Feature article: Watch your weight

Max Kvidera | January 01, 2010

Jones, who owns a 48-foot spread axle flatbed with a tarping system, says knowing your trailer’s center point is critical in determining placement. If you’re handling full-length cargo such as lumber or steel, you can load it evenly on the trailer from front to back. If you have a “split load,” such as pallets, you can start at the center and load to the front and to the back.

Bulking up in the middle, however, can lead to another problem. “When going around corners, especially in mountains, the trailer will [sag] in the middle and have a tendency to tip over,” Jones says. “It’s more work, especially with a tarp, but if you spread out the load and put half in front and half in the back over your tires, it rides better, the trailer doesn’t bounce in the middle and it corners easier” and more safely.

Bow keeps notes about machines he’s loaded on his two double-drop trailers. “With a double-drop trailer, you have to belly-load,” he says. “If you can get a machine at one end, you can load miscellaneous equipment and parts at the opposite end to balance it out.”

Loading vans

Heavy loading and unloading wears out a floor. On a wood floor, fibers break down, Ehrlich says. In metal, fatigue shows up as cracks.

Axles often last 20 years or more before fatigue is apparent. However, if an axle is repeatedly overloaded or subjected to corrosion from road materials, fatigue could come early.

Much trailer damage happens during loading and unloading, especially when forklifts are employed. A loaded forklift can put severe stress on floors and walls. Ehrlich says a forklift falls through a floor somewhere daily.

Occasionally, poor weight distribution on rear trailer axles will cause fishtailing, says Adam Hill, of Great Dane Trailers’ engineering department. “I’ve seen it when they move product from the front to rear as they offload product so it’s more convenient for the next stop,” Hill says.

The problem occurs in air ride suspension when there is upward and backward movement of an axle on one side of the trailer and a downward and forward movement on the other side. n


Bridge law determines legal weights

Federal Bridge Formula, also known as bridge law, has been an integral part of trailer load distribution since 1975, when Congress enacted the formula to limit the weight-to-length ratio for a vehicle crossing a bridge.

The formula establishes the maximum weight a set of axles may carry on the Interstate Highway System.

Compliance with the formula can be achieved by one of two approaches – either spreading weight over additional axles or extending the distance between axles.

Beyond the formula, U.S. law limits single axles to 20,000 pounds and tandem axles closer than 96 inches to 34,000 pounds. At 121 inches apart, two axles may carry 40,000 pounds and three axles may hold 52,000 pounds. Gross vehicle weight is still limited to 80,000 pounds regardless of axle spacing.