Drive to Survive: Energy

By Chris Daly

Editor's Note: Click here to read "Drive to Survive: An Introduction," which appeared in NYSAFC’s SIZE UP magazine Issue 2 • 2015. Look for more articles in this series in future issues of SIZE UP and The NYSAFC Bugle.

It is extremely important that a driver understand the energy associated with a moving vehicle. Any vehicle that is moving down the road has "kinetic energy." Don’t be intimidated by the big words. Kinetic energy is simply a function of how fast the vehicle is traveling and how large it is. Take a 30,000-pound fire truck traveling at 50 mph and you can imagine the amount of energy that is associated with the vehicle as it is weaving through rush hour traffic. It is this energy that causes tires to skid, metal to crush, and at times, injury and death.

STOPPING: As a vehicle travels down the road, it gathers kinetic energy. How much energy depends on how fast the vehicle is going and how large it is. In order for a driver to bring a vehicle to a stop, he must first find a way to dissipate this kinetic energy.

Imagine that your fire truck is a bucket full of water. The water inside the bucket is the energy that the fire truck has created as it is driving down the street. In order to slow the vehicle down, the driver must get rid of some of the kinetic energy or dump some of the water out of the bucket. If the driver wants to bring the vehicle to a stop, he must get rid of all of his kinetic energy or dump all of the water out of the bucket.

The problem is that we can’t just make kinetic energy disappear; it has to be converted into a different type of energy. Almost all the time, a vehicle’s kinetic energy is safely converted into heat energy when the driver applies the brakes. As the brake pads rub against the disc or the drum, it creates tremendous amounts of heat energy. The heat created by the brakes is actually the kinetic energy of the vehicle being "burned off." As this energy is burned off, the vehicle slows down and eventually comes to a stop.

SKIDDING: Normally, the friction of the brake pads rubbing on the discs or drums will "burn off" a vehicle’s kinetic energy and bring it to a stop. But what happens when a car pulls out in front of you and you have to slam on your brakes, causing your wheels to lock? In this case, the wheels stop turning and your brake pads are no longer rubbing on the discs or drums. As a result, the vehicle must find another way to burn off the energy so that it can come to stop. How does this occur? As the locked tires slide across the roadway, the friction between the tire and the road surface creates the heat, which burns off the kinetic energy.

The problem with this scenario is that as the tire slides across the road surface, it creates tremendous amounts of heat. While there is a lot of discussion in the crash reconstruction world as to what a skid mark is, one of the more commonly held beliefs is that the heat from a skidding tire liquefies the oils in an asphalt roadway and causes them to float to the surface. As these oils float to the surface, the vehicle begins to slide on a thin sheen of oil with no steering control. This scenario results in what we commonly refer to as a "skid mark." A skid mark is the mark left by the residual oils that have been brought to the surface of the road by heat energy. Some of the skid mark is also created by the rubber from the tire. The take home message is that once your vehicle begins to skid along the road surface (if not equipped with anti-lock brakes), you will lose all steering control of your vehicle.

In this scenario, if you are lucky, your vehicle will skid in a straight line and eventually come to a stop once it burns off all the kinetic energy. But if you aren’t lucky, your vehicle may slide into a fixed object or another vehicle on the road.

CRUSH: Ideally, you will "burn off" your kinetic energy by applying the brakes and coming to a stop. If you aren’t careful, you may slam on your brakes and lock your tires, causing you to skid to a stop. But what happens if you aren’t able to burn off all your energy before another car pulls out in front of you? In this case, you strike the other vehicle while your vehicle still has kinetic energy. The remaining energy present when the vehicles collide is dissipated by crushing the metal of the vehicles. What you see in the aftermath of a motor vehicle crash (i.e. the crushed cars) is actually evidence of how much kinetic energy was left in the vehicles at the time of the crash. The bigger the vehicle and the faster it is traveling, the more crush damage the vehicles will sustain during a crash.

Vehicle drivers must understand how the size and weight of their vehicle, in conjunction with the speed that they are traveling, directly affects the amount of energy that they must dissipate in order to bring a vehicle to a safe stop. In an ideal situation, this energy will be safely dissipated by applying the brakes. In less than ideal situations, the energy may be dissipated by skidding the tires or crushing the metal of the vehicle. Hopefully, we can all stick to using the brakes.

About the Author:
Chris Daly is a 23-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 recently lectured for NYSAFC during the 109th Annual Conference & FIRE 2015 Expo at the Turning Stone in June. He has been a contributing author to Fire Engineering and FireRescue1.com. Daly has a master’s degree in safety from Johns Hopkins University. Email Chris regarding his "Drive to Survive" program or assisting with crash reconstructions.