Some departments experience apparatus problems because they don't understand how vehicle weight ratings are determined or because they think they are exempt from weight restrictions. The result often are poor performance, broken components or even accidents.

A basic understanding of weight ratings can help avoid many problems when departments specify new apparatus, as well as after they put the apparatus in service. Here are some basic ratings and what they mean.

Gross axle weight rating. The GAWR is the amount of weight a particular axle is designed to support. It is determined by the lowest weight rating of the frame, axle, suspension, steering (for the front axle), hubs, brakes, wheels and tires. The GAWR is usually stamped on a plate inside the cab door frame.

Gross axle weight. The GAW is the actual weight on a particular axle. It is determined by weighing the front or rear axle of a fully loaded vehicle. The GAW must be less than the GAWR to be safe.

Gross vehicle weight rating. The GVWR is the sum of the front axle and rear axle GAWR minus any allowances for frames and other factors. It is the total weight a vehicle is designed to support. The GVWR also is usually stamped on a plate inside the cab door frame or elsewhere on the vehicle.

Gross vehicle weight. The CVW is the actual weight of a vehicle. It is determined by summing the actual weights on each axle of a fully loaded vehicle or by weighing the entire vehicle at one time. The GVW must be less than the GVWR to be safe.

Gross combination weight rating and gross combination weight. Tractor-trailer vehicles, such as tillered aerials and some rescue, hazmat and tanker configurations, use CCWR and GCW instead of GVWR and GVW. The methods of determination are similar.

Side-to-side distribution. Weights must be distributed evenly on both sides of a vehicle to be safe. Putting too much weight on one side can overload the components on that side, even if the total weight is within limits. NFPA 1901 sets standards for variations in side-to-side weight distribution.

Front-to-rear distribution. Vehicle manufacturers define how the total vehicle weight must be distributed between the front and rear axles. NFPA 1901 sets additional standards. Underloading axles can cause as many problems as overloading them.

Fully loaded vehicle. A fully loaded vehicle includes the weights of the chassis and body, plus the weights of all the hose, equipment, tools, supplies, personnel, water, foam and fuel. All tanks must be full. All equipment and hose must be carried in its specified place to give accurate weights.

Where departments go wrong

Looks pretty simple, doesn’t it? Everything has a weight rating, and the actual weights can't exceed those ratings. Despite this fairly straight-forward approach, it’s easy for departments to go wrong. Here are some examples.

Ignoring weight restrictions because they don’t apply to fire departments. This oft-used excuse is wrong. NFPA standards include several weight restrictions and are usually accepted by courts. Chassis and apparatus manufacturers also specify weight restrictions. Even if vehicle laws don’t apply, the laws of physics do. Overloaded vehicles, or those with poor weight distributions, are potentially subject to poor handling, poor braking, component failures and accidents.

Carrying too much equipment. Many departments don’t take the time to weigh all their equipment. Instead, they rely on the miscellaneous equipment allowance specified in the NFPA standards. Sometimes that isn’t sufficient — especially when equipment is installed or added after the apparatus is delivered. Annex C of NFPA 1901 and the FAMA Weight/Cube Calculator can help departments make more accurate weight allowances for equipment.

Selecting replacement components based on fit or finish, rather than weight ratings. Departments need to be careful when replacing components such as tires and wheels to ensure that the new components have equal or greater weight (load) ratings than the old ones. Rated tire loads depend on the design, size, inflation pressure, brand and other factors. Rated wheel loads depend on the material, design, offset and brand. Compare the ratings before you buy.

Looking at the whole, rather than the parts. Some departments simply check the overall weight of a vehicle, rather than checking the weight on each axle separately. Individual axle weights and the front-to-rear and side-to-side weight distributions also are important and need to be checked.

Modifying older vehicles. Many smaller departments use older commercial trucks and surplus military vehicles to build fire apparatus. They feel if a truck was good enough for one service, it’s good enough for another. That may be right, or it may be wrong. Departments that build their own apparatus need to consider many factors — weight is one of them.

NFPA weighs in

The new NFPA 1911, Inspection, Maintenance, Testing, and Retirement of In-Service Automotive Fire Apparatus, 2007 Edition, requires that all frontline and reserve fire apparatus, regardless of their year of manufacture, must be weighed once a year. It defines how the apparatus is to be loaded and requires that each axle be weighed separately and then the entire apparatus be weighed as a whole. If the weight on any axle, or on the vehicle as a whole, exceeds the corresponding GAWR or GVWR/GCWR, then equipment must be removed or re-arranged until the weights are below the rated values. If that cannot be done, the vehicle must be taken out of service immediately.

What was once just a good idea is now a requirement. Departments that continue to run overweight or poorly balanced vehicles will not only incur potential problems, they may also incur potential liabilities.