BY CHRIS DALY
In this series of articles, we will examine some of the more serious safety issues faced by the fire apparatus operator. By reviewing recent case studies, it is apparent that our first topic should be that of fire apparatus rollovers. Rollovers are a common cause of fire apparatus crashes.
Many of these rollovers are the direct result of excess speed or an inappropriate steering maneuver. When a fire apparatus with a high center of gravity is combined with excess speed and harsh steering, disaster often results. Unfortunately, many driver training programs lack an in-depth explanation of rollover dynamics.
The first step in understanding rollover dynamics is to understand the concept of a “rollover threshold.” A vehicle’s rollover threshold is a numerical relationship between the height of the vehicle’s center of gravity and the track width (distance between the rear wheels). The higher the vehicle’s rollover threshold, the more stable the vehicle. The rollover threshold of a vehicle is calculated using the “Equation 1” formula above.
As evidenced by this formula, a vehicle with a low center of gravity is more stable because it has a higher rollover threshold. Therefore, automobile manufacturers strive to design vehicles that have low centers of gravity. If the design and function of the vehicle prevents the center of gravity from being lowered any further, the only way to increase the vehicle’s rollover threshold is to make it wider. A good example of this concept is a quarry truck. Although the vehicle’s center of gravity is high off the ground, the vehicle is relatively stable because it is so wide.
Unfortunately, fire apparatus manufacturers are limited in how wide they can build a vehicle. This is because fire apparatus drive on public highways and must fit within a travel lane. Because a fire apparatus can only be built so wide, any increase in the center of gravity height reduces the vehicle’s stability and increases the risk of rollover.
National Fire Protection Association (NFPA) 1901, Standard for Automotive Fire Apparatus, outlines minimum requirements for fire apparatus rollover thresholds (Table 1). The NFPA requirements are based on two testing methods. The first method is a tilt-table test. In a tilt-table test, the apparatus is chained to a large platform, and the platform is tilted until the vehicle begins to roll over. The angle at which the vehicle begins to roll over is used to calculate the rollover threshold. The second method is to determine the height of the vehicle’s center of gravity and calculate the rollover threshold using Equation 1.
Table 2 provides the average rollover thresholds of different types of vehicles. As evident in Table 2, fire apparatus are relatively unstable when compared with passenger cars and light trucks. This poses a training issue for a firefighter who drives to the firehouse in a small, stable vehicle only to jump behind the wheel of a large fire apparatus with a high center of gravity. The firefighter must understand the significant change in vehicle dynamics that is associated with driving a fire apparatus and adjust his driving tactics. Failure to do so risks disaster.
The moral of the story is this: Because of the high center of gravity and relatively short distance between the rear wheels, a fire apparatus will have a low rollover threshold compared with an average civilian vehicle. This low rollover threshold will make the vehicle inherently unstable and more likely to roll over—especially while rounding curves or making evasive maneuvers. We will discuss this concept further in Part 2.
CHRIS DALY is a 20-year police veteran, currently serving as a patrol supervisor in West Chester, Pennsylvania. He has served 27 years as both a career and volunteer firefighter, holding numerous positions, including the rank of assistant chief. He is an accredited crash reconstructionist (ACTAR #1863) and a lead investigator for the Chester County (PA) Serious Crash Assistance Team. Daly is a member of the Fire Apparatus & Emergency Equipment editorial advisory board. Daly has also developed an emergency vehicle driver training program called “Drive to Survive,” which has been presented to more than 18,000 firefighters and police officers at more than 440 emergency service agencies across the United States.