BY CHRIS DALY
By now, you may be sick of hearing about lateral g-force. Too bad—it’s important. Understanding these advanced concepts is what separates the professional fire apparatus operator from a simple steering wheel holder.
At this point, our regular readers should understand the evil nature of lateral g-force. Lateral g-force can cause a fire apparatus to roll over or lose control. This rollover or loss of control usually happens while rounding a curve or conducting an evasive maneuver. Remember that as speed increases, or the curve gets sharper (the radius decreases), the amount of lateral g-force acting on the apparatus will increase accordingly.
Keep in mind that “curve radius” defines more than just a curve in the road. A vehicle will traverse a “curve in the road” any time the driver turns the steering wheel and causes the vehicle to change direction. The radius of this “artificial” curve is directly related to how sharply the driver turns the steering wheel.
Many fire apparatus crashes are the result of the driver drifting off the road and then overcorrecting to regain control of the vehicle. When the driver overcorrects, he turns the wheel and creates an artificial curve in an otherwise straight road. If the driver turns the wheel too sharply, the vehicle may experience a g-force that is greater than the rollover threshold of the vehicle. As a result, the vehicle will roll over. These types of crashes usually occur at high speeds, which is why it is so important for a driver to understand the danger of excess speed and the proper use of smooth steering control.
The National Institute for Occupational Safety and Health Firefighter Fatality Report System is full of fire apparatus rollover crashes, many of which occurred on a straight road. By examining these crashes in more detail, it is easy to recognize a common scenario. The driver of the doomed apparatus drifts off the road because he is distracted, tries to move out of the way to avoid an oncoming vehicle, or finds some other reason to turn the steering wheel and drop the tires off the asphalt road surface. Once the outside wheels drop off the paved surface, the fire apparatus will often become “trapped.” The inside of the tire will scuff and drag along the pavement edge as the fire apparatus continues traveling forward. Because the tires are unable to remount the pavement edge, the driver turns the wheel more and more, increasing the steering angle, until suddenly the steering tires have turned far enough to ride over the pavement edge and remount the asphalt roadway.
Once the tire remounts the roadway, the steering axle is pointed across the road and the fire apparatus shoots into the oncoming lane. As the driver enters the oncoming lane, he turns the wheel hard in the opposite direction in an attempt to get back into his own lane. When the driver turns the wheel, he creates an artificial “curve” in an otherwise straight road. If the apparatus is traveling fast enough, and the steering wheel is turned hard enough, the driver will create enough lateral g-force to exceed the rollover threshold of the vehicle. As a result of this overcorrection, the vehicle will roll over.
So how do we prevent this type of overcorrection crash? First, don’t drive off the road! Keep two hands on the wheel, let the officer work the radio and the siren, and don’t become distracted. The most important thing the driver can do is keep the vehicle safely within its lane of travel. The other thing to remember is to slow down BEFORE entering a curve. If the driver enters the curve too fast, the vehicle may understeer or oversteer, and the outside tires may drop off the road surface. This loss of steering control will trigger the very scenario we just talked about.
If you drive off the road, stop. There is no reason to fight to get the apparatus back onto the road and risk an overcorrection that results in a crash. The few moments saved by not stopping are not worth the risk of rolling the truck over. Come to a safe stop, reboot your brain, and then safely and slowly get back onto the road.
CHRIS DALY is a 20-year police veteran, 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.