Reference

The words a weight argument is conducted in

Most disagreements about a load turn out to be disagreements about a word. This is every term used across this section, defined once, with the arithmetic behind it where there is any.

58 terms
Alphabetised, with anchors
Updated August 2026

A to Z

Every term, alphabetised

Jump to a letter, or read it through — the definitions are written to be read in order as well as looked up, and several of them explain each other.

A

Air suspension: A suspension that carries the axle load on air bags rather than on steel springs. Because the pressure in a bag is proportional to the load it is carrying, air suspension is what makes onboard air-pressure scales possible, and it is why a mechanically suspended axle cannot be weighed the same way.
Axle group: Two or more axles close enough together to be treated as a single unit for weight purposes and weighed as one. A drive tandem is an axle group, and so is a spread pair or a tri-axle. Limits are applied to groups, not to individual axles alone.
Axle spacing: The distance between the centres of two axles, measured on the vehicle rather than taken from a specification sheet. It is the L in the bridge formula, and one foot of measurement error can be worth a thousand pounds of legal capacity on an interior group.

B

Bobtail: A tractor driving with no trailer attached. It is still subject to axle limits and it is still weighed, and a tractor whose centre of gravity sits well forward can put more weight on its steer axle bobtail than it does coupled to a loaded trailer.
Bridge formula: The Federal Bridge Gross Weight Formula: W = 500 × ((L × N) / (N − 1) + 12N + 36), where L is the spacing in feet and N is the number of axles. The result is rounded to the nearest 500 lb, capped at 80,000 lb, and capped again at 34,000 lb for a pair spaced 8 ft or less. It applies to every set of two or more consecutive axles on the combination.
Bypass: A service that lets a pre-screened vehicle pass a weigh station without stopping, based on the carrier's inspection and safety record. It is a privilege earned by a record, not an exemption from any weight limit.

C

CG also: centre of gravity: Centre of gravity. The single point at which a load's weight can be treated as acting. Every axle weight calculation reduces the freight to this point and its distance from a support, which is why moving one heavy item can change every number on the plan.
CG height: How high the centre of gravity sits above the road. It appears nowhere in the axle weight arithmetic and it governs the rollover threshold, which is why a load can be legal on every axle group and still be the wrong shape to take a curve at speed.
CSA: Compliance, Safety, Accountability, the US programme that scores carriers using roadside inspection and violation data. Weight violations recorded at an inspection attach to the carrier's record and stay visible to customers and insurers for a long time.
Cubed out: A trailer that has filled up by volume before it has filled up by weight. The opposite case, weighed out, is where the axle limits bind first, and the two need completely different kinds of planning tool.

D

Deadhead: Running empty, usually to reposition for the next load. It burns fuel and duty hours and earns nothing, which is why a run that has to be re-planned after an overweight costs more than the citation.
Detention: Time a driver spends waiting at a shipper or a receiver. It consumes the fourteen-hour duty window at exactly the same rate as driving does, which is why three hours of detention can cost a delivery that eleven hours of driving would have made.
Divisible load: Freight that can lawfully and practically be split into smaller parts. A divisible load that is over an axle limit is a rearrangement problem, and the correct answer is to move something rather than to apply for a permit.
Drive tandem: The pair of powered axles under the rear of the tractor. Subject to the 34,000 lb federal tandem limit, and the group that most overweight loads fail on, because it carries most of the kingpin reaction.
Dunnage: Blocking, bracing, airbags and packing used to stop freight moving in transit. It is genuine weight, it goes on at the last minute, and it is routinely missing from the plan the load was checked against.
Duty window: The fourteen hours that begin when a driver comes on duty, after which driving is not permitted. It runs on the wall clock and does not pause for breaks, meals, fuelling or detention. It is the rule that most planning tools model incorrectly.

E

ELD also: electronic logging device: Electronic logging device. Records driving and duty time automatically and reports it against the hours-of-service rules. A plan built as though the duty window pauses is a plan the device will not accept.
Escort also: pilot car: A pilot vehicle required to accompany an oversize or overweight load, usually specified as a condition on the permit. Escorts change both the cost of a run and its hours, because the escort has its own availability.

F

Fifth wheel: The coupling on the tractor that the trailer's kingpin locks into. On most tractors it slides fore and aft in fixed increments, and moving it changes how the kingpin load divides between the steer axle and the drive tandem.
Flatbed: A trailer with a deck and no walls or roof. Freight is held by chains, straps and blocking rather than contained, so securement weight is larger and its position matters more than on an enclosed trailer.

G

GCW also: gross combination weight: Gross combination weight: the tractor, the trailer, the fuel, the driver and the freight, all together, as the vehicle sits. The US federal limit on the Interstate System is 80,000 lb.
Gross weight: What the whole combination weighs. It is the number that is easiest to obtain and it is not the number anyone is cited on, because enforcement is per axle group and gross weight cannot see distribution at all.
GVWR also: gross vehicle weight rating: Gross vehicle weight rating: the maximum operating weight the manufacturer states for a vehicle. It is an engineering limit rather than a legal one, and on some equipment it is lower than the statutory limit, in which case it is the one that binds.

H

HOS also: hours of service: Hours of service. The rules limiting how long a commercial driver may drive and remain on duty. In the United States, for property-carrying drivers: 11 hours of driving, a 14-hour duty window, and a 30-minute break after 8 cumulative driving hours.

J

Jurisdiction: The authority whose rules govern a particular road at a particular moment. Weight limits, bridge tables, geometry rules, units, date formats and time zones all follow it, and a single run can cross several of them.

K

Kingpin: The pin under the front of a semi-trailer that locks into the tractor's fifth wheel. It is one of the two supports the trailer is solved about, and the point through which every pound of freight reaches the tractor.
Kingpin reaction: The downward force the trailer applies to the tractor through the kingpin. It is the only thing about the freight that the tractor knows, so two entirely different loads producing the same kingpin reaction give identical steer and drive weights. A negative kingpin reaction means the load as described cannot exist.
Kingpin-to-rear-axle also: KPRA: The distance from the kingpin to the centre of the trailer's rear axle group. Several jurisdictions limit it — California's 40 ft rule is the best known — and it is a geometry limit rather than a weight one, so a load with no weight problem at all can still fail it.
KPRA: The usual abbreviation for kingpin-to-rear-axle. Worth knowing as an abbreviation because it appears on permits and in state regulations more often than the full phrase does.

L

Lift axle: An axle that can be raised clear of the road when it is not needed. When it is up it carries nothing and it does not count towards N in the bridge formula, so a check that reads the axle count from the equipment record rather than from the axle's state will report a limit that does not apply.
Load securement: Chains, straps, binders, blocking and bracing that stop freight moving. It adds real weight, and that weight lands wherever the securement is rather than at the load's centre of gravity.

M

Moment: A weight multiplied by its distance from a reference point. Summing moments about a support and dividing by the span is the entire arithmetic of axle weight, and everything else on this page is a consequence of it.

N

Net weight: Gross weight minus tare weight, which is the freight. It inherits every error in the tare, so a net computed from a stored tare recorded years ago is only as good as that record.
Non-divisible: Freight that cannot be split without destroying its value or its usefulness — a transformer, a press, a single casting. A non-divisible overweight load is a permit case rather than a rearrangement case, because there is nothing to rearrange.

O

Out-of-service: An order placing a vehicle or a driver off the road until the violation or defect is corrected. An overweight axle can attract one, and the truck stays where it is until the load is made legal, which usually means a second vehicle and a crew.
Oversize and overweight permit: Authorisation from a jurisdiction to move a load that exceeds a legal dimension or weight. Permits carry conditions: a specified route, permitted hours, escort requirements and a validity window that a delayed trip can run past.

P

Payload: The freight, as distinct from the vehicle carrying it. The payload ceiling implied by the axle limits is usually lower than the one implied by the gross limit, and the difference is capacity that exists on paper and cannot be carried at any position.
Pintle: A hook-and-eye coupling used for trailers that are towed rather than coupled through a fifth wheel. Load transfers through a pintle differently from a kingpin arrangement, which is why a solver written for tractor-semitrailers does not simply carry over.

R

Reefer: A refrigerated trailer. It carries a refrigeration unit and its own fuel, both at the nose, so its tare is higher than a dry van's and that extra weight sits where it loads the kingpin most.
Rollover threshold: The lateral acceleration at which a vehicle will roll rather than slide. It is governed by track width and centre-of-gravity height, and it is completely invisible to axle weight arithmetic, so a perfectly legal load can be a poor shape for a curve.

S

Scale ticket: The printed record of a weighing. A certified ticket carries the scale's identification and a weighmaster's attestation and is evidence; an uncertified reading is useful for your own decisions and is not. Either way it records one moment, not a run.
Slider: A movable axle assembly that pins into a rail at fixed intervals — either the trailer tandem or the fifth wheel. Moving one changes the geometry the load is solved over, which is why a correction expressed in holes is actionable and one expressed in inches is not.
Split sleeper: Taking the required ten hours off duty in two parts: one of at least seven consecutive hours in the sleeper berth, and another of at least two consecutive hours off duty or in the sleeper, totalling at least ten. Neither qualifying period counts against the fourteen-hour duty window, which is the whole value of the provision.
Spread axle: Two trailer axles spaced more than 8 ft apart. The pair escapes the 34,000 lb tandem cap and takes the bridge formula result instead, which at 10 ft is 40,000 lb. The extra statutory capacity does not come with extra axle or tire rating, so one of those often binds first.
Statics: The branch of mechanics dealing with bodies that are not accelerating. Every axle weight on a stationary truck is a statics problem, solved by taking moments about a support, and it is ordinary arithmetic rather than anything exotic.
Statutory limit: The weight limit set by law for an axle, a group or the whole combination. It is one of three limits that apply to every axle group, alongside the tire rating and the manufacturer's axle rating, and it is not always the lowest of the three.
Steer axle: The front axle of the tractor. Subject to a 12,000 lb federal limit, and frequently limited below that by the tire rating, because two standard steer tires at 600 lb per inch of tread width can come out at around 10,200 lb.

T

Tandem: Two axles close enough together to be treated as one group, spaced more than 40 in and up to 96 in apart. Subject to the 34,000 lb federal limit. Below 40 in the pair is treated as a single axle group instead, which is a different rule again.
Tare weight: The weight of the vehicle without the freight. A live tare is weighed empty on the day; a stored tare is a historical figure held against the unit number and is wrong the moment a fuel level, a tool box or an auxiliary power unit changes.
Tire rating: The load a tire may legally carry, commonly expressed as 600 lb per inch of tread width and multiplied by the number of tires on the group. On a steer axle with standard tires this is frequently the lowest of the three limits and therefore the one that actually applies.
Tractor: The powered unit of a combination. For weight purposes it is solved as a beam supported at the steer axle and the drive tandem, carrying its own tare at its own centre of gravity plus the kingpin load at the fifth wheel.
Trailer tandem: The pair of axles under the rear of a semi-trailer, usually mounted on a slider. Subject to the 34,000 lb federal tandem limit, and one of the two supports the trailer is solved about.
Tri-axle: Three axles in a single group. It takes a bridge formula limit rather than the tandem cap, and several jurisdictions give it its own statutory treatment, so a tri-axle trailer is not simply a tandem with one more axle on it.

V

Van: An enclosed box trailer, the most common type on the road. Freight is contained rather than strapped down, which makes securement lighter and placement discipline more important, because nothing stops a pallet being put wherever it fits.

W

Weigh station: A fixed enforcement facility where commercial vehicles are weighed and inspected. Which one you meet and when is a routing question, because a load that is over at the pump can be legal four hundred miles later as fuel burns off.
Weighmaster: The person responsible for a certified weighing, licensed individually in some jurisdictions. Their attestation is what turns a printed number into a legal record rather than a receipt.
Weight distribution: How a load's weight divides between the axle groups. It is decided by where the freight sits and by the geometry of the specific tractor and trailer, and it is the subject that everything else on this page serves.
Wheelbase: On a tractor, the distance from the steer axle to the centre of the drive tandem. It is the span the tractor is solved over, and it sets how a change in the kingpin load divides between the steer axle and the drives.

A planner, a loader and an enforcement officer can all use the same word and mean three different things. That is where most weight arguments start.

The four federal limits, to scale

Steer axle12,000 lb
the single axle at the front of the tractor
Any single axle20,000 lb
one axle that is not part of a group
Tandem34,000 lb
two axles more than 40 in and up to 96 in apart
Gross combination80,000 lb
the cap the bridge formula is measured against

United States federal limits on the Interstate System, which is what every entry above quotes unless it says otherwise. Off the Interstate the governing limits are the state's, and several states differ.

A note on the numbers

Which limits these definitions assume

Several entries quote a figure. Here is where those figures come from, so that nobody carries them into the wrong country.

United States federal limits, Interstate System

12,000 lb on a steer axle, 20,000 lb on a single axle, 34,000 lb on a tandem, and 80,000 lb gross. Off the Interstate System the governing limits are the state's, and several states differ. Canadian and European limits are separate regimes with their own numbers.

Definitions are not legal advice

These entries describe how the terms are used in practice, with enough arithmetic to be useful on a dock. Where a decision turns on the precise wording of a regulation, read the regulation. Assessed from publicly published documentation as of August 2026.

Where a decision turns on the precise wording of a regulation, read the regulation. These entries are here to make the argument possible, not to settle it.

Where these terms get used

Three pages that put most of this vocabulary to work on an actual load.

Guide

Axle weight basics

Kingpin reaction, moment, statics and tare weight, worked through on one combination.

Read

Guide

The bridge formula, explained

Axle group, axle spacing, spread axle and the tandem cap, with all ten subsets enumerated.

Read

Guide

Hours of service for planners

Duty window, split sleeper, detention and the logging device, on two worked timelines.

Read

For jurisdiction, units and date formats treated as data rather than as settings, seejurisdictions, units and formats.

Next step

Put the freight-weight glossary against one real load.

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

Bring one lane Open AxleGuard