Editor’s Note: "Drive to Survive" is 9-part series that will be published in SIZE UP magazine and The NYSAFC Bugle. Click to read the previous stories in this series: "Drive to Survive: An Introduction," "Energy," "Anti-Lock Brake Systems," "Stopping Distances," "Stopping Distances (Continued)," "Curves," and "Hydroplaning."
The dangers of overweight vehicles are often overlooked in the fire service. But in recent years there have been a number of crash-related fatalities that were the direct result of an overweight vehicle. Ensuring that your fire apparatus do not exceed safe weight ratings is a crucial part of any effective safety program.
When a manufacturer designs a vehicle, they will take great strides to determine how much equipment, water, hose, and manpower you intend to carry. Based on this information, the manufacturer will specify the correct axles, suspension, brake chambers, and tires that will be needed to safely operate the vehicle. Unfortunately, firefighters are notorious for adding additional equipment once the vehicle arrives at the station. And while we have the best intentions at heart, this added equipment can create a dangerous situation.
Vehicles that are traveling down the road have kinetic energy. The amount of energy is directly related to how fast the vehicle is traveling and how much it weighs. In order to slow it down or bring it to a complete stop, this energy must be burned off.
The safest and most common way to dissipate kinetic energy is to convert it to heat energy, which is most often accomplished through the use of the braking system. The friction of the brake pad rubbing on the brake disc or drum creates heat and burns off the vehicle’s kinetic energy.
However, if the vehicle is going too fast or weighs more than it is supposed to, there may be more kinetic energy than the brakes were designed to handle. As the brake pad rubs against the disc or drum attempting to burn off the excess kinetic energy, it will overheat. If the brakes overheat too much, the brake pad may begin to disintegrate or the brake drum may begin to expand. When this occurs, the braking efficiency of the vehicle is reduced, resulting in longer stopping distances and perhaps even complete brake failure. This situation is known as brake fade.
In a vehicle that is equipped with air-operated drum brakes, an application of the brake pedal will send air traveling through the brake lines. When this air reaches the brake chambers, it will press against a rubber diaphragm. In turn, this diaphragm will push against a plate and a push rod. As this push rod pushes out from the brake chamber, it will spread the brake shoes and force them to come in contact with the brake drum. The end result is the creation of friction, which converts the vehicle’s kinetic energy to heat, slowing the vehicle down.
The amount of braking force available is directly related to the distance that the push rod can travel, otherwise known as "stroke." If the brakes are properly adjusted, the brake shoes are able to spread far enough apart so that they come in full contact with the brake drum. However, if there is too much kinetic energy to burn off, the brake drum may overheat and begin to expand. If the drum expands too far, the push rod will travel as far as it can and run out of stroke, yet the brake shoes are still not in full contact with the now oversized drum. When this occurs, there is significantly reduced braking efficiency and maybe even complete brake failure.
In a vehicle with disc brakes, brake fade can occur for other reasons. If a disc brake begins to overheat due to excess weight or speed, the binding agents that keep the brake pad together may begin to disintegrate. Obviously, if your brake pads begin to disintegrate, you will lose your braking efficiency and possibly all of your braking power.
Overweight vehicles can lead to brake failure and a subsequent crash. For this reason, making sure that your vehicles are not overweight is an essential part of any safe vehicle program. In fact, the NFPA 1911 standard REQUIRES that fire apparatus be weighed once a year. While this may seem like a difficult process, it is actually easier than you think.
Most districts have some type of scrap or junkyard that has a certified vehicle scale. Also, many local and state police departments have portable scales that they would be more than willing to bring to your station. Just remember, not only must you weigh your vehicles, but also make sure that you DOCUMENT the results. If it’s not documented, it never happened!
Also remember to not only weigh the entire vehicle, but also each axle separately. Once you have weighed each axle, make sure that it is within the weight ratings provided by your manufacturer. When it doubt, call your sales or service representative and ask for assistance.
Not only does an overweight vehicle lead to brake fade, but also it can lead to a tire blowout. This will be the topic of the final part of this series.
About the Author:
Chris Daly is a 24-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 and lectures nationally on the prevention of emergency vehicle crashes. Daly has been a contributing author for numerous national fire service publications. He has a master’s degree in safety from Johns Hopkins University. Click here to email Chris Daly for more information.