By Chris Daly

I’m sure everyone has had this experience: You are riding shotgun on the way to a call, and you’ve thrown your helmet onto the dashboard so that you can read the map book.

As the fire apparatus operator rounds the curve, your helmet starts to slide across the dashboard and nearly goes out the window before you are able to grab it. That was close; you would have missed a job if you had no helmet to wear. The sneaky culprit that almost stole your helmet was inertia, and it’s another key concept that professional fire apparatus operators must understand.

What most drivers don’t realize is that every curve in the road has what’s called a “critical speed.” If you take the curve faster than this critical speed, your vehicle will break traction and continue in a straight line instead of negotiating the curve. As a result, the vehicle will travel off the road and crash.

Outside Forces

Fire apparatus operators must understand that as we round a curve, there are two major forces working on our vehicle. The “bad” force is centrifugal force, which makes our vehicle want to continue in a straight line off of the roadway. The “good” force is the traction between our tires and the road surface.

As long as we have more traction than centrifugal force, our vehicle will hold the road, and we will make it through the curve-no questions asked. However, when we drive too fast for the road conditions, we allow centrifugal force to overwhelm our tires’ traction. When this happens, the vehicle breaks traction, and we lose control.

The speed at which we lose control depends on three major factors: the radius of the curve (how sharp it is), the coefficient of friction of the roadway (how “sticky” it is), and the superelevation (“bank”) in the road. Problems arise as your speed increases, the sharpness of the curve decreases, or the stickiness of the road decreases with bad weather.

When you think about it, the curve and bank in the road will never change; however, the stickiness of the road will change based on the weather. As the road gets slicker with rain, snow, or ice, the critical speed of the curve will go down.

Critical Speed of a Curve

Let’s go back to the example of your fire helmet. As the fire truck rounds the curve, the traction of the tires on the dry road surface is more than the centrifugal force trying to make the fire truck continue in a straight line. In this case, the fire truck maintains traction with the road and safely negotiates the curve. However, your plastic helmet is resting on a freshly polished, vinyl dashboard. The “stickiness” that is keeping the helmet from sliding around is considerably less than the stickiness between the tires and the road. In this case, the centrifugal force experienced as the vehicle rounds the curve is more than the coefficient of friction between your helmet and the dashboard. As a result, your helmet breaks traction with the dashboard and tries to keep traveling in a straight line, attempting to exit the window.

The reason for this can be shown scientifically. To figure out the critical speed of a curve, you need only three things: the radius of the curve, the coefficient of friction of the roadway, and the superelevation of the road. By plugging these three values into the following formula, we are able to calculate the critical speed of a curve.

3.86 √ R × (f ± e)

“R” is the radius of the curve. The roadway’s coefficient of friction is represented by “f,” and “e” is superelevation of the roadway.

As

By Chris Daly

I’m sure everyone has had this experience: You are riding shotgun on the way to a call, and you’ve thrown your helmet onto the dashboard so that you can read the map book.

As the fire apparatus operator rounds the curve, your helmet starts to slide across the dashboard and nearly goes out the window before you are able to grab it. That was close; you would have missed a job if you had no helmet to wear. The sneaky culprit that almost stole your helmet was inertia, and it’s another key concept that professional fire apparatus operators must understand.

What most drivers don’t realize is that every curve in the road has what’s called a “critical speed.” If you take the curve faster than this critical speed, your vehicle will break traction and continue in a straight line instead of negotiating the curve. As a result, the vehicle will travel off the road and crash.

Outside Forces

Fire apparatus operators must understand that as we round a curve, there are two major forces working on our vehicle. The “bad” force is centrifugal force, which makes our vehicle want to continue in a straight line off of the roadway. The “good” force is the traction between our tires and the road surface.

As long as we have more traction than centrifugal force, our vehicle will hold the road, and we will make it through the curve-no questions asked. However, when we drive too fast for the road conditions, we allow centrifugal force to overwhelm our tires’ traction. When this happens, the vehicle breaks traction, and we lose control.

The speed at which we lose control depends on three major factors: the radius of the curve (how sharp it is), the coefficient of friction of the roadway (how “sticky” it is), and the superelevation (“bank”) in the road. Problems arise as your speed increases, the sharpness of the curve decreases, or the stickiness of the road decreases with bad weather.

When you think about it, the curve and bank in the road will never change; however, the stickiness of the road will change based on the weather. As the road gets slicker with rain, snow, or ice, the critical speed of the curve will go down.

Critical Speed of a Curve

Let’s go back to the example of your fire helmet. As the fire truck rounds the curve, the traction of the tires on the dry road surface is more than the centrifugal force trying to make the fire truck continue in a straight line. In this case, the fire truck maintains traction with the road and safely negotiates the curve. However, your plastic helmet is resting on a freshly polished, vinyl dashboard. The “stickiness” that is keeping the helmet from sliding around is considerably less than the stickiness between the tires and the road. In this case, the centrifugal force experienced as the vehicle rounds the curve is more than the coefficient of friction between your helmet and the dashboard. As a result, your helmet breaks traction with the dashboard and tries to keep traveling in a straight line, attempting to exit the window.

The reason for this can be shown scientifically. To figure out the critical speed of a curve, you need only three things: the radius of the curve, the coefficient of friction of the roadway, and the superelevation of the road. By plugging these three values into the following formula, we are able to calculate the critical speed of a curve.

3.86 √ R × (f ± e)

“R” is the radius of the curve. The roadway’s coefficient of friction is represented by “f,” and “e” is superelevation of the roadway.

As

By Alan M. Petrillo

Take a look at a pumper or aerial in the United States, then look at a similar vehicle in the United Kingdom or a European country. As the French say, “Vive la différence,” an expression of approval meaning, “Long live the difference.”

Those differences in structural firefighting apparatus go well beyond the outward appearance of the vehicles and reflect differences in the geography of the areas covered, the types of structures protected, street layouts, firefighting tactics, and a host of other issues.

1 A typical pumper in Europe is built on a commercial chassis and has high compartmentation with highly organized interior spaces. (Photo courtesy of Pierce Manufacturing Inc.)

Size Influences

Sam Itani, vice president of international and government sales for E-ONE, says that in Europe fire apparatus are more compact than comparable vehicles in North America. “The European apparatus is shorter, narrower, and tighter in design than what we see here,” Itani says. “In the United States, we usually have larger, wider roads and highways, so we don’t need the tighter designs in most cases.”

The structures and types of buildings in Europe also have an influence on fire apparatus, Itani points out. “European lifestyle revolves around major cities, while their suburbia is an extension of the city,” he says. “There are a lot of narrow streets in their cities, many of them one way. And with the different building construction, they have to design vehicles to go into narrow, congested areas, yet still allow firefighters access to the tools and hoses on their trucks.”

2 Pull-out steps are common on European fire apparatus to allow easy access to the top of compartments on vehicles. (Photo courtesy of Pierce Manufacturing Inc.)

Lisa Barwick, director of business development and product management for Pierce Manufacturing Inc., agrees that because of the infrastructure of European cities, their fire apparatus have to be smaller and more agile. “The roads and streets are much narrower over there,” Barwick says, “so their apparatus, especially their engines, are much smaller than those over here. They also tend to use more commercial chassis, like MAN and Mercedes, rather than custom chassis.”

She adds, “While some Northeastern cities in the U.S. have low overall height and length restrictions and tight streets that might require smaller, more maneuverable apparatus, usually in North America everything seems to be on a grander scale because we have the room, as well as the types of our buildings, and much wider streets.”

Custom vs. Commercial

Dave Reichman, national sales manager for Rosenbauer, says the apparatus chassis is the most noticeable difference between the United States and Europe. “In the States, we’ve seen a huge increase in the use of custom chassis,” Reichman says. “In the last two to three years, 75 to 80 percent of our chassis are custom, while in Europe, about 95 percent of chassis are commer

By Alan M. Petrillo

Take a look at a pumper or aerial in the United States, then look at a similar vehicle in the United Kingdom or a European country. As the French say, “Vive la différence,” an expression of approval meaning, “Long live the difference.”

Those differences in structural firefighting apparatus go well beyond the outward appearance of the vehicles and reflect differences in the geography of the areas covered, the types of structures protected, street layouts, firefighting tactics, and a host of other issues.

1 A typical pumper in Europe is built on a commercial chassis and has high compartmentation with highly organized interior spaces. (Photo courtesy of Pierce Manufacturing Inc.)

Size Influences

Sam Itani, vice president of international and government sales for E-ONE, says that in Europe fire apparatus are more compact than comparable vehicles in North America. “The European apparatus is shorter, narrower, and tighter in design than what we see here,” Itani says. “In the United States, we usually have larger, wider roads and highways, so we don’t need the tighter designs in most cases.”

The structures and types of buildings in Europe also have an influence on fire apparatus, Itani points out. “European lifestyle revolves around major cities, while their suburbia is an extension of the city,” he says. “There are a lot of narrow streets in their cities, many of them one way. And with the different building construction, they have to design vehicles to go into narrow, congested areas, yet still allow firefighters access to the tools and hoses on their trucks.”

2 Pull-out steps are common on European fire apparatus to allow easy access to the top of compartments on vehicles. (Photo courtesy of Pierce Manufacturing Inc.)

Lisa Barwick, director of business development and product management for Pierce Manufacturing Inc., agrees that because of the infrastructure of European cities, their fire apparatus have to be smaller and more agile. “The roads and streets are much narrower over there,” Barwick says, “so their apparatus, especially their engines, are much smaller than those over here. They also tend to use more commercial chassis, like MAN and Mercedes, rather than custom chassis.”

She adds, “While some Northeastern cities in the U.S. have low overall height and length restrictions and tight streets that might require smaller, more maneuverable apparatus, usually in North America everything seems to be on a grander scale because we have the room, as well as the types of our buildings, and much wider streets.”

Custom vs. Commercial

Dave Reichman, national sales manager for Rosenbauer, says the apparatus chassis is the most noticeable difference between the United States and Europe. “In the States, we’ve seen a huge increase in the use of custom chassis,” Reichman says. “In the last two to three years, 75 to 80 percent of our chassis are custom, while in Europe, about 95 percent of chassis are commer