In the July 2007 issue of In Service Online, we discussed why it is important to understand vehicle weight ratings. That isn’t enough, however. Departments also need to understand how the distribution of that weight determines a vehicle center of gravity, which can affect vehicle performance and safety.

Most people know that some of the vehicle weight is carried on the front axle and some on the rear axle. That’s known as fore-aft weight distribution and it needs to be matched to the size and weight ratings of the front and rear tires, wheels, hubs, brakes, axles and suspensions.

Weight can also be distributed towards one side or the other of a vehicle because of the location of various components and equipment. That’s known as side-to-side weight distribution and it needs to be equalized to avoid overloading one side. NFPA 1901 allows a maximum side-to-side weight variation of only 7% to be safe.

The third weight distribution is in the up-down direction. Like the fore-aft and side-to-side weight distributions, it is determined by the locations of various components and equipment, such as the body, water tank, aerial device, hose load and so on. The up-down, or vertical, weight distribution needs to be kept low to prevent the vehicle from overturning on side slopes or when making turns.

The center of gravity of a vehicle depends on all three weight distributions. Technically, it is the point where all the weight could be concentrated for purposes of calculating vehicle performance. More practically, it is an indication of how well the weight is distributed. Of the three weight distributions, the location of the center of gravity in the vertical direction is the most important because it affects stability — high is bad, low is good.

Low Loads

There are several ways to keep a vehicle center of gravity low. Many of them need to be considered when specifying a vehicle, and others need to be considered when adding equipment.

One factor in determining the vertical center of gravity of a vehicle is the height of the frame. This is determined by the height of the front and rear suspensions and tires, as well as the size of the frame rails. Specifying low-profile tires rather than standard profile tires is one common way to lower frame height; specifying smaller diameter tires rather than large diameter tires is another way. For example, using 22.5-inch low-profile tires instead of 24.5-inch standard profile can lower the frame height by one inch. You will need to adjust the rear axle ratio accordingly to achieve the same top vehicle speed. Using a lowered rear suspension can shave another two inches off the frame height. Frame rails with higher cross-sections in the middle and lower cross-sections at the front and rear give good frame strength without contributing to the frame height.

Another factor in determining the vertical center of gravity is the size, shape and location of the water tank. For a given tank capacity and tank length, a rectangular cross-section tank can lower the center of gravity by as much as 15 inches compared to a round cross-section tank. Variations on tank shapes include T-section tanks and L-shaped tanks, which increase capacity while either improving side-to-side slosh damping or reducing tank length. The weight of the tank and water are major contributions to vehicle weight, and departments need to discuss their effect on the center of gravity with the manufacturer before the apparatus is built. This is especially critical with tankers.

The height of the aerial device also determines center of gravity. The lower the turntable height and overall travel height of an aerial, the lower the center of gravity. Aerials represent a large amount of weight, and the lower they can be mounted, the better the vehicle side stability.

The placement of hose and other equipment is also important. For example, 800 feet of 5-inch jacketed supply hose can weigh as much as 800 pounds, which can seriously affect the vertical center of gravity if an apparatus has a high hosebed. A hydraulic rescue tool with pump and hose reel can weigh almost 300 pounds. The standard pumper compliment of ground ladders can weigh 150 pounds or more and a smoke ejector can weigh 100 pounds. The average structure pumper may carry 2,000 to 2,500 pounds of loose equipment. Ideally, departments should carry the heaviest equipment on slide-out trays in the lowest compartments. Consult Annex C of the latest edition of NFPA 1901, Automotive Fire Apparatus, for sample weights of various equipment.

Wide Tracks

Although lowering the center of gravity is the most common way of improving vehicle side stability, another is to utilize wide-track axles. Depending on the cab and body configuration, axle make and weight ratings and several other factors, departments may be able to specify axles that place the tires further apart. This provides a wider base for the vehicle and increases the angle at which the vehicle would tip over when traversing a side slope or undergoing side forces during turns or sudden evasive maneuvers. Wider track axles may also allow wider bodies, which could possibly lower the overall vehicle heights.

Wide-track axles have several restrictions, however, and departments should discuss the subject with the manufacturer before the vehicle is built.

It's a Matter of Safety

More than anything, paying attention to vehicle center of gravity is a matter of safety. Rollover accidents involving tankers, aerials and other vehicles with high centers of gravity are commonplace and often fatal — we’ve reported many of them in past issues of In Service Online. Before you specify your next apparatus, or before you throw yet one more piece of equipment on top of your current apparatus, think about how the weight distribution affects the vehicle center of gravity.