There are two different types of weight limits that can apply to the axles found in cars and trucks. One limit refers to amount of weight that each axle is rated to carry safely, while the other indicates the most weight a roadway or bridge can bear from one axle. Axle weight limits that refer to the heaviest load a vehicle can safely carry are typically referred to as gross axle weight ratings (GAWRs), and are determined by vehicle manufacturers. The other axle weight limits are primarily imposed on large vehicles by governmental entities to prevent the sort of stress that can lead to excess wear on a road surface or the failure of a bridge structure.
All cars and trucks receive a GAWR at the time of manufacture, which refers to the amount of weight the vehicle can safely carry. The GAWR refers to a fully loaded condition, including the weight of all passengers and cargo in addition to the vehicle itself. Some vehicles, such as mini motorhomes, can be very close to the GAWR before any passengers or cargo are even loaded. For the purposes of determining axle weight limits, each set of wheels is referred to as an axle. This means that a passenger car with four wheels has two axles, even if there is no physical connection between each set of wheels.
The other type of axle weight limit is typically a regulation or law instituted by a government entity. Many jurisdictions enforce these limits with mandatory weigh stations on major roadways. In these areas, all large trucks may be required to pull over and submit to a weight inspection. If any of the axles are found to be overweight, fines or other punishments may be administered, and the vehicle may not even be allowed to continue until the excess weight is offloaded.
In most cases, axle weight limits imposed by governments refer to individual axles just like a GAWR. The steering axles of a tractor trailer are typically subject to lower axle weight limits than fixed axles are. In some cases the limit for steering axles will be less than half of the regular limit. Some tractor trailers have adjustable fifth wheel hitches, or rear axles, so that the load distribution can be altered.
Axle weight limits are often determined by the axle configuration of a vehicle. A lone axle with single tires typically has the lowest limit, while tandem axles with dual tires often have higher limits. This is due to the different way these axles distribute their loads onto the surface of a road. A single axle with single tires concentrates the entire load onto two points, while a tandem axle with dual tires spreads it over a wider area.