Life and Family

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Weight and Balance

by Jim Park

We covered this issue to some extent in our April edition (Careers, ‘Axle Weights and Staying Legal’), but it seems I was a little off the mark in my theory for loading to legal maximums. Ray Camball of Trailmobile Canada called me to say that while I was on the right track, I was wrong in simply suggesting that drivers parcel out the load and place so much here and so much there. He kindly agreed to help out in providing a more precise approach to the exercise. So, with Ray’s help and perspective, we’re going to take another look at axle weights here. And for clarity’s sake, we’ll be using American axle-weight limits in most of our examples. Some Canadian examples appear in the chart on p34/35.

Axle-weight violations are costly, and they do affect a carrier’s safety ratings in many jurisdictions. But there are other aspects to the issue as well, like equipment spec’ing and vehicle handling characteristics, to name just a couple. Trailer makers use weight-distribution software to calculate axle weights under different payload and vehicle configuration conditions when engineering trailers. It’s easy to load a truck right to the pound in a computer simulation, but it’s a bit tougher to eyeball the load to the legal maximum. With the help of Trailmobile Canada’s Ray Camball’s, we’ll explain how to load for better axle-weight distribution. Knowing how to load could save you a bit of dough on your next trip across a scale or when spec’ing your next truck or trailer. In aviation, this exercise is called the ‘weight and balance’ calculation. It’s the art and science of properly distributing the weight of the passengers and their baggage on an aircraft. You can’t, for example, put all the passengers in the front of the plane because you’d never get the nose to lift for take-off. Where weight is placed on a trailer can also make an incredible difference, like 5000 to 10,000 lb in lost payload capacity in a worst-case scenario, Camball says. You might think that distributing the weight evenly from front to back in the trailer will balance the weight borne by the drive and trailer axles, but that’s usually not the case. Placing the load on the trailer in such a way as to split the weight evenly on either side of the trailer’s mid-point will usually result in a larger portion of the load’s total weight being borne by the trailer axles. Knowing where to ‘center’ your load on the trailer can make all the difference between a legal load, and an overweight fine.

Axle-Weight Analysis
Before we start mapping out load placement, it would be worth looking at some of the factors that affect payload capacity and weight distribution. Trailer kingpin placement is one of the more significant ones. You’ll have probably noticed that multi-axle trailers have shallow pin placements, 24 in. or so, less in some cases, while the lead trailer of a set of B-trains tends to have a deeper kingpin setting – commonly around 42 in., with some as deep as 60 in. or more.

The shallow setting causes more of the weight to ride on the trailer axles, which is desirable in a tri- or quad-axle application. The deep setting on a B-train lead is designed to keep the weight on the tractor and off the center axle grouping. Often, the pup has a very shallow setting, intended also to keep the weight off the center axles by letting the pup tandem do most of the work. This is especially true in the case of a seven-axle train, while it’s not so much of an issue with the eightaxle combinations.

“The 36-in. kingpin setting typical of 53-ft trailers is a compromise, and a throwback to the era of 48-ft trailers. The 36-in. pin worked well with the 48-footers, but the 53-footers really should have a 42-in. pin setting,” Camball says. “When 53-ft trailers were introduced, a few jurisdictions had old regulations on their books limiting the kingpin to 36 in. and we pretty much decided to live with that rather than go through a long and tedious process of convincing them to change their rules. Now we have to compromise by placing the center of the payload [weight-wise] ahead of the center line of the trailer to overcome the effects of the ‘wrong’ kingpin setting.”

By loading ahead of center, you remove weight from the trailer axle and place it toward the front where, by sliding the fifth wheel forward, you can shift weight off the drives and onto the steer axle.

There are obvious limits to the weight one can stuff onto a trailer; your payload will be dictated by the gross combination weight (GCW) limit minus the tare weight of the truck and trailer. This creates pressure to maximize payload by spec’ing the absolute lightest unit possible – using fewer crossmembers or vertical posts, for example, or spec’ing a lighter pick-up plate, etc. But by making better use of your existing carrying capacity, Camball says you can often get away with spec’ing a heavier and more durable tractortrailer combination.

To gain a better understanding of how much weight you’re actually hauling, and how that weight is distributed on the axles, every load should be weighed on an axle scale. You might find that you typically run light on the steer axle, light on the drives, but heavy on the trailer. This would suggest the weight is sitting too far back in the trailer. If you’re heavy up front and light on the back, the weight might be too far forward. The difference between the lightly loaded axle and its maximum carrying capacity is lost payload potential that could be accommodated by simply rearranging the freight. Being aware of how the weight is distributed ‘naturally’ is the first step toward learning how to position the weight where it should be to stay legal and to maximize capacity.

While a few overweight tickets may prompt equipment buyers to lurch toward a lighter spec, simply learning to load the truck properly could be a wiser, less expensive alternative to the lighter spec.

Referring to the payload capacity chart on p. 34/35, the examples also illustrate how much payload capacity can be pulled out of two identical units just by loading them properly. As illustrated in example 13, switching from steel to aluminum wheels and hubs, going with low-profile tires, and skimping on the weight of the structural parts, wouldn’t make up one-quarter of the difference gained by just positoning the load forward a couple of feet.

The same applies to overweight fines: examine the tickets to see where you’re heavy, and adjust your loading strategy to overcome the problem. Of course, if you’re over gross, there will need to be some other adjustments made, which we can’t help you with.

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