Specialized trailers explained: lowboys, RGNs, spread axles and the bridge formula

0:00Why do specialized trailers sit so low?

Why do specialized trailers sit so low? Because the limit that matters most is often not weight. It's height. There's no federal height limit for trucks. Each state sets its own: 13 feet 6 inches in most of the East and Midwest, 14 feet across much of the West, and 14 feet 6 inches in a few states, like Colorado and Nebraska.

Every inch of deck height is an inch your freight can't use. A flatbed deck sits about 5 feet off the ground, so under a 13-foot-6 limit, your freight can stand about 8½ feet tall. A step deck's main deck sits about 3 to 3½ feet up, which buys you about 10 feet. And the well of a lowboy rides just 18 to 24 inches off the road, leaving room for freight close to 12 feet tall, without an oversize permit.

0:53Step deck, double drop, lowboy or RGN: what's the difference?

What's the difference between a step deck, a double drop, a lowboy and an RGN? A step deck has one drop. A short upper deck rides over the tractor, then the main deck steps down and runs all the way back over the trailer's axles. A double drop has two. It drops behind the gooseneck into a low well, then steps back up over the axles at the back. That well is where tall freight rides, and on most trailers it's about 24 to 30 feet long, which often decides whether a machine fits.

Lowboy is the heavy-haul name for that same double drop shape. And an RGN, a removable gooseneck, is a lowboy whose front comes off, so machines can drive straight on from the front. So you're never choosing between a lowboy and an RGN. An RGN is a lowboy. The real question is how its gooseneck works.

1:45How does a removable gooseneck work?

How does a removable gooseneck work? There are three main designs. A fixed gooseneck is built on for good. It's the lightest and simplest, but a machine has to be lifted on by crane, or climb over the back on ramps. A mechanical gooseneck comes off with pins and latches, using the tractor's own movement. It's lighter than a hydraulic one and has nothing to leak, but it's slower to work. A hydraulic gooseneck is the most common in heavy haul. Hydraulic cylinders raise and lower the deck, powered either from the tractor's own hydraulics, called a wet kit, or by a small engine built into the neck, called a pony motor.

The basic sequence: park straight on level ground, lower the deck, disconnect the air and electrical lines, release the locks, and drive the tractor forward with the neck still attached. The front of the deck now rests on the ground and becomes the ramp.

You'll also see "non-ground-bearing" on spec sheets. On some designs, a ram pushes down on the ground to lift the neck, and it can sink into soft dirt. A non-ground-bearing neck pushes against the tractor and the deck instead, so nothing has to touch the ground.

2:59What does a "55-ton" trailer really carry?

What does a 55-ton trailer really carry? On a normal road, less than you'd think. 55 tons, 110,000 pounds, is a strength rating, measured with the weight concentrated in a 12-foot span of the deck. It tells you what the frame can hold, not what the road allows.

A standard tractor and 3-axle lowboy already weigh about 35,000 to 43,000 pounds together, and the road limit is 80,000. That leaves roughly 40,000 pounds of legal payload. Everything above that takes permits and more axles. And adding axles doesn't make the trailer any stronger. It makes the load legal, by spreading it over more of the road.

3:41Where does the weight go?

Where does the weight go? Think of the trailer as a beam with two supports: the kingpin at the front, which sits on the tractor, and the trailer's axles at the back. A load is shared between them by distance. The closer it sits to one end, the more that end carries.

Here's an example. A 55,000-pound excavator, centered 18 feet behind the kingpin, on a trailer with 40 feet between the kingpin and its axles. The kingpin end carries 22/40 of it, about 30,000 pounds, and nearly all of that goes down through the tractor's drive axles. Add the weight of the tractor and trailer themselves, and the drive axles carry about 45,000 pounds, against a limit of 34,000, while the trailer's axles carry about 39,000. Slide the machine back and the drives get lighter, but the trailer's axles get heavier. At 97,000 pounds in total, there's no spot on the deck that makes this load legal. It needs a permit and more axles.

And while we're on axles: there's no federal 12,000-pound limit on the steer axle. States can hold it to 20,000 pounds, or to what the axle and tires are rated for, whichever is lower. 12,000 is simply what common steer axles and tires are rated for.

5:02What is the bridge formula?

What is the bridge formula? Start with the three numbers everyone knows. On the Interstate, a single axle can carry 20,000 pounds, a tandem 34,000, and the whole truck 80,000. A tandem is two axles more than 40 inches, and no more than 96 inches, apart. Congress set those limits in 1975. Before that, they were 18,000, 32,000 and 73,280.

When Congress raised them, it added the bridge formula, because a bridge feels how much weight is packed into each foot of truck, not just the total. The formula sets the most weight any group of axles can carry, from how many axles there are and how far apart the first and last ones sit. Take that distance in feet, times the number of axles, divided by one less than the number of axles. Add 12 times the number of axles. Add 36. Multiply by 500.

For a standard 5-axle tractor-trailer with 51 feet from the steer axle to the last trailer axle, that works out to 80,000 pounds. That's why an 18-wheeler needs about 51 feet between those axles to carry the full limit.

There's one exception. Two tandems in a row can carry 34,000 pounds each, if their first and last axles are at least 36 feet apart. The formula alone would allow only about 66,000. The lesson: to legally carry more, add axles, or spread them out.

6:37Why do spread axles carry more?

Why do spread axles carry more? Because of that 96-inch line. Push two axles more than 96 inches apart and they stop being a tandem. Each becomes a single axle, good for 20,000 pounds, and the pair answers to the bridge formula. At 10 feet apart, the formula allows 40,000 pounds on the pair, instead of 34,000 on a closed tandem. That's why you hear about 10-foot spreads. Just past 8 feet, it's only about 38,000.

But there are catches. The 36-foot tandem exception doesn't apply to a spread, so a spread-axle trailer has to pass the formula on its own numbers. Spread axles scrub their tires in tight turns. And many states limit the distance from the kingpin to the rear axle. California, for example, holds trailers over 48 feet to 40 feet from the kingpin to the rearmost axle, which is harder to meet with a spread. States set their own spread rules too, so check every state on your route.

7:39How do jeeps, boosters, stingers and flip axles add capacity?

How do jeeps, boosters, stingers and flip axles add capacity? They add axles, and they add distance. A jeep is an axle group that goes between the tractor and the trailer. The trailer's gooseneck hooks to a fifth wheel on the jeep, so the jeep takes part of the kingpin weight off the tractor's drive axles. A booster is an axle group pinned to the back of the trailer. In heavy haul, many people call that same rear unit a stinger. The name changes by region and by builder. Just don't confuse it with a stinger-steered car hauler. That's a federal term for a car carrier whose hitch sits behind and below the tractor's rear axle, and it's a different machine. A flip axle is a single axle pinned to the rear that flips up onto the trailer when it's empty, keeping the rig short and the tires off the road.

Many boosters hold their share of the load with nitrogen. Hydraulic cylinders press against a nitrogen-charged accumulator, which acts like a spring, so the added axles keep their set load as the road rises and dips. On long trailers, the rear axles can steer, either on their own as the trailer turns, or with hydraulics, so a long combination can make a corner without dragging its tires.

Michigan shows where this leads. Since 1967, it has allowed trucks with up to 11 axles and 164,000 pounds, because what wears out a road is the load on each axle, not just the total.

9:10What makes a load oversize, overweight or a superload?

What legally makes a load oversize, overweight, or a superload? Oversize means wider than 8 feet 6 inches, taller than the state's height limit, or longer than its length or overhang limits. Overweight means over 80,000 pounds, or over any axle or axle-group limit, even when the total is under 80,000. Either way, you need a permit from every state on the route. Routine permits are often issued online the same day.

Superloads are a different level, and there's no federal definition. Each state sets its own. In Pennsylvania, it's over 16 feet wide, 160 feet long, or 201,000 pounds. In California, over 15 feet wide, 135 feet long, or 17 feet high. Texas treats loads over 254,300 pounds as super heavy. Superloads can need route surveys and bridge engineering reviews, and take weeks to approve.

Escorts follow size, and the thresholds vary by state. Commonly, one pilot car starts around 12 feet wide, two around 14, and police escorts around 16. Most states also limit oversize moves to daylight, and many restrict weekends, holidays and rush hours.

10:35What hauls the biggest loads?

What hauls the biggest loads? Past the multi-axle lowboy, the trailer starts to wrap around the load. A perimeter frame trailer has its frame along the outside edges, so a transformer can sit down between the rails, close to the ground. A beam trailer hangs the load between two long beams that run from a front set of axles to a rear one. A Schnabel trailer goes further. The load itself becomes part of the trailer, held between a front end and a rear end that can lift it, or shift it sideways around obstacles.

Dual-lane trailers widen to spread their weight across two lanes, so bridges can take it. And hydraulic platform trailers are built from rows of axles, each on its own hydraulic suspension, so every tire carries its share, even over uneven ground. Self-propelled versions drive themselves and steer every axle.

In January 2022, one of these moves crossed Pennsylvania: a 294-ton superload, 213 feet long, carrying an empty steel tank from a decommissioned prototype reactor about 400 miles, mostly on Interstate 80, taking two lanes and moving mostly at night.

11:50Check your load

Before you ship anything big, check the limits. Our state limits pages list the legal width, height, length and weights for every state, at freightsidekick.com/resources/state-dot-regulations. Our trailer guide shows what each trailer can carry, at freightsidekick.com/resources/trailer-guide. Rules vary by state and change, so check every state on your route.

When you're ready, get a quote at freightsidekick.com/freight-quote, or call (877) 345-3838. Allison, our AI freight agent, answers around the clock.