.


I’m sure everyone has that particular driver in the station who thinks he can drive better than most Indy race car drivers. The firefighters in the back seat end up grunting and groaning like fighter pilots as they try to breathe during the high “G” turns. The problem is that a fire truck is not an F-16. Fire trucks typically have high centers of gravity and heavy water loads, which make them relatively unstable during tight cornering situations.

What most drivers don’t realize is that every curve in the road has what’s called a “critical speed.” If you take the curve faster than this critical speed, your vehicle will continue in a straight line instead of negotiating the curve. As a result, the vehicle may travel off the road and strike whatever happens to be on the side of the road. It doesn’t matter how long you have been driving or how good you think you are; if you travel faster than the critical speed, you will lose control of your vehicle.


(5) Heavy truck tires are designed to carry weight and increase durability. For this reason, they have lower coefficients of friction (a.k.a., they are “more slippery” than standard automobile tires). This leads to increased stopping distances and decreased braking efficiency.

Two main forces are acting on a vehicle when it rounds a curve. The fancy words are “centrifugal” and “centripetal” forces. Centripetal force is the stickiness of your tires to the road surface. This force allows you to maintain traction and safely negotiate the curve without driving off the road. Centrifugal force is the opposing force trying to keep your vehicle in a straight line instead of safely rounding the curve. As long as these forces are equal and the centrifugal force isn’t more than the centripetal force, your tires maintain traction and you safely round the curve.


6) Drivers must slow down well in advance of an approaching curve. Waiting until the last minute to try to bring the vehicle down to a safe speed can lead to disaster.

Problems arise as your speed or the sharpness of the curve increases. These two factors increase the centrifugal force acting on your vehicle. Eventually, if you are going too fast or the curve is too sharp, the centrifugal force of the curve will overwhelm the centripetal force keeping your tires on the road. The rear end of your vehicle will begin to break traction and rotate around the vehicle’s center of mass. This is known as a “yaw.” As your vehicle is rotating around its center of mass, it is still traveling in a straight line and not following the track of the curve. If the driver is unable to recover control of the vehicle, it will continue on a straight line and drive off the side of the road. Think of trying to make a curve on a snow-covered parking lot. If you go too fast, your car will “fishtail” or you will lose control. The same thing happens on dry roads at higher speeds.


(7) Drivers cannot trust posted advisory speeds for curves. Unstable vehicles with high centers of gravity, such as fire apparatus, may lose control even when traveling at the posted speed. Slow down.

Now, let’s go back to the example of your fire helmet. As the fire truck rounds the curve, the traction of the tires on the road surface is still equal to the centrifugal forces trying to keep the vehicle in a straight line. In this case, the fire truck maintains traction with the road and safely negotiates the curve. However, your plastic helmet is resting on a freshly polished, vinyl dashboard. The centripetal force (a.k.a stickiness) keeping the helmet from sliding around is considerably less than the stickiness between the tires and the road. In this case, the centrifugal force is more than the centripetal force acting on your helmet. As a result, your helmet breaks traction with the dashboard and tries to keep traveling in a straight line. In other words, your helmet starts to slide out the window.


(8) Curves kill firefighters. Slow down, especially during wet or inclement weather. The critical speed of a curve can decrease drastically during wet weather conditions, catching drivers by surprise.

To figure out the critical speed of a curve, you need only two things, the radius (or sharpness) of the curve and the coefficient of friction of the roadway. By plugging these two values into a formula, you can calculate the critical speed of a curve. This also means that as the curve gets sharper or the road more slippery, the critical speed goes down. In other words, if it’s raining, you can’t drive through the curve at the same speed as if it were dry. You also can’t round the curve at the same speed at which you are approaching it. You must SLOW DOWN well in advance and adjust your driving according to weather conditions.

Consider a curve with a 150-foot radius. This is a pretty common curve for most of our districts. On a dry day, with a coefficient of friction of 0.9, the critical speed for the curve is 44 mph. Drive faster than 44 mph, and the truck will fail to stay in the travel lane and safely negotiate the curve. Instead, the vehicle will continue to travel in a straight line and strike whatever happens to be along the roadside. Now let’s say it’s raining and the coefficient of friction for this same curve is 0.4; this “more slippery” road condition will lower the critical speed to 29 mph. As we discussed before, it doesn’t matter how long you have been driving or how good you are, if you exceed 29 mph, your vehicle will start to slide off the road.

To understand the concept of a “critical speed,” it is important to understand what is happening to your vehicle as you begin to round a left-hand curve. Centrifugal force begins to push the weight of your vehicle onto the outside tires, especially the front, passenger side tire. This weight shift causes the body of the vehicle to deflect down onto the suspension (shocks and springs) and shifts the center of mass toward the outside of the curve. In most cases, especially when dealing with a standard automobile, this weight transfer has little effect on the vehicle, the curve is safely negotiated, and you continue on your way. Sometimes, if you are going too fast for the curve, the weight of the vehicle will shift onto the outside tires, and you will start to hear your tires squeal. Again, most drivers let off the gas and continue on their way. But what about when you are driving a vehicle with a high center of gravity, namely a fire truck?


This sketch demonstrates how the center of gravity shifts as a vehicle enters a curve. Centrifugal force causes the body of the vehicle to deflect down on the springs and suspension. This deflection results in the center of gravity shifting toward the outsides. If the center of gravity shifts too far, a rollover or loss of control can result.

When you are driving a fire truck through a curve, critical speed is not the only thing you have to worry about. A vehicle with a high center of gravity is less stable during cornering situations because the center of mass shifts as the suspension deflects down and out in the direction of the centrifugal force. If the center of mass shifts too far, the vehicle may tip over. Sound familiar? Or, the vehicle might not tip over completely but may drift off the road just enough that the driver is forced to recover and get the wheels back on the road. This is another common cause of fire apparatus crashes as the driver overcorrects and causes the fire truck to roll over in the opposite direction. This is known as an “induced yaw.” In other words, by turning the wheels too hard to correct the vehicle’s path of travel, the vehicle is put in an artificial cornering situation for which the vehicle is going too fast. As a result, the vehicle “trips” as it rotates around its center of mass and ends up rolling over.

Another factor that must be considered when rounding a curve is that most fire trucks carry several hundred gallons of water. Now, not only do you have to worry about a high center of gravity and the weight of the vehicle shifting to the outside tires, but you have to worry about the water “surge” in your tank. This surge of energy will also add to the forces trying to tip you over or at the very least cause you to drive off the road. This is one of the main reasons water tankers (tenders for the West Coast readers) are the most common type of apparatus involved in serious crashes.

We’ve talked about some big words in this article, namely “centrifugal” and “centripetal” forces. It’s not important to remember these words at 2 in the morning when you are driving to a working fire. What is important is that you remember that Mother Nature is trying to force you off the road as you drive to the call. Apparatus operators must be thoroughly familiar with their districts and remember to slow down well in advance of curves in the road. Apparatus operators must also remember that rain, sleet, or other slippery conditions will lower the critical speed of a curve. That means that you can’t drive the same way on a rainy day as you would on a sunny day. Only by slowing down and respecting the forces of nature can the number of fire apparatus crashes be reduced.

• • •

This article discusses some of the most common causes of apparatus crashes. We must also remember that many firefighter fatalities are the result of not wearing seat belts. There is no excuse for this; if nothing else, it’s the law! Each member, each driver, and each officer should be held personally responsible for ensuring that everyone on the rig is wearing their seat belt. You wouldn’t enter a burning building without an SCBA, so why would you drive to an automatic alarm without a seat belt?

Although it’s true that we kill an average of 25 firefighters each year in apparatus crashes, how many other crashes do we never hear about? Countless civilians and innocent motorists have been killed by fire apparatus traveling at unsafe speeds or under unsafe conditions. If nothing else, remember these six words that may save your life: SLOW DOWN, BUCKLE UP, AND THINK!

Endnotes

1. See “Antilock Braking Systems Revisited,” William C. Peters, Fire Engineering, April 1997.

2. See “Air Brakes and the Driver-Operator.” Terry Eckert, Fire Engineering, March 1998.

3. http://www.newbiedriver.com/ABCsUpdates/AirBrakes101.htm

4. See “Secondary Braking Systems: A Second Chance,” William C. Peters, Fire Engineering, September 1995.

I would like to thank the following people for their assistance with this article: Officer Justin Dimedio, West Goshen (PA) Police Department; Corporal Greg Sullenberger, Pennsylvania State Police; Sergeant Rick Daly, Greenwich (CT) Police Department; and Gerald Dinunzio Sr., chief engineer, Goshen Fire Department.