Pound for Pound
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.”