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Watch That Ground Pressure

by Jim Park

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Over-weight fines really hurt. You've got gross vehicle weight (GVW) restrictions to contend with, then there are the axle weight limits and the registered gross vehicle weight (RGVW) to worry about, and finally, the Federal Bridge Formula if you operate in the U.S. Depending on the circumstances, you may be legal in up to three, but in violation of the other. The result is a ticket, usually made out to the driver. Very occasionally, the shipper is written up too, but that's rare.

So, how do we manage all these demands? The shipper wants to load 50,000 lb of spaghetti noodles on a truck that already weighs 31,000 lb. If you're U.S.-bound, you've got a problem. You're over gross, so you're going to be over on at least one set of axles, maybe more. Depending on your trip routing, the implications could range anywhere from being fined, then allowed to proceed anyway, or you might be fined and then required to get legal before proceeding. That would mean a spaghetti sale, right there at roadside, and then having to account for the shortage when you arrived at your destination.

If the shipper wanted to load 48,000 lb of spaghetti on your 31,000-lb truck, you'd be under gross by 1000 lb, but you could well be overweight on an axle grouping. That would result in a fine too, but you might be able to redistribute the load and make it legal. Axle weights are a challenge in almost every instance. In this chapter of Trucking 202, we'll tackle staying legal on your axles, and under your gross too.

American weight laws are pretty straightforward, while the weights and dimensions regulations in Canada remain a nightmare. Ontario and Quebec boast the most complex weights and dimensions rules in North America (the Ontario 'handbook' runs 233 pages!). Atlantic Canada has come far in harmonizing weights and dimensions among the four provinces, but they aren't quite there yet. Western Canada has enjoyed a high degree of standardization under the Roads and Transportation Association of Canada (RTAC) Agreement on Weights and Dimensions signed in 1988. RTAC, by the way, is now called the Transportation Association of Canada (TAC). The levels of harmonization are improving, but we're still a long way from a national standard for even the tandem axle. As the table illustrates, the east and the west like 17,000 kg tandems, while Ontario and Quebec prefer 18,000 kg. The variation on tridem axles is even worse. Depending on the spread, you can load anywhere from 21,000 to 28,600 kg (this one is from Ontario, as if you couldn't guess) on a tridem, depending where you are.

And the whole shooting match depends on a number of other factors, so for the purposes of what we're going to do here, we'll refer to the U.S. regulations, simply because they're easier to work with than the zillions of possible Canadian configurations.

Axle Weights: Spreads and Spacing

Regardless of where you haul, there are restrictions on allowable axle weights. The U.S. has adopted 80,000 lb as their gross weight for a five-axle combination vehicle, with the steering axle limited to 12,000 lb, and tandem axles limited to 34,000 lb, for use on the Interstate highway system, and on certain other designated highways. That weight limit is by no means applicable to all highways so local limits should be checked before venturing down a path of no return.

The U.S. Department of Transportation created axle-weight regulations in part to minimize the pavement damage an overweight axle can cause, and partly because of the Federal Bridge Formula. The formula limits the weight carried by groups of axles in order to reduce the risk of damage to highway bridges. The allowable weight depends on the number of axles and the distance between those axles. Spreading the weight out allows the pavement to recover from the passing of the first group of axles before flexing under the next.

The formula, which can be found near the front of most motor-carrier-type road atlases, is helpful if you've got a less than typical truck/trailer configuration. In order to haul a gross weight of 80,000 lb, the distance, according to the Bridge Formula, required between the first and last of five axles on the truck must be at least 51 ft. That puts all of today's conventional tractors and 53-ft trailers well within the range.

It becomes a problem for the folks who haul those 20-ft container chassis. A 20-ft chassis pulled by a short-wheelbase tractor might only be allowed something like 73,000 lb. It could also become a problem for a trailer with a long sliding tandem. If the bogey sits fully forward, depending on the spacing, the bridge law may permit only something like 77,000 lb. So if you're rigged to run in some unusual configuration, you had better figure out how to use the Bridge Formula table in your atlas. It's not terribly complicated, but most drivers overlook the implications, focusing instead on their allowable axle weights.

Axle spacing and spreads have some bearing in Canada as well, particularly in Ontario. That province is known for its bizarre combinations and configurations, and most truckers from outside the province can't even conceive how they were arrived at. I hate to say this, but if you're running something out of the ordinary, you should consult with Ontario's weights and dimensions guidelines. And if you can figure them out, you're a better trucker than I am.

Back in the U.S. once again, their basic weight limits are easy to follow:

  • 12,000 lb on the steer axle
  • 20,000 lb on a single axle
  • 34,000 lb on a tandem with a spread of 8 ft or less
  • 38,000 lb on a tandem with a spread of between 8 ft and 9 ft
  • 39,000 lb on a tandem with a spread of between 9 ft and 10 ft
  • 40,000 lb on tandem axles with a spread of 10 ft or more

The gross is limited to 80,000 lb, but the extra allowable weight on the spread trailer tandem gives you more flexibility. For example, if you're grossing 79,000, you may be loaded to 10,500 on the steers, 29,500 on the drives, leaving you 39, 000 lb on your 10ft -1in.-spread-tandem. You're still legal! It's all in the Bridge Formula table.

Watch Your Ground Pressure

Putting on a heavy load when you're nowhere near a public scale is always a nerve-wracking experience. How are you supposed to know if you're legal or not? If you know how to load the truck, you have a better chance of coming away legal than if you just stuff the freight in and hope for the best. Here's how to calculate your axle weights, even before you load.

First, you need to weigh the truck empty (tare weight), but full of fuel. If you drag the same trailer around regularly, you're miles ahead here, but if not, repeat the process with each different trailer configuration. Position your fifth wheel slider so that the kingpin is dead center between the drive axles. Next, set the tandem sliders to center on a point that is the same distance from the rear of the trailer as the distance between the kingpin and the nose of the trailer (on a 53-footer, the two centers should be 41 ft apart, or about 6 ft from either end). In this configuration, your centers of balance (the kingpin and the center of the trailer tandems) are the same distance from the mid-way point in the trailer. Having the fuel tanks full represents a worst-case scenario, weight wise. Anything less than full when you really load, and you're money ahead.

Let's say your empty axle weights come out as follows: steers: 10,500 lb; drives: 14,500 lb; trailer tandem: 8000 lb. Now, let's calculate possible payload.

12,000 34,000 34,000 - 80,000 allowable gross weight
10,500 14,500 8,000 - 33,000 tare weight
1,500 19,500 26,000 - 47,000 possible payload

We now know we can load 47,000 lb on the truck, but the numbers show that the weight cannot be distributed evenly from front to back. To place half the load in the front, half in the back, the drives would be too heavy and the trailer axles too light. You should always load the rear of the trailer heavier than the front because it's lighter at the rear when empty.

The way to work this out is to determine the mid-point of the trailer, halfway between the kingpin and the center of the bogeys. From there, you know you can load 19,500 lb in front of that line, and 26,000 lb behind the line.

This theory will also work with material loaded on top of spacers, as on a flatdeck, if you remember to place the spacers ahead of and behind the center line as indicated.

Experience will make your decisions easier as time goes on, provided you make notes after you load and weight the truck. You'll learn from experience where the weight goes as it's placed on the trailer. For example, if you loaded only the front of the trailer, as illustrated below, you'll see that some of the weight is picked up by the rear axles. Try to think of your weight distribution exercise in terms of percentages, rather than pounds or kilos. It makes the visualization easier.

If 80% of the weight in the front is carried by the drives, then 20% will be transferred to the rear bogey. And since the rear portion of the load sits further forward, the percentage will be different, perhaps 30%. Experience will help in working this out.

This will work with a B-train as well, but you'll have to determine how the weight on the lead and the pup transfer to the tridem. Experience and good notes really help.

When all is said and done, you can fine-tune your weight distribution by sliding the rear bogey and the fifth wheel as needed. By sliding the fifth wheel forward, you'll be transfering weight from the drives to the steering axle. Each time you slide your fifth wheel, make note of the before-and-after weights on the steer axle to determine how much weight is transferred for a given distance. For example, you may find that each inch of forward movement transfers 200 lb to the steer axle, so sliding forward eight inches will transfer 1600 lb to the steer axle. This will vary from truck to truck, so make notes.

The same applies to the trailer bogey. Sliding the bogey forward will transfer weight onto the bogey (lifting it off of the drive axles); sliding it back will lighten it (transferring weight onto the drive axles). Make note of how much the weight shifts for each notch on the slider for future reference. As a starting point, you can assume that an inch of movement, either way, will shift about 100 lb, but each situation will be different.

This may all sound ridiculously complicated, but once you've thought the process through and done it a few times, it will all make sense. Just remember to axle-weigh the truck when it's empty and full of fuel, with the axles centered as described above; remember that the rear always loads heavier than the front. And to distribute the weight evenly, think of loading the front half of the trailer to the weight available on the drives, and think of loading the rear half of the trailer to the weight available on the trailer axles.

Additional W&D Resources

Canadian Transportation Equipment Association
Atlantic Canada
British Columbia
New Brunswick
www.highways.gov.sk.ca/docs/trucking/M006 SkHiwayseightDim11.5.pdf
Ontario/Quebec Agreement

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