Here’s an easy question. What type of fire apparatus is commonly involved in single-vehicle accidents? If you said tanker (tenders for West Coast firefighters), you’re right.
The real question is why. Even when built to National Fire Protection Association (NFPA) 1901 standards, they are usually heavy, big and have high centers of gravity. Sometimes the center of gravity is just too high.
To talk about center of gravity we really need to define what it is. It’s the point where the vehicle’s weight is balanced in all three directions: front to rear; side to side; and up and down.
On Jan. 1, a new NFPA section went into effect to address the problem of high centers of gravity. However, before Jan. 1, fire apparatus designers only had to worry about the center of gravity from side to side and front to back. Engineers only needed to look at how much weight was placed on each of the wheels of the vehicles. The individual wheel set weights can be used to calculate the center of gravity from front to rear and side to side directions, but not in the up and down direction. The limits for how high the weight is on the vehicle have been generally ignored, and NFPA’s limits were very unrestricted prior to Jan. 1.
It’s important to know the up and down center of gravity because the higher it is, the greater the vehicle’s weight will act on the driving dynamics of the apparatus. All vehicles have a front and rear suspension system. Each type of suspension has a design point called the roll center point. An imaginary line between the front and rear suspension roll points is called the roll center line.
The distance to the vertical center of gravity, above the roll center line, determines how much the vehicle’s weight will act on the driving dynamics. This vertical center of gravity height also determines maximum side slope on which the apparatus can safely operate. The greater the distance from the roll center line, the greater the leverage the vehicle’s weight has, and the more the weight acts on the vehicle’s dynamics.
The dynamics include: the forces acting on the front axle system when decelerating or braking; the forces acting on the rear axle system when accelerating or braking; and the forces acting on the front and rear axle systems when cornering.
As of Jan. 1, the revised edition of NFPA 1901 has a new section dealing with vertical center of gravity on fire apparatus. The new limit allows an apparatus to operate at a maximum 26.5-degree side tilt-angle. The rationale for limiting side tilt angle is to reduce poor handling and unsafe fire apparatus, particularly tankers, which have a high center of gravity.
The new standard won’t affect existing apparatus, but it will have an effect on apparatus contracted after Jan. 1. The good news is most newer apparatus already meets the new regulation. There are some exceptions, especially apparatus with tanks so high they extend well above the cab roof and apparatus with larger booster tanks on short wheelbases. Some do-everything-tanker-pumper-rescue-water-tower combinations also have trouble with high centers of gravity.
Calculating Tilt Angle
Other problem apparatus are converted fuel, heating oil and milk trucks. These kinds of commercial trucks were never designed to respond to emergency incidents. Additionally, water is heaver than fuel, causing even greater dynamic stability problems.
To avoid problems with high centers of gravity, most apparatus builders and their suppliers have 3D computer engineering software giving designers the ability to identify the center of gravity of every component used to build an apparatus. Once the center of gravity is located and the rear axle assembly’s track width is known, the tilt angle can be calculated.
There are two methods to determine the center of gravity for a finished apparatus. Neither requires sophisticated computer software.
The first method is to use a tilt bed, which is a device where the apparatus is chained to a platform that physically tilts along with the apparatus. The platform is tilted sideways until one or more tire sets lift off the platform. At that point, the platform angle is recorded, providing the tilt angle number.
The next method is to measure the weight under each wheel set while it’s stationary on level ground. Next, the apparatus is driven up a ramp approximately four feet high, and the vehicle is reweighed. Using the weights and the known ramp height, the center of gravity can be calculated.
Apparatus manufacturers use one of these measurements to prove their calculated methods, as well as to evaluate new designs. The measurements can also be used to evaluate modifications to existing designs.
Fire departments might also consider using the methods of determining center of gravity if they are having apparatus modified or upgraded. Larger water tanks, in particular, can dramatically affect the center of gravity and the driving dynamics of the apparatus.
Even though the new NFPA 1901 standard is not retroactive, it makes sense to check every apparatus suspected to have a high center of gravity. While it’s not cheap or easy to correct high centers of gravity on existing apparatus – and in some cases it might be impossible to fix – it is good information to have. Department officials can decide whether they want to take apparatus that is unsafe and no longer compliant out of service.
Short of completely redesigning new apparatus, adding electronic stability control (ESC) can make an apparatus with a high center of gravity safer.
ESC works by reading data from the engine, anti-skid brakes, traction control systems and vehicle roll sensors. This data is used to control individual brake actuation and engine throttle to stop the vehicle from becoming unstable and out of control.
The system is not cheap, it is not available on all chassis and it is not practical to retrofit on an existing apparatus. Nevertheless, it is an option to consider, especially on new apparatus.
Editor’s Note: Gary Handwerk is global pump product manager for Hale Products. He has been involved with the fire service industry for 36 years working for various fire apparatus or pump manufacturers and has been a member of the National Fire Protection Association (NFPA) Fire Apparatus Standards Committee for 15 years.