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For the last 100 years, dual tires have been used on the non-steering axles of heavy-duty commercial trucks to increase their load capacity and help maintain vehicle drivability in the event of a flat rear tire. Placing two tires on both ends of a single axle nearly doubles the weight that the axle can carry and allows three properly inflated tires to temporarily carry the weight originally allocated to the four tires if a rear tire loses pressure or goes flat.
Recreational vehicle motorhomes (RVs) continue to travel American highways and pickup trucks have evolved from hard-working commercial vehicles to become daily drivers that occasionally tow fifth-wheel trailers in support of personal hobbies. Many RVs and heavy duty, 3/4-ton and 1-ton pickup trucks also use dual tires on their rear axles. By using the same size tires and wheels singly on the steering axle and dualed on the drive axle, the load capacity of the vehicle is increased, all tires can be rotated periodically and the vehicle can be outfitted with a single spare tire and wheel.
However in order to accommodate dual rear tires, these vehicles are equipped with distinctive wheels, as well as unique front and rear axles. All of their wheels feature extreme offsets to allow them to be installed center-to-center on the rear axle. In effect, the inner wheel uses extreme positive offset to allow the tire and wheel to be placed traditionally on the axle (with its backside over the axle), while the outer wheel uses extreme negative offset and is installed on the axle with its center against the inner wheel's center and its "backside" facing outward.
The wheel width and offset are carefully engineered for the vehicle and the selected tire size. This allows them to match the vehicle's requirements while assuring sufficient clearance is maintained between the pairs of tires on the rear axle so their sidewalls don't rub each other under maximum load when cornering or running over large bumps.
While both front and rear axles are heavy-duty with 8-bolt hubs, the front axles are longer than normal to accommodate wheels with extreme positive offset. The rear axles are sized to fit a pair of tires and wheels on each side between the vehicle's frame to the inside and its flared fender on the outside.
Because maintaining dual tire spacing is critical to eliminate the possibility of the tires rubbing (or typical road debris from being lodged between them), the tire sizes specified for the vehicle cannot be upsized. For that matter, even some Original Equipment (O.E.) applications will prevent snow chains from being fitted.
Additional considerations require that all four tires on the rear drive axle be equivalent. Ideally this means tires should be the same model and have identical remaining tread depths. Any discrepancies between the four tires will result in the taller tire(s) being forced to carry more than their fair share of the load.
Light truck tires that are intended to be used in dual applications have two "Max Load" ratings branded on the tire's sidewall. This is because the load capacity rating of a tire serving duty in a "single" application is greater than the exact same tire being used in a "dual" application. For example, a LT235/85R16 Load Range E tire is rated to carry 3,042 pounds when inflated to 80 psi and used in a single application, but only 2,778 pounds when used in a dual tire application at the same inflation pressure.
This reduction in rated load capacity results in slightly larger tires being specified for the application that can better withstand the additional stresses experienced when a single tire goes flat and the three properly inflated remaining tires are required to temporarily carry the load at reduced speeds to remove the vehicle from immediate danger. This load reduction is not a concern for vehicles originally equipped with dual rear tires because the vehicle manufacturer factored it in when they specified the tire size for the vehicle.
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