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Apparatus Purchasing: Center of Gravity

Issue 5 and Volume 17.

Bill Adams

Everybody has an idea of what center of gravity (CG) is, but not too many people know how to evaluate it when purchasing fire apparatus. When you research CG, all sorts of odd sounding nonfirematic terms surface like center of mass, barycenter, finite set of point masses, and gravitation potential energy.

Use a search engine and specifically enter “Center of Gravity in Fire Apparatus.” More than 3.2 million results are generated. Pick one whose title, magazine, manufacturer, or author catches your eye. Industry and fire service pundits write about CG all the time. Use caution-some have an agenda. Go to www.fireapparatus.com, click on past issues, 2009, and January. Look for “High Center of Gravity-High Risk” by Gary Handwerk, the pump guru at Hale Products, Inc. It’s about the best unbiased description out there. It’s technical, but you needn’t be a physicist to understand it. It’s worth reading.

(1) In NFPA 1901, Section 4.13, on vehicle stability, sentence 4.13.1 on rollover stability says if a new rig is not equipped with a "stability control system," it must be tested to 26.5 degrees on a tilt table.
(1) In NFPA 1901, Section 4.13, on vehicle stability, sentence 4.13.1 on rollover stability says if a new rig is not equipped with a “stability control system,” it must be tested to 26.5 degrees on a tilt table. (Photos courtesy of CustomFIRE Apparatus.)

NFPA 1901

National Fire Protection Association (NFPA) 1901, Standard for Automotive Fire Apparatus (2009 ed.), addresses CG. Sentence 3.3.34 defines it as, “The point at which the entire weight of the fire apparatus is considered to be concentrated so that, if supported at this point, the apparatus would remain in equilibrium in any position.” Equilibrium means stability-everything is in a sense of balance. Plainly put, if something is not stable or in balance, it can tip over.

Under Section 4.13 on vehicle stability, sentence 4.13.1 on rollover stability says, if a new rig is not equipped with a “stability control system,” it must be tested to 26.5 degrees on a tilt table. Or, “The calculated or measured center of gravity (CG) shall be no higher than 80 percent of the rear axle track width.” The stability control system will be addressed later. It is unknown how, why, and where the NFPA came up with the 80 percent figure.

If there is no stability control system, the apparatus manufacturer can either physically test the rig on the tilt table or it can calculate the CG. If calculated and the rear tires are 72 inches on center, the center of gravity can’t be more than 58.6 inches from ground level. In simple terms, if the CG is higher than 58.6 inches, there is a good chance of rolling the truck over. In even simpler terms, if it does roll over, the purchaser and whoever built the truck might have a big problem.

(2) When physically tested on a tilt table, results are pass or fail. There is no in between. There is nothing to evaluate or compare. It either passed or it didn't.
(2) When physically tested on a tilt table, results are pass or fail. There is no in between. There is nothing to evaluate or compare. It either passed or it didn’t.

Certification and Interpretation

Sentence 4.13.1.1.1 explains that rollover stability compliance shall be certified by testing, measuring, calculating, or comparing to a similarly compliant apparatus. It goes on to describe what a similar apparatus is and what is supposed to be loaded on it when certified. When writing apparatus specifications, the purchaser has the choice to have an apparatus’s rollover stability physically tested or tested by a calculator.

Regardless of the method, NFPA 1901 specifically states, “…the certification shall be delivered with the apparatus.” How many people have taken delivery of a new rig contracted to be NFPA 1901-compliant and actually received certification of its rollover stability? Take a look; it should be with your delivery papers. My interpretation of NFPA 1901 is that you should receive documentation that says your new rig physically passed the tilt table test or is certified to have been calculated to have a certain CG. Hopefully, you will not need it for a jurisprudential matter.

Pass or Fail

When physically tested on a tilt table, results are pass or fail. There is no in-between. There is nothing to evaluate or compare. It either passed or it didn’t. A calculated CG may be open to speculation and supposition-and not necessarily from the mathematical standpoint. Most advertisements and written articles mentioning CG say a low CG is desirable, especially for aerial devices and tankers (tenders). But, what CG is most desirable?

Use caution when comparing manufacturers’ claims of a low CG. How does a purchaser objectively evaluate a dimensional or percentile difference in the CG from one manufacturer to another? How much safer is Brand A’s pumper-rescue if its CG is 10 inches lower than a competitor’s apparatus?

Brand B’s quint is advertised as having a CG that’s 40 percent less than Brand D’s, 30 percent less than Brand E’s, and 20 percent less than Brand F’s. How much safer is Brand B’s quint than each of its competitors? How do you evaluate it? How can it be proven? Can a monetary value be placed on it? I know of no way to quantitatively compare the differences in inches or percentages between various “acceptable” CGs. If there is, please share it with our readers.

Responsibility

Purchasers should realize the CQ for a given apparatus is not necessarily at the discretion of the apparatus manufacturer. If a purchaser specifies full-depth high side compartments on both sides of a pumper, chances are the tank height is raised. Add a large-diameter hose load laying on top of the tank and coffin compartments on either side of the hosebed, and a point is reached where a responsible manufacturer says, “Enough. It’s too high.” You may be exceeding the maximum allowable height for an acceptable CG. Will you have to lengthen the wheelbase or take something off? Regardless of what is done, you might end up with a rig that just meets NFPA criteria. Don’t blame the manufacturer if you are the one who wants to carry too much too high.

Electronic Stability Control (ESC)

According to the Insurance Institute for Highway Safety (IIHS), “ESC is a vehicle control system comprised of sensors and a microcomputer that continuously monitors how well a vehicle responds to a driver’s steering input, selectively applies the vehicle brakes, and modulates engine power to keep the vehicle traveling along the path indicated by the steering wheel position. This technology helps prevent the sideways skidding and loss of control that can lead to rollovers. It can help drivers maintain control during emergency maneuvers when their vehicles otherwise might spin out. The systems are marketed under various names, including dynamic stability control, vehicle stability control, and dynamic stability and traction control, among others.”

Between the National Traffic Safety Administration and the United States Department of Transportation, the fire service (whose apparatus are considered to be commercial trucks) may be mandated to have an ESC.

A stability control system is not defined by NFPA 1901, except for being mentioned in sentence 4.13.1.2, where it says if you have one, “…it shall have, at a minimum, a steering wheel position sensor, a vehicle yaw sensor, a lateral accelerometer, and individual wheel brake controls.” Annex A, Section A.4.13.1, gives the NFPA’s informational take on the subject. It is worth reading.

Common Sense

Any system that improves firefighter safety is not in question, providing the true intent is not to compensate for fire apparatus that are poorly designed or marginally compliant-regardless if done so at the bequest of the purchaser. Nor should any system be a stop gap measure for inadequate training of apparatus operators. An ESC system will probably cost in excess of $2,000. Although no cost can or should be assigned to an injury or a firefighter’s life, the financial burden for any safety system must be evaluated and justified by the authority having jurisdiction. Let the safety pundits elaborate on that.

BILL ADAMS is a former fire apparatus salesman, a past chief, and an active member of the East Rochester (NY) Fire Department. He has more than 45 years of experience in the volunteer fire service.

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