Axle Load Distribution

Distribution of weight on a tractor-semitrailer

A semitrailer is supported at two points: the kingpin, which rests on the fifth wheel of the tractor, and its own axle group. The weight of the cargo is divided between these two points in inverse proportion to its distance from each. Cargo positioned directly over the trailer axles is carried entirely by them; cargo positioned at the kingpin is carried entirely by the tractor; and cargo midway between them is divided equally.

The tractor in turn divides the weight at the kingpin between its steering axle and its drive axles, according to the position of the fifth wheel between them. The closer the fifth wheel is to the drive axles, the greater the proportion of the kingpin weight they carry.

Effect of the position of the load

Where the load is positioned too far forward, the tractor carries too great a share of it: the drive tandem may exceed its limit of 34,000 lb, or the steering axle its rating, while the trailer tandem remains well within its limit. Where the load is positioned too far to the rear, the trailer tandem may exceed its limit; in the European Union, the driving axles may also fall below the minimum proportion of the total laden weight they are required to bear.

Either condition may arise while the gross vehicle weight is within its limit. The gross weight limit governs the total weight; the axle limits govern how that weight is distributed.

Means of adjusting axle loads

• Repositioning the cargo. Moving the load toward the rear of the trailer transfers weight from the tractor to the trailer axles, and moving it forward has the opposite effect. Heavy items positioned over the axle groups, with lighter items between them, assist in keeping each group within its limit.
• Adjusting the trailer tandem. Many semitrailers in North America have a sliding axle group, which is locked in one of a series of positions. Moving the tandem forward transfers weight from the kingpin to the trailer axles. It also reduces the distance between the drive and trailer axles, which reduces the weight permitted by the bridge formula.
• Adjusting the fifth wheel. On a tractor with a sliding fifth wheel, moving it forward transfers weight from the drive axles to the steering axle, subject to the limit and rating of the steering axle.

Stacking and stability

Heavy items should be loaded on the floor and lighter items above them. A light item beneath a heavy one is liable to be crushed, and a high center of gravity reduces the stability of the vehicle. Each stacked item should be supported by a solid surface beneath most of its base, and items that cannot bear weight should be loaded on top or without other items upon them.

Determining axle loads before loading

A scale ticket records the axle weights only after the vehicle has been loaded. The same weights may be determined in advance from the position of each item, the empty weights of the tractor and trailer, and the positions of the axles. MaxLoader performs this calculation for a complete cargo list: it places the cargo, calculates the load on every axle and axle group, checks each against its limit and identifies positions of the load and the trailer tandem that comply. The calculation is based on the following assumptions.

• Loads are static and the vehicle is on level ground.
• The semitrailer is supported at the kingpin and at the center of its axle group; the tractor at its steer axle and the center of its drive group.
• Weight is distributed equally between the axles of a group, as an equalizing suspension is designed to do.