Calculating the curve
How many times have we heard of a fire apparatus crash that occurred while a vehicle was rounding a curve? There are many reasons for this, most of which revolve around speed.
Every curve in the roadway has a critical speed. If you exceed this critical speed, the vehicle will begin to spin around its center of mass and leave the roadway. Regardless of the driver’s skill level or years of experience, if you exceed the critical speed, your vehicle will lose control.
Critical speed is determined based on how sharp the curve is, how much bank is in the roadway and the road surface’s “stickiness,” or coefficient of friction. By measuring these three factors and plugging them into a formula, the critical speed of a curve can be calculated.
Let’s examine a curve in the road that has a radius of 200 feet, a curve not uncommon to most districts. On a sunny day with dry asphalt, the critical speed of this curve would be approximately 51 mph. A driver traversing this curve at 40 mph would safely navigate the curve and not think twice.
Imagine that the next day it rained. This change in weather conditions would lower the critical speed of the curve to approximately 34 mph. If the same driver, who previously drove through this curve at 40 mph on a sunny day, attempted to do the same thing on a rainy day, he would find himself spinning off the roadway. This is because the rain caused the critical speed of the curve to drop to from 40 mph to 34 mph. By driving faster than 34 mph, the vehicle lost traction with the road surface and the driver lost control.
In addition to critical speed issues, the driver of a vehicle with a high center of gravity has other issues to contend with. When driving a vehicle with a high center of gravity, such as a fire truck, you could roll over well below the critical speed of the curve. As the fire truck enters and begins to round the curve, centrifugal force will cause the weight of the vehicle to shift to the front outside tire. This causes the center of mass of the vehicle to shift as well. If you’re traveling too fast and the center of mass of the vehicle shifts too far, a rollover will result.
Overcorrection is also a common cause of curve-related crashes. How many times have we heard of a fire apparatus crash that was caused when the driver drifted off the right side of the road, attempted to bring the vehicle back onto the road, overcorrected and then caused the vehicle to roll over?
We often hear training programs say that drivers should slow down, regain control of the vehicle and then bring the truck back onto the road surface. I say: Stop the truck. It is much safer to bring the vehicle to a controlled stop and then gently bring it back onto the road surface at a slow and safe speed. There is no need to try and save a few seconds conducting such a risky maneuver as bringing a moving vehicle back onto the road surface. Instead, stop the truck, take a breath, assess the situation and continue on. If you are that concerned about the time it will take to bring the truck to a stop, then slow down and don’t drift off the road in the first place!
Curve-related crashes are a common cause of firefighter injuries and fatalities. Drivers must recognize the hazards of driving a vehicle through a curve, especially during inclement weather conditions or while operating a vehicle with a high center of gravity. Drivers must be thoroughly familiar with their districts so they are able to recognize that a curve is approaching and slow down well in advance of entering it. Realizing that you are going too fast when you are midway through the curve is too late. You’re probably going to go for a terrible ride.
Daly has been a contributing author to Fire Engineering Magazine, Pennsylvania Fireman and Firerescue1.com. He has a master’s degree in safety from Johns Hopkins University. He can be contacted at
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