Feature article: Watch your weight

Max Kvidera | January 01, 2010

Watch your weight

Trailer makers sometimes use a sticker to indicate the optimum location for placing a heavy load, such as a steel coil.

 Knowing how to distribute your cargo when loading will help you maximize payload and keep you running legal.

Years ago, a dry van trailer collapsed near El Paso, Texas. The service manager at the Utility Trailer dealership went to the scene to find out what happened. “There was 177,000 pounds of metal castings that came across from Juarez, Mexico,” recalls Chuck Cole, manager of technical sales for Utility. “It hit a major bump on I-10 and broke in half.”

Though most operators will never pull such a heavy load, today’s trailers can handle loads well above legal limits. They’re also capable of handling weight that is poorly distributed, though not indefinitely. Planning your cargo’s positioning lessens the likelihood of trailer wear and damage. Combined with regular equipment inspections and intelligent driving practices, careful weight placement also will maximize your payloads, reduce component costs and help you avoid overweight fines.

Fred Jones keeps track of past loads to familiarize himself with how a similar product might be positioned on his trailer.

Rod Ehrlich, chief technology officer at Wabash National, says many shippers focus on load planning and distribution. An example is the paper industry, which faces challenges of loading paper rolls that can weigh up to 8,000 pounds apiece into a tractor-trailer that is limited to a 48,000-pound payload. “Those rolls are not going to fill up the trailer,” he says. “How you put those in is very strategic.”

One solution is to position the rolls in a zig-zag pattern, with rolls touching to reduce movement and to achieve optimal weight distribution. “Rolls can be 7 feet to 8 feet tall and subject to falling,” Ehrlich says. “If two of those rolls topple over at the same time, they’ll roll the entire trailer over. That happens fairly often because of a lack of understanding.”

 Cole, who teaches load distribution policies to trailer dealers, says that once an operator knows the weight of his tractor and trailer, the most critical points to consider are the proper locations for the tractor’s fifth wheel and the trailer’s suspension, using a bit of rudimentary math.

He gives the example of an 18,000-pound tractor and a 13,000-pound trailer, which could transport a maximum 49,000-pound load. A typical distribution would be 34,000 pounds to the trailer axles, 34,000 pounds to the tractor’s drive axles and the remaining 12,000 pounds to the steer axle.

When you add the trailer’s 13,000 pounds to the target load weight of 49,000 pounds, you get 62,000 pounds. Subtract the 34,000 pounds of the trailer axles and you have 28,000 pounds, which is the kingpin load. By moving the fifth wheel to the proper position, you will balance the load at 46,000 pounds – drive axles weighing 34,000 pounds and steer axle at 12,000 pounds, which matches the 46,000 pounds when you add the 18,000 pounds of tractor weight and 28,000 pounds of kingpin load, Cole explains.

Fifth wheel placement “is sometimes difficult because you have clearances and other factors to consider,” he says. “The basic equation is X (which equals the ideal fifth wheel location in inches) divided by the tractor wheelbase multiplied by the kingpin load to give you the weight transferred to the steer axle.” Once you weigh the tractor and know the scale weight of the steer and drive axles, you can determine the proper fifth wheel location.

For an example of a tractor with front axle kingpin load of 2,500 pounds, a kingpin load of 28,000 pounds and a 185-inch wheelbase, divide 2,500 by 28,000 and multiply by 185 to get 16.52 inches.

Cole says you first need to know how much your axles weigh with a full fuel tank and your own weight factored in. If you don’t calculate those precisely, you run the risk of being overweight when you hit a scale with your loaded tractor-trailer.

Maintenance for maximum loads

Proper load distribution reduces trailer wear and damage and helps carriers maximize payloads.

The U.S. Department of Transportation requires an annual tractor and trailer inspection by a mechanic trained to DOT standards. You must display a sticker that verifies equipment has been inspected.