Coupling, loading and weight distribution
What this domain tests
This section tests the physical connection between lorry and trailer, the legal weight limits, and how load placement affects stability. Exam questions often combine coupling checks with axle-load calculations.
Coupling checks
- Confirm the coupling is fully engaged and the locking indicator shows locked.
- Connect the breakaway cable or safety chain so it operates the trailer brake if separation occurs.
- Plug in the electrical connector and test brake lights, indicators, and rear lights.
- Raise the jockey wheel and secure it; check the trailer handbrake is released.
- After moving a few metres, stop and recheck the coupling and load.
Weight and distribution
- Stay within the lorry's gross vehicle weight, each axle's plated maximum, the trailer's maximum, and the combination's train weight.
- Distribute load evenly; heavy items go low and over the axles, not behind the rear axle.
- Nose weight adds to the lorry's rear axle. Too much nose weight overloads the rear axle and unloads the front steering axle.
- Too little nose weight causes trailer sway and poor tracking.
Load securing
Use certified straps or chains with sufficient lashing capacity. In normal road transport, secure against forward acceleration of about 0.8 g, rearward 0.5 g, and sideways 0.5 g, using friction and lashings together. Check tension after the first kilometres.
Worked example
A lorry has a plated train weight of 8,500 kg. Its own mass is 7,000 kg and the trailer is 2,000 kg. Each unit is individually legal, but the combination is 9,000 kg. The train weight is exceeded by 500 kg, so the combination must not be driven on the road. The correct action is to reduce load or use a vehicle with a higher plated train weight.
Traps that cost marks
- Checking only the lorry's gross weight and ignoring train weight.
- Forgetting that nose weight transfers onto the rear axle.
- Leaving the breakaway cable unconnected.
- Trusting the coupling without checking the locking indicator.
- Assuming a legal trailer and legal lorry always make a legal combination.