Combination knowledge: coupling, articulation, dimensions and masses
This domain asks you to describe the hardware that holds a combination together and to reason about the numbers on the plates. It is the most technical part of the CE test and the part where careless reading of a question loses most marks.
The coupling hardware
- Tractor and semi-trailer (sættevogn): the fifth wheel (saddel) locks around the trailer's kingpin (koblingsbolt), with a locking jaw and a secondary safety catch. Landing legs carry the trailer when uncoupled.
- Drawbar trailer (påhængsvogn): a drawbar eye or ball coupling, plus a breakaway safety cable that applies the trailer brakes if separation occurs.
- Centre-axle trailers put part of their mass on the towing vehicle's coupling, so the coupling load is part of the towing vehicle's payload.
Dimensions to know
- Maximum width 2.55 m (2.60 m for certain refrigerated bodies), maximum height 4.00 m.
- Maximum length of an articulated combination 16.5 m; a lorry with drawbar trailer 18.75 m. Longer modular combinations exist on approved routes but are not the default answer.
- Kingpin load and axle loads are limited separately from total mass; the lowest of the three decides your payload.
Mass arithmetic
Payload = permitted total mass − kerb mass of tractor − kerb mass of trailer, then corrected for how the load sits. Fifth-wheel load is carried by the drive axle, which is why a correctly loaded semi-trailer usually improves traction.
Worked example - is it legal? Tractor kerb mass 8,200 kg, trailer kerb mass 7,300 kg, permitted combination mass 40,000 kg. Front axle limit 7,500 kg, drive axle limit 11,500 kg. The load gives a kingpin load of 12,000 kg and a front axle reading of 7,000 kg. The tractor's own rear share is about 3,000 kg, so the drive axle carries roughly 15,000 kg - over its 11,500 kg limit even though the total is legal. The fix is to move cargo rearward to reduce the kingpin load until the drive axle is within limits.
Traps that cost marks
- Reporting payload by subtracting only the tractor's kerb mass and forgetting the trailer.
- Treating the fifth-wheel load as weight that does not count against the drive axle.
- Assuming a legal total mass means legal axle loads - they are checked separately.
- Quoting the unladen height or width when the question describes a loaded combination.