Guide
A procedure that produces a legal trailer every time
Most loads are built by weight and checked by hope. This is the other way round: work out the ceiling and the window before a forklift moves, load to them, then verify. It takes about ten minutes longer and it removes an entire category of bad afternoon.
- A derived payload ceiling
- A centre-of-gravity window
- Nine steps, in order
Before the forklift starts
Four numbers you need, and none of them are on the bill of lading
A loading plan built without these is a guess with a document attached to it.
The equipment geometry
Kingpin to trailer tandem centre, tractor wheelbase, fifth wheel position, and where each slider currently sits. These are properties of the specific tractor and the specific trailer, not of a category of truck, and they are the inputs every other number on this page is computed from. If the yard cannot tell you the fifth wheel setting on the unit that is about to be loaded, that is the first thing to fix.
The two tare weights
The tractor as it will run — fuelled, with the driver and their kit aboard — and the trailer with whatever lives in it permanently: load bars, straps, dunnage, a pallet jack. Tare weights that were recorded once when the unit was new are the single most common source of a plan that is wrong by four hundred pounds in the same direction every time.
The binding limits along the whole route
Not the limits at the origin. The limits everywhere the truck is going, including the state it clips for twenty miles. And for each axle group, the lowest of three: the statutory limit, the tire rating at 600 lb per inch of tread width, and the manufacturer's axle rating.
What the freight actually is
Item weights and item dimensions, whether each item is stackable, and whether it is divisible. Divisibility decides what your options are when the arithmetic fails. A pallet of paper can be split across two trucks. A transformer cannot, and for that load the answer is a permit rather than a rearrangement.
Step one
The payload ceiling is lower than the gross limit says
Everyone computes payload as the gross limit minus the tare weights. On most combinations that number is not achievable, and the gap is worth knowing before you promise it to a customer.
Take the combination from the axle weight guide: a 18,000 lb tractor and a 14,000 lb trailer. Subtract both from the 80,000 lb federal gross limit and you get 48,000 lb of payload. That is the number that gets quoted.
Now do it from the axle limits instead. The drive tandem may carry 34,000 lb. It already carries part of the tractor's own weight, so the most it can accept from the kingpin is 27,294 lb. The steer axle imposes its own kingpin cap of 32,000 lb, which is looser, so the drive tandem is the binding one at 27,294 lb.
Everything on the trailer is carried either by the kingpin or by the trailer tandem. The kingpin can take 27,294 lb and the trailer tandem can take 34,000 lb, so the trailer as a whole can carry 61,294 lb. The trailer's own tare of 14,000 lb comes out of that, which leaves 47,294 lb for the freight.
The axle limits are the tighter of the two, so the real payload ceiling on this combination is 47,294 lb. There are 706 lb of payload that exist in the gross arithmetic and cannot be carried at any position, in any arrangement, by this truck.
Payload by gross
48,000 lb
80,000 lb less both tare weights
Kingpin ceiling
27,294 lb
the most the drive tandem can accept
Payload by axle limits
47,294 lb
kingpin ceiling plus tandem, less trailer tare
Payload that is not real
706 lb
on paper only, at any position
There are 706 lb of payload that exist in the gross arithmetic and cannot be carried at any position, in any arrangement, by this truck.
Run this once per combination, not once per load
Step two
The centre-of-gravity window, and how it closes
For any given payload there is a band of positions where the load's centre of gravity may sit. Too far forward and the drive tandem goes over. Too far back and the trailer tandem does. The heavier the load, the narrower the band.
| Payload | Gross | CG no further forward than | CG no further back than | Window |
|---|---|---|---|---|
| 38,000 lb | 70,000 lb | 17.90 ft | 27.68 ft | 9.78 ft |
| 40,000 lb | 72,000 lb | 19.01 ft | 26.30 ft | 7.29 ft |
| 42,000 lb | 74,000 lb | 20.01 ft | 25.05 ft | 5.04 ft |
| 44,000 lb | 76,000 lb | 20.91 ft | 23.91 ft | 2.99 ft |
| 46,000 lb | 78,000 lb | 21.74 ft | 22.87 ft | 1.13 ft |
| 47,000 lb | 79,000 lb | 22.13 ft | 22.38 ft | 0.25 ft |
Read the last column
At 38,000 lb of payload the centre of gravity can land anywhere across 9.78 ft and the truck is legal. Nobody needs a procedure for that load. At 46,000 lb the window is 1.13 ft, and at 47,000 lb it is 0.25 ft.
This is why heavy loads go wrong and light ones do not, and it is also why experience stops being a reliable guide near the ceiling. A planner who has placed a thousand loads correctly has been working inside a window several feet wide. The window on a nearly-full trailer is narrower than the pallet they are about to put down.
What moves the window rather than the load
The two sliders. Taking the trailer tandem back lengthens the trailer's span, which moves both edges of the window forward. Moving the fifth wheel forward raises the kingpin ceiling, which moves the front edge forward. Neither of them touches a single pallet. On a load that will not fit the window as the trailer stands, the correct first move is almost always a slider rather than a forklift.
The centre-of-gravity window closing as the payload rises
ftHow wide the legal band for the cargo centre of gravity is, at each payload in the table above. It is 9.78 ft at 38,000 lb and 0.25 ft at 47,000 lb. The curve is why experience stops being a reliable guide near the ceiling.
Three ceilings on payload, and the lowest one is the answer
lb- By the steer axle, through the kingpin52,000 lb
the steer axle is the looser of the two tractor constraints
- By the 80,000 lb gross limit48,000 lb
the figure that gets quoted to a customer
- By the drive tandem, through the kingpin47,294 lb
the lowest of the three, so this is the real one
Each bar is what one constraint alone would allow, computed from the same geometry. The truck has to satisfy all three at once, so the real ceiling is 47,294 lb — 706 lb short of what the gross arithmetic promises.
The window on a nearly-full trailer is narrower than the pallet you are about to put down.
The procedure
Loading, in the order that works
Nine steps. The order matters more than any individual step, because several of them are cheap before the load is built and expensive afterwards.
- 01
Confirm the combination and read its settings
Which tractor, which trailer, where the fifth wheel is set and where the trailer tandem is pinned. Write the two slider positions down. Everything downstream is computed against them, and a trailer that came back from a customer with the tandems in a different hole will quietly invalidate the whole plan.
- 02
Establish the jurisdiction and the binding limits
For every state the run passes through, not just the one you are standing in. For each axle group take the lowest of the statutory limit, the tire rating and the axle rating. Write the five binding numbers at the top of the plan. If the tire rating on the steer axle is below 12,000 lb, that is the number that goes on the sheet.
- 03
Compute the payload ceiling and check the order fits
Kingpin ceiling plus trailer tandem limit, less the trailer tare. If the freight on the order exceeds that number, no arrangement will make it legal and you have found out now rather than at the scale. The options at this point are two trucks, a different trailer, or a permit if the freight is non-divisible.
- 04
Compute the centre-of-gravity window for this payload
The drive tandem sets the forward edge, and the trailer tandem sets the rear edge. Mark both limits on the trailer floor if that helps the crew. Two strips of tape are more useful on a dock than a number on a clipboard.
- 05
Place the heavy items first, and place them deliberately
The heaviest items dominate the centre of gravity, so they are the ones worth positioning carefully. Put them near the target position and build the lighter freight around them. Loading nose-first and hoping the tail fills in is how a load ends up with its centre of gravity three feet forward of where it needed to be.
- 06
Solve the axle weights before anything is secured
Run the trailer solve, the kingpin hand-off and the tractor solve on the load as it currently sits. This is the last moment when moving a pallet costs nothing. Once the load is strapped, blocked and braced, moving one item means undoing all of it.
- 07
Adjust with the sliders before you adjust with the forklift
If the solve is out, try the sliders first. The trailer tandem changes the split between the kingpin and the trailer axles. The fifth wheel changes the split between the steer and the drives. Both are a two-minute job. Re-solve after each move, because each one changes more than the group you were aiming at, and record the new settings on the plan.
- 08
Secure, then re-check the weight you just added
Securement adds real weight: chains, straps, load bars, corner protectors, dunnage and blocking. On a heavy load near the ceiling this is not a rounding error. Add it to the plan at the position it actually sits, which is usually spread through the trailer rather than at the centre of gravity.
- 09
Weigh, reconcile, and record the settings
Get a group-by-group ticket rather than a gross one, compare it against the plan group by group, and record the fifth wheel and tandem settings against the load. The settings are what make the load reproducible next week. The reconciliation is what tells you whether your tare weights are still true.
Judgement
Five things that go wrong even when the procedure is followed
The arithmetic is the easy part. These are the failures that come from the world rather than the maths.
The load changes after you finish with it
Fuel is added on the way out of the yard and lands on the tractor. A second pickup boards freight at stop two that changes everything from stop two onwards. A drop at stop three removes weight from one part of the trailer and leaves the centre of gravity somewhere new. A load that was solved once, at the dock, has been solved for one moment of its life.
The slider was not where the paperwork said
Trailers come back from customers with the tandems moved. Drivers slide them to get under a dock, to make a turn, or because a previous load needed it. If the plan assumed one setting and the trailer is on another, every number in the plan is wrong and none of them look wrong. Check it physically, on the trailer, before loading.
Height was ignored because weight was the question
Centre-of-gravity height does not appear anywhere in the axle arithmetic, and it decides the rollover threshold. A load that is legal on every axle and stacked tall is a different vehicle in a curve. Where the freight allows it, heavy low and light high is worth more than the few inches of cube it costs.
The freight is not what the paperwork says it weighs
Declared weights are estimates surprisingly often, and they are usually estimates in the direction that favours the person declaring them. On a load near the ceiling, a five percent error in the declared weight is larger than the entire centre-of-gravity window. Where it matters, weigh rather than believe.
Nobody wrote down what worked
The same lane, the same trailer and the same commodity will run again next week. If the fifth wheel setting and the tandem position that made it legal were not recorded against the load, the crew solves the same problem from scratch, and gets it right at a different rate.
A load that was solved once, at the dock, has been solved for one moment of its life.
Dated, and honest about it
Next
Related reading
The procedure above assumes you can solve the axle weights and the bridge subsets. These pages are how.
Guide
Axle weight basics
The three solves the procedure depends on, worked through with the same combination.
ReadField notes
Nine ways loads go overweight
The specific failures this procedure is designed to prevent, with the numbers behind each.
ReadGuide
Reading a scale ticket
Step nine in full: how to reconcile the ticket against the plan and learn something from it.
ReadFor the placement and the solve happening in the same frame, see theload planner.
Next step
Put the legal-load procedure against one real load.
Bring the route, configuration or scale ticket that made the legal-load procedure matter. We will work through it in plain English and show where AxleGuard changes the decision.