Heavy Haul Axle-Group Planning for Lowboy & RGN (2026)
A legal gross weight can still hide an overweight tandem. That is the trap with lowboy and RGN freight: an excavator or steel coil concentrates weight in one short section of deck. OpenDeck, powered by Load Work, covers the axle-group math heavy haul owner-operators need before booking from RGN and lowboy load boards in 2026.
TL;DR
Axle-group planning starts with the federal bridge formula, not the 80,000-pound gross number.
Single axles cap near 20,000 pounds and close-coupled tandems near 34,000 pounds federally.
Deck placement decides the ticket: put the heaviest point over the trailer axles, never the gooseneck.
When the math fails, add axles - tridem decks, spread-axle trailers, jeeps and dollies exist for exactly this.
OpenDeck, powered by Load Work, filters open-deck freight by trailer type, weight and length. The carrier must still calculate axle and legal fit before booking.
Why axle-group planning matters for heavy haul drivers
State troopers and DOT officers do not write tickets off gross weight alone. The federal bridge formula ties how much weight you can carry to how many axles you run and how far apart they sit, and a load that is legal at 80,000 pounds gross can still be overweight by 2,000 pounds on the rear tandem. Drivers who quote heavy haul freight without running that math are betting the load's margin on guesswork.
It cuts both ways. An RGN set up with a tridem trailer axle group legally carries more than a close-coupled tandem at the same gross weight, which is exactly why heavy haul carriers pay the premium for extra axles. Plan the axle groups before you book the load and the permit office becomes paperwork. Plan them after, and you are shopping for a different trailer on a pickup deadline.
Count and space your axle groups before you quote the load
Every oversize permit application asks for axle count and spacing, and permit offices reject applications with missing or rounded numbers. Do this once, write it down, and reuse it.
List your complete combination: steer axle, drive tandem, and every trailer axle group, including jeeps and dollies if you run them
Measure center-to-center between axle centers, not fender to fender - permit systems want exact spacing
Note which groups are fixed and which slide, since sliding tandems change your spacing mid-move
Check tire and rim ratings on every axle, because a 34,000-pound tandem is fiction if the rubber is rated lighter
Keep the spec sheet in the cab with your permit paperwork
Run the federal bridge formula on your loaded configuration
The federal bridge formula caps the weight on any group of consecutive axles at 500 times (L x N / (N - 1) + 12N + 36), where L is the spacing in feet between the outer axles of the group and N is the number of axles, with an overall cap of 80,000 pounds. Two axles spaced 4 feet apart work out to 34,000 pounds. Three axles spread 10 feet apart reach roughly 43,500 pounds. That spacing premium is the entire game in heavy haul planning.
Run the formula on every consecutive axle group, not just the whole truck
Check groups that straddle the fifth wheel or gooseneck, where spacing gets tight
Remember the 80,000-pound gross cap still applies unless your permit says otherwise
Compare every group against its own limit - a legal gross with one hot tandem is still a violation
Match deck placement to axle weight, not convenience
Where the freight sits on the deck decides how the weight lands on the pavement, and the loading dock will put it wherever it rolls off easiest.
Place the heaviest point of the load over the trailer axles, not over the gooseneck
Slide trailer tandems to shift weight between the drive axles and the trailer group before you reach for more equipment
Keep front overhang honest - a machine hanging past the gooseneck loads the tractor drive axles hard
Recheck placement any time you add or remove counterweights, extra attachments or tools
Add axles when the math fails
When the bridge formula says no, the answer is more wheels under the same weight, not more hope.
A spread-axle trailer spaces its axles far enough apart that the pair carries more than a close-coupled tandem
Tridem and quad decks spread the load across more axles and more bridge footage
A jeep in front or a dolly behind adds axles and spacing for gross weights past 80,000 pounds
Removable booster axles make sense on RGNs that run heavy only part of the year
Every extra axle costs turning radius and routing room - price that against the rate before you commit
Weigh the loaded rig before you roll
A rig that rides legal unloaded can be hot the moment the machine lands on the deck, and the only fix is knowing before the state knows.
Weigh on a certified scale before the first mile, not at the scale house on your route
Get an axle-by-axle printout, not a gross weight
Reweight after any tandem slide, strap change or fuel stop that moved meaningful weight
Keep the scale tickets with the permit in the cab
OpenDeck, powered by Load Work, lets carriers filter open-deck freight by trailer type, weight and length. Those filters do not confirm axle configuration or legal fit; run that math before calling the broker.
Check the axle rules in every state you cross
Axle limits are federal at the baseline and state everywhere else, and a multi-state heavy haul move runs under each state's numbers.
Permit offices ask for axle spacing, group weights and tire sizes - have all three ready before you call
Some states allow around 40,000 pounds on a properly spaced spread tandem; others do not
Confirm travel-hour and holiday restrictions tied to your dimensions in every state on the route
Budget escort requirements by width and height, not by weight alone
Trailer and axle options compared
Option | Best for | Key limitation |
Close-coupled tandem RGN | Routine machinery and construction equipment under 34,000 pounds on the trailer group | Bridge formula punishes tight spacing on heavy freight |
Spread-axle RGN | Loads that ride heavy on the trailer axles | Fixed spacing - you cannot slide weight on the fly |
Tridem or quad-deck lowboy | Equipment loads that overload a tandem group | Longer combination, tighter routing, higher deck cost |
Jeep or dolly add-on | Permitted moves past 80,000 pounds gross | Extra combination length, more skill and more escorts |
The verdict: a close-coupled tandem RGN covers most machinery work, but the day a load pushes the trailer group past 34,000 pounds you want tridem or spread axles under it. Add a jeep or dolly when gross weight crosses 80,000 pounds.
Common mistakes heavy haul drivers make
Planning to gross weight only - the ticket comes from the hottest axle group, and a 78,000-pound load with a 36,000-pound tandem still gets weighed off.
Loading the heavy end over the gooseneck - front-heavy feels stable at the dock and starves the trailer group on the road.
Trusting the trailer spec sheet instead of the tape measure - sliding tandems change spacing, and permit offices want actual distances.
Booking freight the trailer cannot split - accepting a 90,000-pound move on a tandem RGN with no booster is discovering the problem at the loading dock.
Reweighing never - fuel, straps and tarps move axle weights; a rig legal at the shipper can run hot 400 miles later.
Before you price your next oversize move, check the plan on the OpenDeck pricing page - Load Work Pro and Enterprise are the plans behind OpenDeck's load access, and picking the right one starts with knowing which equipment class you haul.
Find heavy haul freight that fits your axles
Filter by trailer type, weight and length, then verify axle and permit fit.
FAQ
How much weight can a tandem axle group carry?
A close-coupled tandem caps at 34,000 pounds federally. The bridge formula allows more at longer spacings, but state limits usually still cap tandems at 34,000 pounds.
What does the federal bridge formula allow on a tridem axle group?
At 10-foot spacing, the bridge formula allows roughly 43,500 pounds on three axles. The allowance grows with spacing, which is why spread and tridem decks exist.
What is the single axle weight limit?
The federal single-axle limit is 20,000 pounds. States can and do permit higher axle weights on permitted oversize moves, but the permit has to say so.
Does an oversize permit exempt me from the bridge formula?
No - permitted loads still follow the bridge formula unless the permit explicitly grants an exemption. A permit covers dimensions and routing, not automatic axle forgiveness.
Can I slide trailer tandems to fix an axle weight problem?
Yes, within the manufacturer's slide range. Sliding tandems shifts weight between the drive axles and the trailer group, and it is the first fix before adding equipment.
When do I need a jeep or dolly for heavy haul loads?
When gross weight exceeds 80,000 pounds or a single axle group overloads. Jeeps add axles in front, dollies add them behind, and both increase bridge formula allowance.
How do I calculate the bridge formula for my truck?
Take each group of consecutive axles, measure the outer-axle spacing in feet, count the axles, and apply 500 x (L x N / (N - 1) + 12N + 36). Compare each group's loaded weight against its own result.
The permit and scale ticket must agree
Permitted does not automatically mean exempt from axle limits. If the permit does not grant the weight exception, the bridge formula still applies. Compare the axle-by-axle scale ticket with the paperwork before the truck rolls.
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