Aerials, Daly, Pumpers, Rescues

Rollover Threshold and G-Force

Issue 2 and Volume 24.

In last month’s article, we discussed how to determine a vehicle’s rollover threshold. Once a vehicle’s rollover threshold is calculated, it is possible to determine at what point it will roll over.

The point at which the apparatus will roll over depends on how much lateral acceleration is placed on the vehicle. Lateral acceleration is the evil force that is constantly trying to “push” the apparatus off of the road or flip it over.

When a vehicle moves from side to side, it will experience “lateral acceleration,” commonly referred to as “g-force.” While lateral g-force sounds like a complicated term, it is actually quite simple. Not only is this concept very simple, it is a key point that every fire apparatus operator must come to understand if he plans to drive safely. Unfortunately, few emergency vehicle operation course (EVOC) programs discuss it.

The amount of g-force a vehicle will experience as it rounds a curve will depend on the speed of the vehicle and the radius of the curve (how sharp it is). If the vehicle’s speed increases, or the curve gets sharper, the lateral g-force acting on the vehicle will increase. If a vehicle experiences a lateral g-force that is greater than its rollover threshold, the vehicle will roll over. Lateral g-force is calculated using the following formula:

Formula One

REAL-LIFE G-FORCE

By using the lateral g-force formula, it is possible to determine how much lateral g-force is acting on a vehicle as it rounds a curve. As an example, consider the curve in photo 1, which has a radius of 80 feet along the middle of the inside travel lane. The amount of lateral g-force experienced by a vehicle while rounding the inside travel lane is shown in Table 1.

Table 1 demonstrates how dangerous it is to round a sharp curve or corner in a fire apparatus. As most fire apparatus have an average rollover threshold over around 0.60 (give or take, no manufacturer will give me exact measurements), the average fire apparatus will roll over while rounding this curve at just 25 to 30 miles per hour (mph). Keep in mind that at 30 mph, the fire apparatus operator will be driving the vehicle at 100 percent of its ability. In reality, most drivers “give up” and panic when the lateral g-force exceeds 0.20.

Table One

he measured radius of the inside lane of this curve is approximately 80 feet. This would be considered a “sharp” curve to most drivers. When a curve has a smaller radius (is sharper), the lateral g-force increases significantly with an increase in speed.

1 The measured radius of the inside lane of this curve is approximately 80 feet. This would be considered a “sharp” curve to most drivers. When a curve has a smaller radius (is sharper), the lateral g-force increases significantly with an increase in speed. (Photos by author.)

The measured radius of this curve is approximately 320 feet. This curve would be considered more sweeping than the curve in Photo 1. Because of the larger curve radius, the increase in g-force is not as severe as the vehicle’s speed increases. However, fire apparatus operators must still be wary while rounding a sweeping curve.

2 The measured radius of this curve is approximately 320 feet. This curve would be considered more sweeping than the curve in Photo 1. Because of the larger curve radius, the increase in g-force is not as severe as the vehicle’s speed increases. However, fire apparatus operators must still be wary while rounding a sweeping curve.

When we examine Table 1, it is important to note that a 5-mph increase in speed results in a significant increase in g-force. This is because the curve is so “sharp.” A vehicle rounding a sharp curve will experience a significant increase in g-force with just a small increase in speed. This explains why so many fire apparatus roll over while rounding a corner at an intersection. If the fire apparatus operator fails to slow down to a safe speed while rounding the corner, the lateral g-force will exceed the rollover threshold of the vehicle, and it will roll over. Rounding a corner is a surprisingly dangerous activity, as evidenced by the number of fire apparatus that have crashed while simply turning from one road onto another. A small increase in speed results in a large increase in lateral g-force.

Now consider a curve with a measured radius of 320 feet along the middle of the inside travel lane (photo 2). Because this curve has a much larger radius, there will be less g-force at each respective speed as compared to the curve with a radius of 80 feet. A more “gentle” curve places less g-force on the vehicle, as evidenced in Table 2.

Table two

A gentler curve, with a larger radius, will be more forgiving than a sharp corner. However, fire apparatus operators must still be wary of rounding a curve in a vehicle with a high center of gravity (fire apparatus). In the case of the more gentle curve, if the fire apparatus operator exceeds 50 mph, the vehicle will most likely lose control and flip over. As already mentioned, we must remember that this assumes that the driver can operate the vehicle at 100 percent of its ability. Most drivers begin to “give up” and panic when they sense more than 0.20 Gs acting on their body. As such, most fire apparatus operators will begin to feel uncomfortable when they round this curve at a speed greater than 30 mph and may begin to panic.

The moral of the story is this: As speed increases, or a curve gets sharper, the amount of lateral g-force acting on the vehicle will increase substantially. Once the lateral g-force exceeds the rollover threshold of the vehicle, the fire apparatus will roll over. Furthermore, the lateral g-force may reach a point that makes the driver feel so uncomfortable that he “gives up” and loses control.


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.