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The gross axle weight rating (GAWR) on any vehicle is defined as the maximum load an axle can support per a manufacturer's specifications. A vehicle's GAWR is the lowest of any ratings for parts of the axle, including the axle itself, tires or other components. Exceeding the gross axle rating can create handling problems and void manufacturer's warranties.
A four-wheeled vehicle such as a passenger car or light truck normally has two axles. The front and rear axles may have slightly different GAWRs, depending on design. Front axle ratings may also include steering components such as tie rods, steering arms or other parts. Tires may be the same on all wheels or different for front and back axles, which will affect the rating.
When the front and back axle ratings are added together, this is the total axle weight rating, but may not be the gross vehicle weight rating (GVWR). Other factors such as the engine or frame design may reduce the gross vehicle weight rating, which is determined by the manufacturer. An owner who needs to know the useful load of a vehicle can subtract the empty weight of the vehicle from the GVWR.
Trucks and cargo vans may have greatly different front and rear axle weight ratings. Higher rear GAWR allows cargo to be added to the vehicle without exceeding the rating for the rear axle. Cargo should be placed both in front and rear of the rear axle when loading the truck or van. This transfers some of the load to the front axle to prevent overloading the rear.
Trailer towing requires knowledge of the GAWR of the tow vehicle. When too much weight is placed on the rear of the vehicle, the axle weight rating may be exceeded before passengers or cargo have been placed in the vehicle. A weight-distributing hitch can be used to transfer some weight back to the trailer axles, and forward to the front axle of the vehicle.
When purchasing a trailer, it can be useful to take the tow vehicle and trailer to a commercial scale. These scales can weigh each axle separately, and this number can be used to check the real weight against the GAWR. Having knowledge of the actual trailer and tow vehicle weights can help in loading the vehicles with balanced loads.
Adding too much weight and exceeding the gross axle or vehicle weight rating can result in an unsafe vehicle. Overloading the rear axle can cause premature wear of tires and bearings, or failure of the wheels or axle. This same load can reduce the weight on the front axle and potentially cause unsafe handling.