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10:11 pm EST        32°F (0°C) in Harrisburg, PA

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I managed to make it here to Pennsylvania without picking up a citation and possible hefty fine for being overweight. (And no, we’re not talking about my gut here. smile) The shipper of this load did such a horrible job of loading the product in the trailer that no matter what I did, I could not get my drive axles below 34,280 lbs. (15,549 kg). I’m going to have to get into a bit of technical trucking detail here to explain.

A bit of federal regulation known as the “bridge law” sets out the various parameters that determine legal axle weights for various configurations of axles. The bridge law states that regardless of the number of axles, the maximum allowable gross weight for any vehicle (or combination of vehicles) on the Interstate system is 80,000 lbs. (36,287 kg) (although a few states, mostly in the Mountain West region, ignore this and allow higher gross weights). Additionally, the bridge law formula dictates that two axles no more than 8’ (2.44 m) apart are defined as a “tandem axle set” and may carry a maximum load of 34,000 lbs. (15,422 kg).

Most standard over-the-road tractor-semitrailer combinations have a total of five axles: one up front, called the steer axle; two at the rear end of the tractor called the drive axles, which also support the front end of the trailer; and two somewhere near the rear end of the trailer, called (appropriately) the trailer axles or “tandems.” The drive-axle and trailer-axle pairs on many trucks and trailers meet the definition of a tandem axle set I mentioned in the previous paragraph, meaning that each of these axle pairs is permitted to weigh up to the aforementioned 34,000-lb. (15,422 kg) limit.

Here, however, there is yet another twist to the bridge law. In order for these four axles to be allowed to carry the maximum 68,000-lb. (30,844 kg) combined load, the front-most drive axle and the rear-most trailer axle must be at least 36’ (10.97 m) apart. This is actually rather easily accomplished, though, because the axle-sliding systems on trailers only go so far forward (and even at the most forward position of the axles, this 36’ requirement is still met).

As I alluded to in the previous paragraph, all tandem-axle trailers are constructed in such a way that the axle pair can be moved back and forth relative to the “box” of the trailer, so that axle weights can be changed without having to move freight around inside the trailer. It is a trade-off: moving the trailer tandems forward takes weight off the drive axles and puts it on the trailer axles, and moving them back shifts more weight to the drive axles. With the trailer tandems in a more forward position, the turning radius of the tractor-trailer combination is reduced, but tail swing becomes more of a concern with more of the trailer’s length behind the rearmost set of axles. Likewise, moving the trailer tandems back increases the combination’s turning radius and decreases maneuverability, but also reduces or eliminates the tail swing problem. Many states impose a maximum limit on how far back the trailer tandems can be set on 53’ (16.15 m) trailers, mostly to keep the turning radii of tractor-trailers from getting insanely large.

In any case, when I first drove up on to the CAT Scale at the Pilot Travel Center location in Monroe, MI, and got my weight, I had nearly 37,000 lbs. (16,783 kg) on the drive axles, and just under 28,000 lbs. (12,701 kg) on the trailer tandems. Obviously, I had to move the trailer tandems forward to shift weight from the drive axles to the trailer axles, but even when I put the trailer tandems in their most forward position, I still had the aforementioned 34,280 lbs. (15,549 kg) on the drive axles. Dispatch told me to run with it that way, and told me I would be covered by the company in the event of any overweight fines, so that’s what I did. I also got them to permit me to run the Ohio Turnpike and Pennsylvania Turnpike at company expense, so that I wouldn’t have to use roads such as Interstate 80 in Pennsylvania on which I would encounter weigh stations. (It was bad enough already having to evade one weigh station on Interstate 75 at Luna Pier, MI, by taking the parallel U.S. Route 24 to bypass most of the Monroe County segment of I-75.)

You might be asking, “what about the steer axle?”, at this point. Since the gross weight limit is 80,000 lbs. (36,287 kg), and the drive and trailer axle pairs are limited to 34,000 lbs. (15,422 kg) each, that by default leaves 12,000 lbs. (5,443 kg) for the steer axle. However, only a handful of states (if memory serves, Louisiana, Arkansas, Mississippi, Montana, North Dakota, and Vermont) have an actual 12,000-lb. limit for steer axles; in most places, you’re fine being over 12,000 lbs. on your steer axle so long as you’re under 34,000 lbs. on the drive and trailer axles and under 80,000 lbs. gross weight.

Readers who are a bit more knowledgeable about trucking will notice that I have neglected to mention two other common features: (a) sliding fifth wheels, and (b) spread-axle trailers — the reason being that my current company has neither of them. For those who have no clue, the fifth wheel is the large (usually cast-iron) plate atop the tractor frame that supports the weight of the front end of the trailer; it also latches securely around the trailer’s kingpin to keep the combination together. A slider system on a fifth wheel allows for changes in weight distribution between the steer axle and the drive axles, although in practice, far larger weight shifts can be accomplished between the drive and trailer axles by sliding the trailer tandems.

A spread-axle trailer is just that — instead of having the two trailer axles stuck together as part of one moveable unit, a spread-axle trailer allows the two axles to move independently. If the driver sets them more than 8’ (2.44 m) apart, they are considered to be single axles that can legally carry 20,000 lbs. (9,072 kg) each, meaning that the rear end of the trailer is now good for a total of 40,000 lbs. (18,144 kg) instead of the 34,000-lb. (15,422 kg) maximum rear-end weight on a standard trailer. Typically, spread-axle systems are used on refrigerated trailers (“reefers”) because the roughly 1,500-lb. (680 kg) refrigeration unit, which is mounted on the nose of the trailer, reduces the available payload capacity in the front end, and spread trailer axles increase rear-end payload capacity to compensate; however, spread axles can also be found on flatbed trailers, which often must carry freight with highly irregular shapes and weight distributions.

After this load comes off, it’s back to Michigan with one more load to deliver Friday morning before a few days off for Christmas.