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Nine ways a legal-looking load goes overweight

None of these involve loading too much. Every one of them takes a truck that is comfortably under 80,000 lb gross and puts an axle group over, and in seven of the nine the gross weight does not change at all.

  • Nine failure modes
  • One reference combination
  • Every number worked, not asserted

Method

One truck, nine ways to get it wrong

Every number below is solved on the same combination, so the nine failures can be compared against each other rather than against nine different trucks.

The reference combination is an 18,000 lb tractor on a 20 ft wheelbase. Its fifth wheel is 17 ft behind the steer axle. It pulls a 14,000 lb trailer with 40 ft from the kingpin to the trailer tandem centre. It carries 44,000 lb of freight with its centre of gravity 21 ft behind the kingpin.

Solved, that gives 11,280 lb on the steer axle, 33,920 lb on the drive tandem, 30,800 lb on the trailer tandem, and 76,000 lb gross. It is legal, with 80 lb of margin on the drive tandem and 4,000 lb of room on the gross. That is the starting point for all nine.

These are worked-example figures for a representative combination rather than measurements of a specific vehicle. Your own geometry will give different constants. The failure modes are the part that transfers.

Twelve thousand pounds on a steer axle, twenty thousand on a single, thirty-four thousand on a tandem, eighty thousand gross. Four limits, and a load has to clear all of them at the same moment.
US federal weight limits on the Interstate System, 23 CFR 658.17

One to three

At the dock, before anything moves

Three failures that are entirely free to fix while the freight is still on the floor, and expensive from the moment the doors close.

01

The load was built from the nose backwards

Loaders fill a trailer the way it is convenient to fill: start at the front bulkhead and work back. On a full trailer that is fine, because the freight ends up spread across the deck. On a trailer that will not be filled, it puts the whole load in the front half, and the front half of a trailer is carried mostly by the kingpin.

Worked example

Worked example for The load was built from the nose backwards.
Cargo centre of gravity21 ft behind the kingpin18 ft behind the kingpin
Kingpin reaction27,200 lb30,500 lb
Drive tandem33,920 lb36,725 lb
Gross76,000 lb76,000 lb

Three feet of centre of gravity is worth 2,805 lb on the drive tandem. The gross weight did not change by a single pound, and the drive tandem is 2,725 lb over.

What to do about it

Give the crew a target rather than a direction. A strip of tape on the trailer floor at the intended centre of gravity, and an instruction to balance the heavy freight around it, costs nothing and removes the single most common cause of an overweight drive tandem.

02

One heavy item went where it fitted

A single dense item — a machine, a coil, a transformer, a pallet of fasteners — dominates the centre of gravity of the whole load. It gets placed at the nose because that is where the forklift could reach it, or because it went on first.

Worked example

Worked example for One heavy item went where it fitted.
Placement of the 6,000 lb item22 ft behind the kingpin4 ft behind the kingpin
Whole-load centre of gravity22.00 ft19.55 ft
Drive tandem32,985 lb35,280 lb

Moving one item of 6,000 lb eighteen feet forward moved 2,295 lb onto the drive tandem and put the truck 1,280 lb over. Nothing else about the load changed.

What to do about it

Place the heaviest item first and place it on purpose. Everything else can be built around it. If the item has to go at the nose for unloading reasons, that constraint belongs in the plan before the load is built, not after.

03

The trailer tandem was not in the hole the record said

Trailers come back from customers with the tandems moved. A driver slides them to get under a dock, to make a tight turn, or to satisfy a bridge limit on the previous run, and nobody updates anything. The next plan is computed against a setting that is no longer true.

Worked example

Worked example for The trailer tandem was not in the hole the record said.
Kingpin to tandem centre40 ft (as recorded)41.33 ft (four holes back)
Trailer tandem30,800 lb29,806 lb
Drive tandem33,920 lb34,765 lb

Four holes at four inches each is sixteen inches, and it is worth 845 lb on the drive tandem. The plan says legal. The trailer is 765 lb over.

What to do about it

Read the slider position off the trailer, physically, as the first step of building the load, and write it on the plan. It is a ten-second check that invalidates or confirms every number downstream of it.

Four to six

On the road, after the plan was signed

Three failures where the load that leaves the yard is not the load that reaches the scale, and nothing in the paperwork records the difference.

04

The truck was fuelled after the plan was built

Diesel is weight, it goes on after the load is planned, and it lands on the tractor rather than the trailer. Because the tanks sit near the middle of the wheelbase, it splits roughly evenly between the steer axle and the drive tandem, which is exactly the axle that has the least margin on a heavy load.

Worked example

Worked example for The truck was fuelled after the plan was built.
Fuel addednoneabout 68 gallons, 480 lb
Steer axle11,280 lb11,520 lb
Drive tandem33,920 lb34,160 lb
Gross76,000 lb76,480 lb

The load was 80 lb inside the limit when it was planned. Filling the tanks put it 160 lb over, and no freight moved.

What to do about it

Plan against the fuel state the truck will actually have at the scale, not the state it had at the desk. On a load with less than a thousand pounds of margin on the drive tandem, the fuelling decision is part of the compliance decision.

05

Freight boarded at the second stop

A milk run is not one load. It is a different load on every leg, and the compliance question has a different answer on each. Freight picked up at stop two goes wherever there is space, and the space left on a part-loaded trailer is usually at the nose.

Worked example

Worked example for Freight boarded at the second stop.
Leg one: 36,000 lb at 24 ftdrive tandem 28,395 lb
8,000 lb boards at 6 ft behind the kingpincentre of gravity moves to 20.73 ft
Leg two: same truckdrive tandem 34,175 lb

Leg one had 5,605 lb of margin on the drive tandem, which is not a close call by any standard. Leg two is 175 lb over, and nobody re-solved anything between them.

What to do about it

Solve every leg of a multi-stop run separately, with the freight that is actually aboard for that leg. A run that is checked at the origin has been checked for the one moment when the least freight was on it.

06

The last forty miles were not on the Interstate

The federal weight limits apply on the Interstate System. Off it, the governing limits are the state's, and several states set them differently for particular road classes, bridges and seasons. A load planned against the federal table can be legal for four hundred miles and illegal for the last forty.

Worked example

Worked example for The last forty miles were not on the Interstate.
Drive tandem, as planned33,800 lb
Federal limit on the Interstate System34,000 lb
Governing limit on the delivery road33,500 lb

The same 33,800 lb is 200 lb inside the limit for most of the run and 300 lb over it at the end, without anything about the truck changing.

What to do about it

Make the jurisdiction a property of each leg rather than a setting on the plan, and check the run against the tightest limit it will encounter. Where a delivery road has a lower limit, the load has to be built to that limit from the start, because there is nowhere to fix it at mile 400.

The load that was solved is not the load that was weighed. Fuel went on, freight came aboard, or the rules changed under the wheels.

Seven to nine

In the paperwork, where nobody is looking

Three failures that are invisible on the load and permanent in the data, so they repeat until somebody goes looking for them.

07

The declared weight was an estimate

Weights on a bill of lading are frequently estimates, and estimates from the party paying for the freight tend to lean in one direction. On a load with plenty of margin this does not matter. On a load near the ceiling it is larger than the whole margin.

Worked example

Worked example for The declared weight was an estimate.
Declared cargo weight44,000 lb44,000 lb
Actual cargo weight44,000 lb46,200 lb, five percent heavy
Drive tandem33,920 lb34,808 lb
Gross76,000 lb78,200 lb

A five percent error in the declared weight is 888 lb on the drive tandem and puts it 808 lb over, while gross is still 1,800 lb inside the federal cap.

What to do about it

Where the margin is thin, weigh rather than believe. Where weighing is not practical, build the plan against a declared weight plus a tolerance, and treat the resulting margin as the real one.

08

The tare weight was recorded once and never again

A tractor accumulates weight over its life: an auxiliary power unit, a heavier bumper, a tool box, chains, a second battery bank, an inverter. None of it is a large number. All of it is permanently in the tare and permanently absent from the plan.

Worked example

Worked example for The tare weight was recorded once and never again.
Tractor tare on file18,000 lb18,000 lb
Tractor tare in reality18,000 lb18,600 lb
Drive tandem, actual33,920 lb34,280 lb

Six hundred pounds of tare that nobody recorded is 360 lb on the drive tandem, on every load that unit ever carries, in the same direction every time.

What to do about it

Re-weigh each tractor and trailer empty at a sensible interval and after any modification. If scale tickets keep disagreeing with the plan in the same direction, treat that as evidence about the tare rather than noise about the load.

09

Only the gross weight was ever checked

This is the failure that contains the other eight. Gross weight is the number that is easy to obtain, so it becomes the number that gets checked, and it is not a compliance check. It cannot detect any of the eight failures above, and it cannot detect the one below either.

Worked example

Worked example for Only the gross weight was ever checked.
Gross79,300 lb, legal
Steer axle11,600 lb, legal
Drive tandem34,000 lb, exactly legal
Trailer tandem33,700 lb, legal
Axles two to five, 38 ft apart67,700 lb against a 67,500 lb bridge formula limit

Every number a planner would look at passes. The load is 200 lb over on a four-axle group that is not printed on a scale ticket, does not have a name in the yard, and appears in very little software.

What to do about it

Check every consecutive axle subset, not the three groups that have names. On a five-axle combination that is ten checks rather than three, and the seven nobody runs are where the formula usually bites.

What each failure puts on the drive tandem

lb
  • Freight boards at the second stop5,780 lb
  • The load is built from the nose backwards2,805 lb
  • One heavy item goes to the nose2,295 lb
  • The declared weight is five percent light888 lb
  • The trailer tandem is four holes back845 lb
  • The tare weight is 600 lb out of date360 lb
  • The truck is fuelled after planning240 lb

The reference load leaves the yard with 80 lb of margin on the drive tandem, which is the marker on every bar. Each of these seven is larger than that on its own. The other two are missing because they do not act on the drive tandem at all: one moves the governing limit rather than the load, and one is an interior bridge group no scale ticket prints.

The pattern

What the nine have in common

They are not nine unrelated problems. They are three problems wearing nine hats.

The first three are about position. The same weight, in a different place, produces a different verdict. Position is free to change at the dock and expensive to change anywhere else, which is why every one of them is a dock problem with a roadside consequence.

The middle three are about time. The load that was solved is not the load that was weighed, because fuel went on, freight came aboard, or the rules changed under the wheels. A single verdict, computed once, describes one instant of a run that has many.

The last three are about inputs. The arithmetic was done correctly on numbers that were not true. A wrong tare or a wrong declared weight produces a confidently wrong answer, which is worse than no answer, because nobody double-checks a plan that says it is fine.

There is no version of this that gets solved by being more careful. A planner cannot look at a trailer and see 800 lb of centre-of-gravity error, and a scale ticket cannot show a four-axle bridge group. Both of those are arithmetic problems, and arithmetic problems get solved by doing the arithmetic every time, on the real geometry, before the doors close.

There is no version of this that gets solved by being more careful. A planner cannot look at a trailer and see 800 lb of centre-of-gravity error.

Dated, and honest about it

Assessed from publicly published documentation as of August 2026. The limits used throughout are the US federal figures on the Interstate System: 12,000 lb steer, 20,000 lb single, 34,000 lb tandem and 80,000 lb gross. All axle weights are worked examples.

Next

Related reading

Three pages that go a level deeper on the arithmetic behind these nine.

Guide

Axle weight basics

The solver every number on this page came out of, worked through by hand.

Read

Guide

The bridge formula, explained

Failure nine in full: all ten consecutive axle subsets, and the one that binds.

Read

Guide

Building a legal load

The procedure that closes off the first three failures before a forklift starts.

Read

For these checks running on your own equipment rather than a reference truck, see theper-axle weight engine.

Next step

Put the overweight-load causes against one real load.

Bring the route, configuration or scale ticket that made the overweight-load causes matter. We will work through it in plain English and show where AxleGuard changes the decision.

Bring one laneOpen AxleGuard