Idaho allows tandem loads of 37,800 pounds under a grandfather weight table up to a gross weight of 79,000 pounds, for trucks carrying raw products (lumber, farm, construction material) on both IS and non-IS highways. There is growing use of wide-base tires in Idaho. With Idaho's 600 pounds per inch of tire width limit, some carriers are removing the inside of the dual sets on one axle in a tandem group. Using 11-inch tires, this leaves 66 inches of tire in a tandem axle. This is more than needed to operate at 34,000 pounds (that is, 66 x 600 = 39,600 pounds).
The Montana limits specified for double-trailer combinations by the ISTEA freeze are 137,800 pounds GVW on I-15 between Shelby, Montana and the Montana/Alberta border and 131,060 pounds on the other Interstate highways and a box length of 93 feet on NN highways. For triple- trailer combinations the GVW limit is also 131,060 pounds, but the box length is longer at 100 feet on NN highways. Permits are required to operate at greater than 80,000 pounds GVW on IS highways. These are obtained across the counter for a fee. Once the permit is obtained, the carrier must at any time be able to demonstrate that the vehicle is in compliance with the axle weight limits and Bridge Formula B to on-road enforcement personnel.
The basis of the 131,060 pounds GVW ISTEA cap is that Montana limits all vehicles to a maximum of nine axles and limits total combination length. When combined with Montana's overall length limit of 105 feet (with cab over tractors) and 110 feet (with conventional tractors), and complying with both the inner and outer requirements of Bridge Formula B, 131,060 pounds was estimated by Montana to be the maximum practical GVW for a nine-axle unit at the time of the freeze.
Split tandems are used on five-axle tractor-semitrailers to achieve 86,000 pounds GVW. They are used in interstate weigh-out operations such as lumber haul and in local gravel haul.
Grain haul is often done in Rocky Mountain A-train doubles. These units use long drawbars (16 or more feet) to stretch the distance between axles to achieve a higher allowable GVW in accordance with Bridge Formula B. One recent development is the use of a cab-controlled air pressure device which can stretch or contract the drawbar while the vehicle is moving.
Large truck-trailer combinations are used in Montana. These combinations employ a long drawbar (up to 20 feet) designed to increase the allowable gross weight in accordance with Bridge Formula B. One example is a combination of a four-axle straight truck, with a single lift axle, coupled with an eighteen-foot drawbar to a four-axle trailer, having two sets of wide-base tires on tandem axles with a total length of as much as 110 feet.