BY CHRIS DALY
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:
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.
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.)
Read more
- 284
- Article rating: No rating