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G-Force: How Much Is Too Much?
BY CHRIS DALY
In “Rollover Threshold and G-Force” (February, 2019), we continued to discuss lateral g-force. Lateral g-force is the evil force that is continually trying to make a fire apparatus flip over or lose control. Fire apparatus operators must have a thorough understanding of lateral g-force so they are better able to operate a vehicle safely.
G-FORCE REVIEW
During February’s discussion, we learned that if the amount of lateral g-force exceeds the rollover threshold of the vehicle, the vehicle will roll over. And if a vehicle rolls over, it clearly indicates that the vehicle was driven in excess of its handling capabilities. As an example, if the rollover threshold of an engine is 0.62, and the fire apparatus operator is rounding a curve at a speed that places 0.61 lateral g’s on the vehicle, he is driving the vehicle at nearly 100 percent of its capability. Under no circumstances should a fire apparatus operator drive so close to the upper limits of the apparatus’s capabilities! Driving in this manner leaves no room for error. Therefore, it is important for the fire apparatus operator to understand how to judge the amount of lateral g-force acting on a vehicle based on the sensation of g-force the driver experiences on his body.
1 An example of a g-meter I purchased online. This is an extremely useful teaching tool when used by a trained instructor. (Photo by author.)
The concept of sensing g-force is known to pilots as “flying by the seat of your pants.” A good pilot is able to safely operate his airplane without reading the instruments on the dashboard. This is because an experienced pilot is able to correlate the sensation of g-force he is experiencing to the performance, behavior, and limitations of the airplane. This understanding of how to read g-force with your body and understand when things are getting uncomfortable and unsafe is a key aspect of safe driving.
TRAINING
So, we must ask ourselves, at what point will the sensation of lateral g-force start to make us feel uncomfortable? Depending on the reference source, many experts believe that once the lateral g-force exceeds 0.20 to 0.30 g, a driver will start to feel uncomfortable. In fact, many highway design manuals recommend limiting curve speeds and curve designs to keep the lateral g-force under 0.20 g. By keeping the lateral g-force under 0.20, it will help to mitigate the feeling of discomfort felt by the driver. This is important because once a driver begins to feel uncomfortable, he may panic and make an inappropriate steering or braking maneuver. It is often this inappropriate steering or braking maneuver that leads to a crash. This is especially true in a vehicle with a high center of gravity, where there is little margin for error.
Fortunately, a safe and professional fire apparatus operator will never allow the
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