01
Legal on gross, and over on the drive tandem
The base case, and the one that accounts for more citations than anything else on this list. Gross weight and axle distribution are independent, and a truck can be thousands of pounds under the federal cap while an axle group is over.
Worked example
Worked example for Legal on gross, and over on the drive tandem.| Gross | 76,000 lb, 4,000 lb under the cap |
|---|
| Centre of gravity | 18 ft behind the kingpin instead of 21 ft |
|---|
| Drive tandem | 36,725 lb against 34,000 lb |
|---|
What misses it
Any tool whose only weight input is a gross figure. It is structurally incapable of seeing this, because it has never been told where anything sits.
02
Legal on every named group, and over on an interior bridge subset
The bridge formula applies to every consecutive axle group, and the interior ones do not correspond to anything with a name. A load can clear the steer axle, both tandems and the gross and still fail on the four-axle group that spans from the front drive axle to the rear trailer axle.
Worked example
Worked example for Legal on every named group, and over on an interior bridge subset.| Steer, both tandems, gross | all legal, with margin |
|---|
| Axles two to five, 38 ft apart | 67,500 lb limit |
|---|
| Actual load on that group | 67,700 lb, 200 lb over |
|---|
What misses it
Any implementation that checks the adjacent pairs and the outer spread. That is three of the ten groups on a five-axle unit, and the seven it skips include the one that usually binds.
03
The tire rating binding below the statutory limit
Three ceilings apply to every axle group and the lowest one governs. On a steer axle fitted with standard tires the tire rating is frequently the lowest, which means the 12,000 lb everybody quotes is not the number that applies.
Worked example
Worked example for The tire rating binding below the statutory limit.| Statutory steer axle limit | 12,000 lb |
|---|
| Two tires, 8.5 in of tread, at 600 lb per inch | 10,200 lb |
|---|
| Solved steer axle | 11,280 lb, legal on statute and over on the tires |
|---|
What misses it
Every tool whose equipment model records an axle count rather than axles. If there is no field for tread width, the tire rating is not being checked, whatever the marketing says.
04
Spread axles gaining capacity as they separate
Two axles more than 8 ft apart escape the 34,000 lb tandem cap and take the formula result instead. This is why spread-axle trailers exist. It is also a trap in the other direction, because the extra statutory capacity does not come with extra axle rating.
Worked example
Worked example for Spread axles gaining capacity as they separate.| Two axles at 10 ft, formula limit | 40,000 lb |
|---|
| The same pair treated as a tandem | 34,000 lb, denying 6,000 lb |
|---|
| Manufacturer rating on the pair | 39,200 lb, which is what actually binds |
|---|
What misses it
Tools that hard-code 34,000 lb for any two trailer axles, and tools that grant the formula result without checking whether the axles are rated to carry it. Both errors are common and they point in opposite directions.
05
A lift axle that is raised
A raised lift axle carries nothing and is not part of the group. N appears in both the numerator and the denominator of the formula, so counting an axle that is off the ground does not slightly inflate the limit. It substantially inflates it.
Worked example
Worked example for A lift axle that is raised.| Four axles across 38 ft, lift raised | 67,500 lb |
|---|
| Five axles across 38 ft, lift counted anyway | 72,000 lb |
|---|
| Capacity that does not exist | 4,500 lb |
|---|
What misses it
Anything that takes the axle count from the equipment record rather than from the axle's current state. The check reports a comfortable margin on a load that is thousands of pounds over.
06
A negative kingpin reaction
Load a trailer far enough aft and the trailer stops pressing down on the fifth wheel and starts trying to lift it. The arithmetic keeps working and returns numbers that look calm, because the steer axle and the drive tandem both read low.
Worked example
Worked example for A negative kingpin reaction.| Cargo centre of gravity | 46 ft behind the kingpin, past the tandem centre |
|---|
| Kingpin reaction | -300 lb, which is upward |
|---|
| What a naive check reports | steer 7,155 lb, drive 10,545 lb, both comfortable |
|---|
What misses it
Solvers that report the numbers without sanity-checking them. A negative kingpin reaction is not a tight load, it is a load that cannot exist as described, and the correct output is an error rather than a verdict.
07
Two jurisdictions on one leg
A leg that crosses a state line is governed by two rule sets, and the limits, the bridge table and sometimes the units all change at the boundary. There is no moment at which the truck is subject to only one of them.
Worked example
Worked example for Two jurisdictions on one leg.| Drive tandem, as planned | 33,800 lb |
|---|
| Governing limit for the first 260 miles | 34,000 lb, legal |
|---|
| Governing limit for the last 40 miles | 33,500 lb, over by 300 lb |
|---|
What misses it
Tools where jurisdiction is a global setting rather than a property of the load and the leg. They give one confident answer for a run that has two correct ones.
08
Fuel burn, and where the weigh station is
Diesel comes off the tractor as it burns, roughly evenly across the steer axle and the drive tandem. A load that is over at the pump gets lighter every mile, so whether it passes depends on which scale it meets and when.
Worked example
Worked example for Fuel burn, and where the weigh station is.| Drive tandem at the pump | 34,200 lb, 200 lb over |
|---|
| After 120 miles, 137 lb of fuel burned | 34,131 lb, still over |
|---|
| After 400 miles, 458 lb burned | 33,971 lb, legal |
|---|
What misses it
Anything that computes one verdict at the origin. The load has a different answer at every point on the route, and the only points that matter are the ones with a scale on them.
09
A load that is legal at no slider position at all
There is a payload ceiling implied by the axle limits, and it is lower than the one implied by the gross limit. Above it, no arrangement of freight and no slider setting produces a legal truck. The right output is not a correction, it is a refusal.
Worked example
Worked example for A load that is legal at no slider position at all.| Payload from the gross limit | 48,000 lb |
|---|
| Payload the axle limits actually allow | 47,294 lb |
|---|
| A 47,500 lb order | 206 lb beyond what this combination can carry |
|---|
What misses it
Tools that always return a suggested fix. Offering to move the tandems on a load that cannot be made legal wastes an hour on the dock and ends where it started. The honest answer is a different trailer, two trucks, or a permit if the freight is non-divisible.
10
Lateral imbalance and centre-of-gravity height
Neither of these appears anywhere in the axle arithmetic. Move a thousand pounds from the left side of the trailer to the right and every axle group reads exactly the same, because the group totals have not changed. The truck is a different vehicle in a curve.
Worked example
Worked example for Lateral imbalance and centre-of-gravity height.| Effect on axle group weights | none at all |
|---|
| Effect on the rollover threshold | material, and it grows with stack height |
|---|
| Where it is caught | wheel-by-wheel weighing, or a curve |
|---|
What misses it
Almost everything, because axle compliance and stability are treated as separate subjects. A load model that records lateral offset and centre-of-gravity height can at least warn; one that records weight and position along the deck cannot.