When is an articulated lorry most likely to jack-knife?
- A When it's carrying a high load
- B When its load is carried at the rear of the trailer
- C When it's unloaded
- D When it's fully loaded
Why this is the answer
When the trailer is loaded, the weight of the load presses the trailer tyres firmly onto the road surface, so improving the tyres' grip. Unladen, there is no load bearing down on the tyres, and under heavy braking the trailer wheels can lock or slide across the road surface, making the trailer more difficult to stop. Severe braking can result in jack-knifing: the trailer does not slow as quickly as the tractor unit and this causes it to pivot around the coupling.
Why the other answers don't fit
- A
- When it's carrying a high load — A high load raises the centre of gravity, which risks rolling rather than jack-knifing.
- B
- When its load is carried at the rear of the trailer — Weight at the rear is a loading fault with different consequences.
- D
- When it's fully loaded — A full load presses the drive axle down and helps it grip.
Where this comes from
- Driving Goods Vehicles — section 3
Rule numbers link to The Highway Code on gov.uk.
Where this question comes from
The question, its options and the explanation are reproduced word for word from the DVSA revision question bank under licence. The licence requires them to appear unaltered, so we cannot reword them and neither can anyone else — that is why you will meet the same wording on other sites. Everything else on this page is written by Theory Hub: the explanation of why each wrong answer is wrong, the topic guide and the revision notes.
Asked in: Lorry & Bus (LGV/PCV)
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