# Calculating trailer capacity

05 March 2019

Editor’s note

This is the second article in a two-part series about considerations that need to be made when calculating trailer capacity.

To select the right trailer, the one that will require the least amount of maintenance, provide the greatest lifespan and deliver the highest possible return on investment, it is vital to understand the most important factor of all – capacity.

Carriers can vary gooseneck lengths to achieve proper steer weight and drive axle weight, and alter the distances between axles and axle groups to hit max weights and remain in compliance with state laws.

Taking up where we left off in the February 2019 Rigging Review column, a trailer’s safety rating also comes into play when discussing capacity. Without understanding the safety rating, it’s difficult for a carrier to get a clear, comprehensive picture of true capacity.

A safety rating is an indicator of how much stress a trailer can safely handle. It encompasses the strength of the raw materials and components that a manufacturer has put into the trailer’s design and construction, how the beams and cross members are configured and so on.

A widely accepted average magnification of payload weight on a trailer due to road dynamics is 1.8 to 1 ratio. When a driver is rolling down the road, the truck is hitting chuckholes, bouncing over bumps and crossing railroad tracks along the way. On average, the stress placed on a 50-ton-rated trailer by a 50-ton load when the rig hits those bumps, chucks and tracks equals 1.8 times 50 tons, or 90 tons. It’s important to keep in mind that the 1.8 multiplier is only an average. On any given haul, the stress placed on the trailer can go above that level multiple times. If no cushion is built in to the trailer to handle those spikes in stress, there will be more potential for long-term, progressive structural damage.

For the most capacity and smallest impact on weight, some manufacturers use a T1 material with 100,000 psi minimum yield. T1 has maximum strength versus ductility.

Since there is no universally prescribed level for safety ratings, they vary from manufacturer to manufacturer. They range from a safety factor of 1.8 to 1, which allows for no margin, to an industry high of 2.5 to 1, which is considered an ample cushion for even the most extreme road dynamics a trailer might encounter.

Not only does the safety rating tell a carrier how strong his trailer is, it’s also a very good indicator of potential life. The greater the difference between the static design safety factor, be it 2.5 to 1, 2 to 1 or some other factor, and the dynamic 1.8 average multiplier, the longer one can expect a trailer’s useful life expectancy to be.

Most trailer manufacturers do not like to advertise safety ratings, often for good reason. To understand why, it’s helpful first to comprehend what a safety rating is not. A safety rating should never be used to determine how much weight can be added over the capacity rating and still keep the load, the driver and other drivers safe. In other words, a 2.5 to 1 safety rating on a 50-ton lowbed should not be used to justify loading a trailer with 125 tons of cargo. Just as the deck rating indicates, that trailer can safely handle a 50-ton payload under dynamic conditions.

The sum of the parts

Safety factors are strongly related to the quality of the components incorporated into the trailer, and that gets down to the nitty-gritty of the steel and the deck material.

Manufacturers have several options when choosing steel. For the most capacity and smallest impact on the trailer weight, some manufacturers use a T1 material with 100,000 psi minimum yield. T1 not only has maximum strength versus ductility, but also equates to a lighter, stronger trailer frame over other materials.

A trailer’s decking is continually exposed to the elements, making durable decking with a long wear life also crucial. Apitong decking provides a tougher, longer lasting wood in comparison to other varieties, such as oak or pine. Tightly woven and incredibly dense, apitong is less susceptible to chipping and cracking and provides some amount of traction in comparison to a smooth metal surface.

Sometimes trailer owners or operators don’t completely understand the capacity rating and mistakenly put more weight on the trailer than it is built to handle, or more weight in a concentrated area than it can handle.

Apitong decking provides a tougher, longer lasting wood. Tightly woven and dense, apitong is less susceptible to chipping and cracking.

Overloading a trailer once is not likely to make it break. Overloading it twice probably won’t either. But do it consistently over time and eventually it will likely fail.

When a trailer breaks, the manufacturer hears about it. But often the complaint will be that a 50-ton trailer broke when it was hauling less than a full payload. For example, a 50-ton trailer might break when it’s only hauling a 40-ton payload. The reason is not in that 40-ton load, but the number of times the trailer previously was overloaded and by how much each overload exceeded the yield strength of the trailer’s load-bearing beams and cross members.

Yield strength refers to the amount of stress that the steel can withstand and still return to its original shape when unloaded. The first time a trailer is overloaded, the steel will return to something close to the shape it was in when it came off of the manufacturing line, but not exactly to that original shape. Each time it’s overloaded, the steel will return to something less than the shape it was in the time before. Eventually, the steel, and therefore the trailer, will break.

So in our example, even though the trailer only carried a 40-ton payload, it is likely that it had been overloaded to some extent at one or more points in its past. That’s when the damage was done, and that is what ultimately caused the failure. The steel got weaker and weaker and, on the day the trailer failed, it happened to be rolling with 40 tons on the deck.