Aerials, Apparatus, Daly, Pumpers, Rescues, Tankers

Apparatus Rollovers, Part 6: Vehicles with Low Centers of Gravity

Issue 8 and Volume 24.

For the past several articles, we have discussed lateral g-force and how it can cause a fire apparatus to roll over. When the amount of lateral g-force exceeds the rollover threshold of the vehicle, the g-force will “push” the vehicle over. Most fire apparatus have a high center of gravity, and it does not take much lateral g-force to push them over.

But, what about a vehicle that has a low center of gravity? These types of vehicles may include your personal car, squads, or command vehicles. If you drive a vehicle with a low center of gravity, are you immune from the worries of too much lateral g-force? NO! You must still have a thorough understanding of how lateral g-force can cause a crash.

LOW CENTERS OF GRAVITY

The average personal vehicle or sport utility vehicle has a rollover threshold that is nearly twice that of a fire apparatus. This is because smaller vehicles tend to have a low center of gravity in relation to the track width (width of the vehicle). Because of this low center of gravity, it requires more lateral g-force to induce a rollover. However, if the driver takes a curve too fast or turns the steering wheel too far, it is still possible to lose control and slide off the road. This loss of control will occur when the lateral g-force acting on the vehicle exceeds the traction of the tires.

DRAG FACTOR

A tire’s ability to grip the road is limited by the drag factor of the roadway. The drag factor of the roadway will depend on the weather conditions and the grade of the roadway. A dry road with a significant positive superelevation (bank in the road) will have a higher drag factor than a wet road with no superelevation. Most flat, dry roads have an average drag factor of around 0.75.

So if the drag factor of the road is 0.75, and the vehicle rounds a curve at a speed that generates a lateral g-force of greater than 0.75, the tires will start to lose traction and slip. If the front tires lose traction first, the vehicle will “understeer,” and the vehicle will push off the road. If the rear tires lose traction first, the vehicle will “oversteer,” and the rear end will try to spin around. NASCAR fans will probably recognize an understeer as a “push,” while an oversteer describes a “loose” rear end.

What does this mean to us as drivers? It means that lateral g-force can still cause a crash even if you are driving a stable vehicle with a high rollover threshold. If the driver enters the curve too fast, he may generate a lateral g-force that will exceed the drag factor of the road. If this happens, the tires on the vehicle will start to lose their grip on the road and begin to slide. As most travel lanes are only 10 to 12 feet wide, there is not much room for the tire to slide before it drops off the roadway. Once the tire drops off the roadway, the vehicle many continue traveling off the road where it strikes a fixed object such as a tree or guardrail. In many cases, the vehicle will slip off the road and strike a fixed object or soft surface, which induces a “tripped rollover.”

A tripped rollover is exactly as it sounds. As the vehicle is sliding off the road, the tires will strike an object on the ground such as a curb or tree. This object, which is low to the ground, will stop the tires, while inertia will cause the top half of the vehicle to continue to travel. If the top half of the vehicle travels far enough, the vehicle will trip itself and roll over. These are usually very serious crashes, as the vehicle will have a tendency to strike other fixed objects as it rolls. I’m sure many readers have been to a crash just like this, where a vehicle traveled off the road, began to roll, and struck a tree along the roof line.

As you can see, lateral g-force is not just an issue for the fire apparatus operator. Lateral g-force is a safety issue for anyone who drives a vehicle, even a vehicle with a low center of gravity. Fire department members must remember to slow down well in advance of a curve and take the curve at a safe speed. Traveling too fast into a curve may generate enough lateral g-force to cause the vehicle to lose its grip on the road and crash.

CHRIS DALY is a 22-year police veteran, serving as a patrol supervisor in West Chester, Pennsylvania. He has served 29 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 22,000 firefighters and police officers at more than 500 emergency service agencies across the United States.