Your truck is under 40 tonnes and still illegal
A weighbridge does not weigh your paperwork. It weighs each axle. A combination can run at 39,000 kg, well under the maximum, and put 13,500 kg on the driving axle. That is an offence, and no document shows it — only where the load sits does.
Why the total tells you nothing
A semi-trailer rests on two points and only two: the kingpin that couples into the tractor's fifth wheel, and the tri-axle at the back. Between those two supports, every item splits its weight like a plank resting on two bricks.
to the tri-axle = P × d / B to the kingpin = P × (B − d) / B
Where d is the distance from the kingpin to the item and B the distance from the kingpin to the centre of the tri-axle, usually somewhere between 7,000 and 8,100 mm. A pallet sitting directly over the tri-axle loads it almost entirely. The same pallet against the nose loads the kingpin instead, and from there it all goes onto the tractor.
Move one pallet a metre and the axle figures change, with the total weight identical. That is the whole business.
Spanish limits are not the European ones
This catches out plenty of people in the trade, and it is one of the things that costs real money. Directive 96/53/EC sets the maxima for international traffic. Each country may set different ones for traffic inside its borders, and Spain raised its own with Orden PJC/780/2025.
| Spain, domestic | International | Gap | |
|---|---|---|---|
| Single non-driving axle | 10,000 kg | 10,000 kg | none |
| Driving axle | 12,000 kg | 11,500 kg | 500 kg |
| Semi-trailer tri-axle | 27,000 kg | 24,000 kg | 3,000 kg |
| Combination, 5 axles or more | 44,000 kg | 40,000 kg | 4,000 kg |
The practical consequence is ugly: a truck loaded to the Spanish maximum can be in breach the moment it crosses the border. Not because anyone did anything wrong, but because the truck changed regime without changing its load.
If you run international, the number that binds you is in the narrow column, not the wide one.
The warning almost nobody gives: the driving axle has a floor
Everyone watches the ceiling. Almost nobody watches the floor.
Annex I of Directive 96/53/EC, point 4.1, requires that the weight on the driving axle or axles is not less than 25 % of the total laden weight. It is a minimum, not a maximum, and you breach it by doing exactly what looks prudent: shifting the load back to relieve the tractor.
Beyond the letter of it, this is elementary mechanics. With no weight over them, the drive wheels do not grip. You notice on a wet climb, and you notice on a painted roundabout.
Load too far forward and you blow the driving axle through its ceiling. Load too far back and you drop under the 25 % floor. Between those two mistakes there is a good band, and finding it is the entire job.
Three rules that fall out of the formula
These are not received wisdom. They follow from the two equations above.
1. The centre of gravity goes between the kingpin and the tri-axle. Always. Look at the kingpin formula: if d is greater than B, the result is negative. Translated: load behind the tri-axle lifts the kingpin instead of loading it. The trailer levers against the tractor. That is not merely inefficient; it is dangerous.
2. Put the heavy items near the middle of the run. An item halfway between kingpin and tri-axle splits its weight evenly. The further you move it from there, the more it skews the split — and a heavy item skews it far more than a light one. If you can only place one thing well, place the heaviest.
3. Spread it across the width too. The left-to-right split appears in no axle limit, but an off-centre load raises rollover risk in a bend and overloads one side's tyres. That is not a fine; that is an incident.
What to do before loading
- Get the real dimensions for your equipment. Tractor tare, trailer tare, wheelbase, and above all the kingpin to tri-axle centre distance. They are on the plate. That last one varies most from trailer to trailer, and guessing it is what dirties the whole calculation.
- Try the arrangement before you move it. The axle weight calculator lets you drag the load along the deck and watch each axle live, and it proposes the position that leaves the most headroom on the worst axle.
- Flip the switch to international if the trip leaves Spain, even once. Better to find out on screen than at the border.
- Then weigh it. No calculation replaces a weighbridge. What a calculation does is stop you arriving at one with a plan you already knew would fail.
Where the model stops
The statics of a beam on two supports explain the split between kingpin and tri-axle, and the tractor's geometry explains the split between its two axles. What they do not explain is how the load divides inside the tri-axle group, which depends on the suspension and any lift axle, or what a three-axle tractor does, where the trailing axle changes the arithmetic.
Nor does it model gradient, braking, or a load that shifts. It is for planning, and for throwing out a bad plan before you load it. The figure that counts at an inspection is the weighbridge's, every time.