Less weight, more stable
Byline: Chris Cavette, Senior Editor
Some fire apparatus seem to be getting bigger and heavier – more water, more hose, longer aerials and more equipment. Speccing an apparatus that is too big or too heavy can hurt its performance and reduce the amount of equipment it can carry. And when the vehicle weight exceeds the axle weight ratings or is concentrated too high, it can make the apparatus unsafe.
Shed the pounds
The most efficient way to reduce weight on apparatus is to start with the heaviest components.
On pumpers, the heaviest component is usually the water. For example, some rural departments carry 750 to 1,000 gallons on their pumpers to give them enough water for an initial attack while they establish a tanker shuttle or set up a hose relay. In most states, however, the Insurance Services Office requires departments to start flowing 250gpm within five minutes of arrival at the fire scene and then maintain that flow rate (or higher) for two hours. If a department can’t assemble a sufficient number of tankers, pumpers, hose or personnel to sustain the required flow-rate for the full two hours, then carrying a few hundred extra gallons of water on their first-in pumper isn’t going to help. In that case, reducing the water tank capacity from 750 to 1,000 gallons down to 500 gallons would save roughly 2,000 to 4,000 pounds.
Other departments may want to take this idea one step further. The latest edition of NFPA 1901, Automotive Fire Apparatus, requires pumpers and quints to have a minimum water tank capacity of only 300 gallons. Departments using tank water strictly for trash, grass and auto fires should find that 300 gallons is plenty – especially if they use Class A or B foam. With 300 gallons, they can even knock down a good-sized structure fire with a 30-second blast from a preconnected blitz line, then lay a supply line to a hydrant to finish the job. Reducing the water tank capacity from 500 gallons down to 300 gallons will save almost 1,700 pounds.
Don’t forget the weight of the tank itself. One manufacturer estimates that a 500-gallon polypropylene tank can be as much as 600 pounds lighter than a steel tank of the same size. That weight savings gets proportionally larger with a 1,000- to 2,000-gallon poly tank on a tanker. This is one of the reasons why many apparatus builders use poly tanks on all their products.
Another component that offers significant potential for weight reduction is the engine. The largest fire apparatus engines usually tip the scales at about 2,600 pounds, with the smallest weighing in at about 1,300 pounds. In some cases, there may be a 700-pound difference between two engines with the same horsepower.
Changing from a high horsepower rating to a lower one may save as much as 1,300 pounds. And with a lower horsepower engine, a department can realize significant additional weight savings by specifying a smaller and lighter transmission, driveline, front axle and auxiliary retarder to match the engine. Total weight savings for the entire powertrain/axle package could be 2,000 to 3,000 pounds.
Several other vehicle components offer possibilities for shedding pounds. For example, switching from steel to aluminum wheels can save up to 200 pounds on a typical two-axle pumper or medium rescue, and specifying aluminum for the cab, body, wheel hubs, fuel tank, battery box and air tanks also can help shave pounds off a rig. In fact, an aluminum body can be up to 3,000 pounds lighter than one made of galvanized or stainless steel. Many of these options carry an increased price, so departments have to decide which ones offer the most weight reduction for the money.
Even loose equipment offers weight-savings opportunities. Some departments carry 800 feet or more of 5-inch supply hose and 400 feet of 2H-inch hose on their pumpers and quints. All that large-diameter hose may not be needed, however. Departments with good water systems and adequately spaced hydrants can flow 1,000gpm up to 300 feet from a hydrant using a single 2H-inch supply hose, which means that changing the load to 1,200 feet of 2H-inch hose could save about 400 pounds. If a department needs to use large-diameter supply hose, they might consider reducing the amount they carry or switching to lighter weight, single-jacket hose.
Other weight-saving equipment options include lightweight rescue tools and power units, pto-driven generators, aluminum booster reels, lightweight shoring, aluminum ladders, and spiral-coil hard suction hoses.
Lower the center of gravity
The center of gravity is the theoretical point at which all the weight could be concentrated when calculating the forces acting on the vehicle.
A comparison of the height of the center of gravity to the width across the rear tires helps determine how stable the vehicle will be during turns and on a side slope. If the center of gravity is too high compared to the width across the tires, the vehicle is said to be “top-heavy,” which means it has too much weight near the top and could roll over. Picture a small car with a grand piano strapped to the roof and you get the idea.
There are several ways to lower a vehicle’s center of gravity and improve stability.
Lower the vehicle. This is probably the easiest technique, but it produces only limited results. For example, changing from standard 11R24.5 tires to 11R22.5 tires on the front and rear axles will lower the vehicle about one inch. Changing from 11R22.5 tires to low-profile 275/70R22.5 tires could lower the vehicle about 1.3 inches more. The weight and speed ratings for these lower-profile tires are usually comparable to the larger ones, and the reduced rolling radius can be offset by specifying a numerically lower rear-axle ratio. In some cases, lower-profile tires can be combined with lowered front or rear suspensions to produce an even lower center of gravity.
Lowering the entire vehicle also has the advantages of lowering the steps, compartments, ladders and hosebeds for easier access. At the same time, it has the disadvantages of reducing ground clearances and lowering the angles of approach and departure. This can be offset, in part, by specifying angled profiles on the front bumper and a body similar to those found on wildland rigs.
Redistribute weight so the heavier components are closer to the ground. This is often more difficult to do, but it can produce more dramatic results. For example, using a full-width rectangular water tank instead of a round or elliptical tank could effectively lower the mass of the tank and water by as much as 10 to 15 inches. Depending on the chassis and body weight, this could lower the overall center of gravity by 5 inches or more.
This same approach also applies to aerials. Mid-mount aerials generally position the weight of the aerial device lower on the vehicle than rear-mount aerials. As a result, the vehicle’s overall center of gravity is lowered. Likewise, three-section aerials generally position the weight of the aerial device lower than four-section aerials, and four-section aerials are generally lower than five-section units.
Reduce the weight of the body, tank, aerial, equipment and anything else positioned above the truck frame. This is similar to the second technique, except you don’t have to move things around. Using a smaller and/or lighter water tank, lighter-weight body material, or lighter hose load reduces the weight of the upper part of the vehicle and effectively lowers the center of gravity.
If you can’t lower the center of gravity, you might try widening the tire track. Some departments have had good success using wider-profile tires with stiffer sidewalls on the front and rear. This broadens the vehicle base and reduces the amount of tire roll-under during turns to produce a noticeably more stable apparatus.
Figure it out
Reducing a vehicle’s weight and center of gravity can produce a more stable apparatus with better handling, but it also can affect other performance areas. For example, specifying a lower horsepower engine to reduce weight may affect the vehicle’s acceleration or hill-climbing speed. For many applications, this isn’t a significant consideration because the lower-horsepower engine still will have plenty of power, but for others it may be a problem. Likewise, reducing the tire size may adversely affect the vehicle’s ground clearance depending on the application.
Before you reject any of these options, you need to learn about them. One source of help is your apparatus manufacturer, as many have engineering staffs that can give you accurate weight and center of gravity figures for your proposed design. In some cases, they may be able to recommend additional ways to reduce the vehicle weight and center of gravity.
Another source of help is the new Fire Equipment Weight and Cube Calculator published by the Fire Apparatus Manufacturers’ Association. (See sidebar.)
All other considerations being equal, making an apparatus lighter and lower can result in a vehicle that has better handling, more equipment capacity, faster acceleration, greater hill-climbing speed and greater side-slope stability.
In some cases, it also can result in a vehicle that costs less because it uses smaller tires, lower-horsepower engines, and lower-capacity axles. Lighter, lower and less expensive – that’s a combination worth thinking about.
The Fire Apparatus Manufacturers’ Association recently introduced a computer program designed to assist fire departments in calculating the weight and volume of loose equipment carried on emergency response vehicles. It allows users to specify the additional weight allowance and compartment space required on new vehicles.
The FAMA Fire Equipment Weight and Cube Calculator is configured in an easy-to-use Microsoft Excel spreadsheet format that includes the weight and volume requirements for more than 600 categories of loose fire equipment normally carried on pumpers, aerials and rescues. This valuable program determines the weight and cube requirements for the equipment carried in each compartment and then automatically summarizes the total weight and cube requirements for the entire vehicle.
It may be downloaded at no charge from www.fama.org.
NFPA on weight and stability
NFPA 1901, Automotive Fire Apparatus, and NFPA 1906, Wildland Fire Apparatus, require that the axle loads and center of gravity of a fully loaded apparatus not exceed those specified by the vehicle manufacturer. NFPA 1906 has additional requirements for vehicle center of gravity location and side-slope capability depending on the GVWR.
Both standards specify requirements for baffles or swash partitions in all water tanks to control the dynamic movement of water. Unless controlled, the sideways surge of water can cause an apparatus to roll over in a turn or during a sudden steering correction, even if the apparatus is not top-heavy or overweight.
Both standards specify minimum weight allowances for miscellaneous equipment depending on the type of apparatus. These are only minimum allowances, and departments should weigh or calculate the actual weight of the equipment they intend to carry.
Both standards also specify an unequipped personnel allowance of 200 pounds for each seating position on the apparatus, not each person. Some departments may feel this figure is too low. Again, departments should use figures that represent the actual weight of personnel.
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