Guide

Where the weight goes, and why it is never where you put it

Put a pallet in the middle of a trailer and it does not load the middle of the trailer. It loads the kingpin and the trailer axles in one ratio, and the kingpin then loads the steer axle and the drive tandem in another. This page works that through with numbers, twice.

A full solve, step by step
Three limits per axle
US federal figures, August 2026

The premise

Gross weight is a receipt, not a verdict

It is the one number everybody has, and it is not the number anyone is cited on.

A scale at the dock will tell you a load weighs 76,000 lb. That is genuinely useful information and it settles exactly one question: whether you are under 80,000 lb gross. It says nothing about the five axle-group limits that get enforced. The same 76,000 lb can be arranged into a legal or illegal truck without adding or removing a pound.

The reason is leverage. A trailer is a beam. So is a tractor. Weight put anywhere on either of them is divided between its supports in proportion to where it sits, and the proportions are not intuitive. A pallet placed two feet further forward can move several hundred pounds onto the drive tandem, and the paperwork will look identical.

What follows is the whole calculation on one combination, with every number shown. It is not complicated arithmetic. It is just arithmetic that nobody does at 5am with a forklift running.

The same 76,000 lb can be arranged into a legal truck or an illegal one without adding or removing a single pound.

The combination

The truck we are going to solve

A conventional tractor and a 53 ft van. These are worked-example figures for a representative combination, not measurements of a specific vehicle — but they are the figures the rest of the page uses.

Geometry and tare weights used in the worked example.
Input	Value	Why it matters
Kingpin to trailer tandem centre	40 ft	The trailer's span. Every trailer moment is divided by it.
Trailer tare	14,000 lb	Acts at its own centre of gravity, 22 ft behind the kingpin.
Cargo	44,000 lb	Placed with its centre of gravity 21 ft behind the kingpin.
Tractor wheelbase	20 ft	Steer axle to drive tandem centre. The tractor's span.
Tractor tare	18,000 lb	Acts 12 ft behind the steer axle, fuelled.
Fifth wheel position	17 ft	Behind the steer axle. This is the lever that splits the kingpin load.

The method

Three solves, in this order

You cannot solve the tractor first. The tractor does not know what it is carrying until the trailer has been solved, and the trailer does not care about the tractor at all.

01
Solve the trailer about the kingpin
The trailer is a beam supported in two places: the kingpin at the front and the trailer tandem at the back. Take moments about the kingpin. Every weight on the trailer contributes its own weight multiplied by its distance behind the kingpin. Divide the total moment by the kingpin-to-tandem distance and you have the trailer tandem reaction. Subtract that from the total weight on the trailer and you have the kingpin reaction.
02
Hand the kingpin reaction to the tractor
The kingpin reaction is not a number the trailer keeps. It becomes a downward point load on the tractor, applied at the fifth wheel. This is the only thing the tractor knows about the freight. Two completely different loads that happen to produce the same kingpin reaction will produce identical steer and drive weights.
03
Solve the tractor about the steer axle
The tractor is also a beam, supported at the steer axle and at the drive tandem. It carries its own tare at its own centre of gravity, plus the kingpin load at the fifth wheel. Take moments about the steer axle, divide by the wheelbase, and you have the drive tandem reaction. Subtract from the total on the tractor and you have the steer axle.
04
Apply the limits, and take the lowest
Now you have five axle-group numbers. Each one is checked against the statutory limit, the tire rating and the manufacturer's axle rating, and the lowest of the three is the one you have to meet. A verdict that only checks the statute is a verdict that will occasionally be confidently wrong.

Worked

The solve, with the numbers in it

Cargo centre of gravity 21 ft behind the kingpin. This is the arithmetic in full.

Step one: the trailer

Moments about the kingpin. The cargo contributes 44,000 lb at 21 ft, which is 924,000 ft·lb. The trailer's own tare contributes 14,000 lb at 22 ft, which is 308,000 ft·lb. Together that is 1,232,000 ft·lb.

Divide by the 40 ft span and the trailer tandem carries 30,800 lb. The total on the trailer is 58,000 lb, so the kingpin carries the rest: 27,200 lb.

Step two and three: the tractor

The tractor now carries its own 18,000 lb at 12 ft behind the steer, plus 27,200 lb at the fifth wheel 17 ft behind the steer. Moments about the steer axle come to 678,400 ft·lb. Divide by the 20 ft wheelbase and the drive tandem carries 33,920 lb. The steer axle takes what is left: 11,280 lb.

The verdict, before any limits are applied

Gross is 76,000 lb, which is 4,000 lb under the federal cap. The trailer tandem is 30,800 lb against 34,000 lb. The steer is 11,280 lb against 12,000 lb. And the drive tandem is 33,920 lb against 34,000 lb, which leaves 80 lb of margin on a truck that is four thousand pounds under gross.

That is the whole point of the exercise. Nothing about the paperwork suggests this load is 80 lb from a citation.

Where that gross weight actually sits

15%

45%

41%

Steer axle11,280 lb
Drive tandem33,920 lb
Trailer tandem30,800 lb

One number on the paperwork, three on the ground. The drive tandem carries 45 percent of the load and has 80 lb of room left, on a truck that is 4,000 lb under the federal gross cap.

Steer axle

11,280 lb

against a 12,000 lb statutory limit

Drive tandem

33,920 lb

80 lb of margin to 34,000 lb

Trailer tandem

30,800 lb

3,200 lb of margin to 34,000 lb

Gross

76,000 lb

4,000 lb under the federal cap

Sensitivity

What one foot is worth

The useful thing about doing the algebra once is that it gives you a rule of thumb you can trust on this specific truck.

Move the cargo centre of gravity from 21 ft to 22 ft behind the kingpin and re-run all three solves. The trailer tandem goes to 31,900 lb, the kingpin reaction drops to 26,100 lb, the drive tandem falls to 32,985 lb and the steer to 11,115 lb.

So on this combination, every foot the load's centre of gravity moves aft is worth:

1,100 lb off the kingpin reaction
935 lb off the drive tandem
165 lb off the steer axle
1,100 lb onto the trailer tandem

The four numbers sum to zero, which is the check that the arithmetic is right. Nothing appeared and nothing vanished; it moved.

Why the ratios are what they are

The kingpin number is the cargo weight divided by the trailer span: 44,000 ÷ 40 = 1,100 lb per foot. The fifth-wheel position sets the split between the tractor's two axles. It sits 17 ft along a 20 ft wheelbase. That sends 17/20 of a kingpin change to the drive tandem and the remaining 3/20 to the steer. This is why relieving the drives barely touches the steer, and why the two are not interchangeable levers.

Change the truck and these ratios change with it. A tractor with a shorter wheelbase, or a trailer with a different kingpin setting, has a different constant. This is why a rule of thumb learned on one unit quietly misleads on another.

The drive tandem as the cargo moves along the deck

Cargo centre of gravity measured behind the kingpin, with the same 44,000 lb of freight in every case. Forward of 20.9 ft the drive tandem goes over 34,000 lb; aft of 23.9 ft the trailer tandem does. The whole legal band on this combination is about 3.0 ft wide, and the gross weight is identical across all of it.

The four numbers sum to zero. Nothing appeared and nothing vanished. It moved.

Moving the sliders is not the same as moving the freight

Sliding the trailer tandem changes the span the trailer is solved over, so it changes the split between the kingpin and the trailer axles. Sliding the fifth wheel changes where the kingpin load lands on the tractor, so it changes the split between the steer and the drives. Moving freight changes the moment itself. Three different levers, three different effects, and only the third one changes the gross distribution of the load in the trailer.

The limits

Statute, tire rating, axle rating — and the lowest one wins

Three separate ceilings apply to every axle group, they come from three different places, and the binding one is often not the one people quote.

The steer axle on this tractor

Three limits applied to the steer axle.
Limit	Value
Statutory limitUS federal, steer axle on the Interstate System	12,000 lb
Tire ratingtwo tires, 8.5 in of tread each, at 600 lb per inch	10,200 lb
Manufacturer axle ratingstamped on the axle, from the tractor's own specification	13,200 lb

The binding limit is the tire rating at 10,200 lb, not the 12,000 lb everybody quotes. Our solved steer axle is 11,280 lb, which is 1,080 lb over the limit that actually applies while being 720 lb under the limit that gets talked about.

The drive tandem on the same tractor

Three limits applied to the drive tandem.
Limit	Value
Statutory limittandem, spaced more than 40 in and up to 96 in apart	34,000 lb
Tire ratingeight tires, 8.5 in of tread each, at 600 lb per inch	40,800 lb
Manufacturer axle ratingthe tandem's own rating	40,000 lb

Here the statute is the lowest of the three, so 34,000 lb is the real answer. This is the common case, which is exactly why the uncommon case on the steer axle catches people: the habit of quoting the statute is right most of the time.

Where each limit comes from

The statutory limit

Set by law, and it is jurisdictional. The US federal figures on the Interstate System are 12,000 lb on a steer axle, 20,000 lb on any single axle, 34,000 lb on a tandem, and 80,000 lb gross. States set their own limits off the Interstate System and some of them differ, so the jurisdiction a load is running in is an input to the calculation rather than a setting on the software.

The tire rating

Commonly expressed as 600 lb per inch of tread width, multiplied by the number of tires on the group. A steer axle with two tires carrying 8.5 in of tread each is limited to 10,200 lb by this rule, which is below the statutory 12,000 lb. Fit a wider steer tire — 11 in of tread, say — and the tire limit becomes 13,200 lb and the statute becomes the binding constraint again. This is one of the few compliance problems you can solve with a purchase order.

The manufacturer's axle rating

Stamped on the axle and printed in the vehicle specification. It is an engineering limit rather than a legal one, but exceeding it is both unsafe and, in most jurisdictions, an offence in its own right. It binds below the statute more often than people expect on spread-axle trailers, where the statutory allowance for two widely separated axles can exceed what the axles themselves are rated to carry.

Why the lowest one binds

These are three independent constraints, not three opinions about one constraint. The load has to satisfy all of them at once, so the effective limit on any axle group is the smallest of the three. A tool that reports only the statutory number will tell you a load is legal when the tires it is sitting on are not entitled to carry it, and that distinction is not academic at a roadside inspection.

Twelve thousand pounds on a steer axle, twenty thousand on a single, thirty-four thousand on a tandem, eighty thousand gross. The statute is one of three ceilings, and it is not always the lowest.

US federal axle limits, 23 CFR 658.17

Dated, and honest about it

Assessed from publicly published documentation as of August 2026. The geometry, tare weights and tire specifications in this page are worked examples chosen to be representative. Your own numbers will differ, and the whole argument here is that they matter.

Put the axle-weight guide against one real load.

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

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