Aerials, Apparatus, Daly, Pumpers, Rescues, SOC Specialized, Tankers

Apparatus Rollovers, Part 4: Weight Shift and G-Force

Issue 5 and Volume 24.


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.

1 As the fire apparatus rounds the curve, lateral g-force will start to “push” on the vehicle. This will cause the body of the vehicle to shift toward the outside of the curve. (Photo by author.)

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 reducing the force of the liquid surge pushing on the inside of the water tank, the vehicle is not as prone to weight shift. If the vehicle does not experience as much weight shift, vehicle stability is increased.

This is the reason it is so important to drive the apparatus only with a completely full or completely empty water tank. Doing so will reduce the empty void space in the tank, which will reduce the amount of free space available for the water to slosh around. For those departments that must drive with a partially loaded water tank, such as those that practice pump-and-roll operations at wildfires, use extreme caution to reduce the amount of lateral g-force acting on the vehicle. Lateral g-force will cause the water in the partially loaded tank to slosh, which may lead to a rollover.

So, remember, as a vehicle travels faster through a curve, the lateral g-force acting on a vehicle will increase. This increase in lateral g-force will result in more weight shift toward the outside of the curve. When the vehicle’s weight shifts toward the outside of the curve, it will reduce the track width of the vehicle on that side. This reduction in track width on the one side of the vehicle will reduce the rollover threshold of the vehicle and make it more prone to rollover. Therefore, fire apparatus operators must slow down and reduce the lateral g-force acting on the vehicle. Reducing the lateral g-force will reduce the chance of rolling over.


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.