By Chris Daly

We’ve spent the past three articles discussing the fact that if a fire apparatus operator places too much lateral g-force on the vehicle, it will cause a rollover. The reason we have spent so much time on this topic is because rollovers are one of the most common types of fire apparatus crashes. Therefore, a thorough understanding of lateral g-force should be a key part of any fire department crash prevention program.

One of the many ways that lateral g-force can contribute to a rollover crash is by inducing a “weight shift.” It is common to hear driver trainers and crash investigators refer to “weight shift.” But what is “weight shift,” and why is it bad? Understanding why a vehicle’s weight will shift while rounding a curve, attempting an evasive maneuver, or any other time the driver turns the wheel is an important training concept that every fire apparatus operator must understand.

When a vehicle rounds a curve or makes an evasive maneuver, lateral g-force will cause the vehicle’s weight to shift toward the outside of the curve. This is because a vehicle rides on a suspension system, which is supported by springs, shocks, and rubber tires. When lateral g-force “pushes” against the vehicle, it will cause the vehicle to lean to one side as the vehicle settles into the suspension system. As the vehicle leans, the center of gravity will shift toward the outside of the curve. The shifting center of gravity will reduce the vehicle’s track width on that side and cause a reduction in the vehicle’s rollover threshold (see “Fire Apparatus Rollovers, Part 1” in the January 2019 issue). By reducing the vehicle’s rollover threshold, it will take less lateral g-force to roll the vehicle over. This is the reason lateral g-force is evil. Not only will it cause a fire apparatus to roll over, but it will actively reduce the vehicle’s rollover threshold and make it easier to do so.

In addition to causing the body of the vehicle to shift on its suspension, lateral g-force will cause the liquid in the water tank to surge in one direction or another. Just as the body of the vehicle leans, or “pushes out,” as the vehicle rounds a curve, so will the water inside the tank. This phenomenon is known as “liquid surge” or “slosh.” As the fire apparatus rounds a curve, inertia will cause the water in the tank to try and continue traveling in a straight line. However, because the water is confined inside the tank, it has nowhere to go. Instead, the water pushes up against the inside of the water tank and, in a partially loaded tank, the water will ride up the inside wall of the tank. This sloshing or surging effect of the water can cause the vehicle’s center of gravity to shift back and forth on the vehicle’s suspension system. This shifting center of gravity can significantly reduce the rollover threshold of the vehicle, thus reducing the stability of the vehicle.

To help prevent this liquid slosh, the National Fire Protection Association requires all fire apparatus water tanks to have baffles. Tank baffles help to absorb the energy created by the sloshing liquid and reduce the force of the water pushing on the inside of the tank. By reduc