Pound for Pound

| December 11, 2002

If your body is carrying around a few too many pounds, your doctor might politely tell you that you’re out of shape. It’s common in medical speak to refer to people who are “apple” or “pear” shaped. The description is a not-so-cute reference to the outline of your bulges. And it can have serious medical consequences because it refers to places where there is potentially dangerous excess fat in your body. But fat is better, healthwise, in some places than others.

The same is true with your tractor-trailer load. What matters is not only how much weight you’re carrying, but where you’re carrying it.

Jim Feddern, manager of motor carrier enforcement of the Ohio State Patrol and chair of the CVSA vehicle committee, says being overweight on just one axle is likely to net you the same fine as being over gross in most jurisdictions. That’s one reason why troopers so often use portable scales that weigh just one wheel position at a time.

Peter Powell, vice president of marketing at Air-Weigh, adds what you may have already found out that the fine is usually in direct proportion to the amount you’re overweight. Even if your shipper and your addition are right when you figure a load will leave you under 80,000 pounds, you still have to position that load and your axles just right to avoid problems. Poor weight distribution won’t just leave you vulnerable to the wrath of law enforcement – it’ll take a toll on your equipment.

Reviewing the whys
Powell says the reason the DOT created axle weight regulations is partly because of the dramatic impact that an overweight axle has on the infrastructure of the roadbed, and partly because of the Federal Bridge Formula.

“This formula limits the weight on groups of axles in order to reduce the risk of damage to highway bridges,” he says. “Allowable weight depends on the number of axles and the distance between those axles. A bridge is like thin ice on a pond. Walking on the ice concentrates a person’s weight on the small area covered by the individual’s feet, and the ice may break. Lying down, however, spreads the same weight over a much larger area, and the ice is less likely to break.”

Spreading out the weight with axles that are more widely spaced is like spreading the weight out by lying down. Getting axle loadings right won’t just benefit the nation’s infrastructure, it will also benefit you.

Jim Rushe, product engineering manager at Hendrickson International Trailer Suspension Systems says, “When overloaded, air springs or suspension beams can’t hold the trailer as high as it should be. It ends up bottoming out, riding on rubber bumpers instead of air. This means the advantages of air ride have been taken away.”

With either air or spring suspension, overloading will defeat the suspension’s ability to reduce the frequency of the vibrations coming up from the road. This is a basic key to a smooth ride, which minimizes the stresses on both the trailer components and the load itself. Overloaded suspensions are also more likely to create vibrations that would tend to shift the load, which could aggravate the situation further.

Hendrickson Trailer Division’s Howard Adkins, customer and technical service manager, adds, “The weight carried should be well within the limits of the suspension on rough roads. Especially when you move off road, the loading of the components becomes much more dynamic.”

For example, says Adkins, a trucker who runs off road would do well to use a 30,000-pound suspension for an axle he will use to carry 20,000 pounds because the higher capacity of the components “can ease the danger of running over a tree trunk at 30 miles per hour.”

Dave Acker, vice president of sales and marketing of Fontaine Trailer Co., says of overloading axles, “The life of equipment will definitely be shortened. How much depends upon the degree of overloading and the operating environment.”

Air-Weigh’s ComLinks are mounted near the suspension on either the tractor or trailer chassis. This electronic device interprets the signal and displays the actual weight of the axle group digitally so the driver can see it while loading.

Feddern sees the effects of loading while inspecting trucks. “Lightly loaded vehicles last longer, including the braking system. There is little safety factor for exceeding 80,000 pounds.” Feddern points out that one reason to block and brace loads is to keep them from shifting and turning legal weight distribution into illegal axle weights.

Sometimes too little weight can be as much of a danger as too much. Brake system components are geared to axle ratings. Load light and an axle’s brakes will lock prematurely in a panic stop. Even if that axle has an antilock braking system, performance and component life may be degraded.

Acker points out another advantage of specifying higher than standard axle ratings on your tractor. “Increasing the gross axle weight rating of a tractor axle allows more load to be carried by the tractor without exceeding the manufacturer’s rated capacity on its components. On vehicles carrying permit loads, this extra load carrying capacity may allow operators to transport larger overall payloads as well as providing increased flexibility in locating payloads. However, please note that the increased component ratings will not allow for increased axle group allowance under the Federal Bridge Formula used to determine normal legal loadings.”

A similar principle applies with trailer axles spec’ed for higher-than-standard loadings when it comes to permit loads as we’ve already mentioned. But there are downsides to spec’ing a spread axle to take advantage of those higher limits, so you need to make a balanced decision when ordering a trailer. Acker says, “Axle spacing effects trailer span which in turn impacts not only capacities and load distribution characteristics, but empty weights. Increasing axle spacing increases a trailer’s off-tracking and turning radius. It also increases tire scuff wear.”

Now that you know the basic limits, we get into the biggest loading challenge: making it all balance out right.

Hendrickson’s Rushe says that when you shift trailer axles on a slider, “as you move the axle under the load, it always carries more. As you shift the axle rearward away from the load, it always shifts more weight to the tractor.”

When considering trailer axles with separate spring suspensions, “the axle closer to the load carries more.” This happens not only because of leverage, but because of a slight deflection in the trailer chassis where the weight is, which will compress springs a bit farther and increase the force they exert.

Rushe points out that, “When you use air ride, it normally equalizes the load between the trailer axles,” because you have a common height control valve and air supply. The pressure to the air springs will then be the same on both axles, which means exactly the same downforce on both, and the same weight carried. This is the correct method of plumbing the air supply, and it helps simplify loading. Rushe adds, however, that this will only happen if the air ride “is set up correctly. If the trailer has a setup issue, it may not happen.”

Acker laid out some general rules on weight distribution:

“Weight distribution characteristics are determined by two criteria – one, the overall trailer span (i.e. the distance from the center of the kingpin to the center of the trailer running gear), and, two, the location of the payload within this envelope.

“The amount of payload transferred either to the kingpin or the trailer running gear is inversely proportional to the distance from that component to the center of gravity of the payload. In effect, this is a very commonsense property – the farther the center of gravity is from the kingpin or running gear, the less weight that component receives.”

Sliding the fifth wheel shifts tractor dynamics in a similar way. “Weight distribution between a tractor’s steer axle and drive axles functions exactly as trailer weight distribution does,” says Acker. “Sliding the fifth wheel forward is just like moving a center of gravity forward within a trailer span.

“From a practical standpoint, I would suggest drivers think in terms of percentages of the total wheelbase. For example, if the driver is operating a 200-inch wheelbase tractor, a 2-inch movement of the fifth wheel will transfer 1 percent of the total load on the tractor from the trailer kingpin. So if 25,000 pounds is being imposed onto the truck from the trailer kingpin, moving the tractor’s fifth wheel 2 inches forward on a 200-inch wheelbase tractor will add approximately 250 pounds (1 percent of 25,000 pounds) onto the steer axle, and remove the same amount from the drive axles.”

The same basic math would apply, of course, to the trailer wheelbase. Suppose you had a wheelbase from trailer kingpin to the center of tandem trailer axles of 432 inches or 36 feet. If you had a 36,000-pound load and moved it a foot closer to the drives than to the trailer tandems, you’d shift approximately 1,000 pounds forward. In other words, you’d have 19,000 pounds on the drives and 17,000 pounds on the trailer tandems.

Air-Weigh’s Powell says, “Experience on a given vehicle is the only sure way to know, but generally one notch on the fifth wheel slide is worth about 150 pounds. One notch on a 53-foot trailer slider is worth about 500 pounds, but it really differs from one case to another, particularly if the slider hole spacing is different, which it normally is from tractor to trailer and from make to make.”

One problem that crops up is a load like a machine that’s not uniform from front to back. As Adkins of Hendrickson points out, in these cases it’s very difficult to calculate axle weights with complete accuracy, “You would need to learn the trailer weight and know where its center of gravity is. You would also need to know the load weight and where its center of gravity is.” Doing all that can be a tall order.

Hendrickson’s Universal Load Scale Kits offer a simple way to convert air suspension pressure to axle weight to take the guesswork out of loading.

Fine tuning
With a standard suspension system, you must first load the trailer, trying to get the weight as close as you can, and then drive the rig to a truck scale. Once you have the axle weights, you can redistribute it as necessary, shifting the load in the appropriate direction as described. Of course, there is the risk of repeated trips and maybe a ticket before you get set.

When it comes to weight distribution, air suspension can offer special advantages because it adjusts to the weight carried by increasing the pressure in the air springs to maintain a constant height. The pressure is always the same for a given load.

Hendrickson offers a device called the “Universal Load Scale Kit.” It’s an air pressure gauge system that includes a valve to isolate and protect it from air pressure except when the system pressure is stable during readings. It comes in two forms: a liquid filled gauge, and a simple, air-filled pressure gauge.

The air-filled gauge actually reads in pounds per axle. After installing the system, you load the trailer as close as you can to the maximum legal load. Then you drive it onto a scale and weigh the total trailer axle load, and finally divide the total by the number of axles. The gauge is then calibrated by rotating a screw until it reads the actual weight per axle. With the liquid filled gauge, no calibration is necessary. Air-Weigh offers a complete, onboard electronic scale system that can be installed on vehicles with either air or mechanical suspensions. One sensor, which converts the air pressure to an electronic signal with a device called a “transducer,” is installed for each group of air springs controlled by a height control valve. On mechanical suspensions, load cells sense the changing weight using transducers that measure spring deflection. With this system an electronic device interprets the signal and displays the actual weight of the axle group digitally so the driver can see it while loading.

It also communicates with an in-cab electronic scale display through multiplexing and keeps the axle group’s calibration data in its memory. This technology enables operators to drop and hook different trailers without any driver input or recalibration. Once calibrated by initially entering certified empty and loaded weights, the scale then displays the actual on-the-ground weight of an axle group to within 200 to 300 pounds on its digital readout.

Driver Jon Osburn, who is leased to R.B. High Tech Transport of San Carlos, Calif., says he grew tired of driving to a truck scale to get a tare weight, loading, and then returning to the scale to get a loaded weight for the customer. He estimates the cost of his Air-Weigh system as “less than $1,000.” In his view, cost justification involves not only all those scale tickets at $7.50 each, but eliminating what it costs to have to sit until you dump or unload something after having been found to be overweight.

So, what’s clear is that loading for legal weight distribution is a real science. Understanding what’s involved will help you do it better and faster. The bottom line is that it pays to make sure before you hit the highway that you don’t move too many extra pounds in the wrong places.


By the Numbers

The first step is to understand the basic legal limits on the interstate system.
For those running the standard, five-axle tractor-trailer with normally spaced tandem drives and tandem trailer axles, the formula is simple enough. It’s 12,000 pounds on the steer axle and 34,000 pounds on each of the tandems, or 17,000 pounds on each axle in a tandem arrangement. This, of course, adds up to 80,000 pounds.

Where things get more variable is when dealing with spread axles. These allow a substantial redistribution of the weight so you can carry a more concentrated load toward the rear of a spread-axle trailer without exceeding per-axle ratings. They also will help keep weights within per-axle limits when hauling loads where you procure a permit to allow hauling at more than 80,000 pounds gross combination weight.

While the Federal Bridge Formula gets a little dicey as you look at an entire rig, when you look at just two axles, it’s not such a nightmare. Dave Acker summed up the rules this way:

“The Federal Bridge Formula does not establish a minimum distance between axles; instead it rewards longer lengths by providing increased weight allowance. This holds for specific axle groups and overall configuration, but the Bridge Formula does cap gross weight at 80,000 pounds regardless of length.”

The basic weight limits:

  • 12,000 pounds on steer
  • 20,000 pounds on a single axle
  • 34,000 pounds on a tandem with a spread of 8 feet or less
  • 38,000 pounds on a tandem with a spread of more than 8 feet up to 9 feet
  • 39,000 pounds on a tandem with a spread of more than 9 feet up to 10 feet
  • 40,000 pounds on tandem axles spread of 10 feet or more

As you can see, having a spread axle trailer allows you more latitude in distributing your 80,000 pounds provided your trailer axles are rated for the added weight. For example, if you had a long load that was heavy in the rear, you could end up with 40,000 pounds (total) on your two trailer axles, 12,000 pounds on the front axle and 28,000 pounds on the drives.

Comments are closed.

OverdriveOnline.com strives to maintain an open forum for reader opinions. Click here to read our comment policy.