Fifth wheel
The coupling that joins an articulated tractor unit to its semi-trailer, and carries part of the trailer’s weight.
The fifth wheel is the horseshoe-shaped plate on the back of a tractor unit. The trailer’s kingpin drops into it and locks, and the joint it forms both tows the trailer and carries a share of its weight through the tractor’s rear axle. That weight transfer is why fifth-wheel position affects axle loading.
Coupling correctly is a safety-critical sequence and a recurring exam subject: the pin must be locked, the locking bar or handle in the right position, and the connection checked by attempting to pull away against the trailer brakes before relying on it.
It also needs greasing. A dry fifth wheel makes steering and coupling heavier and wears both halves, and the plate is one of the walk-round checks a driver is expected to make rather than something left entirely to the workshop.
10 practice questions on this
Each one shows the answer, and our note on why every other option is not it.
Lorry & Bus (LGV/PCV)
- You've finished recoupling. How should you make sure that the tractor and trailer are secure? Vehicle loading
- You're part-loading a lorry with an empty ISO container. Where should you position it on the trailer? Vehicle loading
- Which articulated trailer is most at risk of jack-knifing? Vehicle loading
- When should your trailer be fitted with a kingpin or drawbar lock? Drivers' hours and rest periods
- When should your lorry coupling system (fifth wheel) be checked and lubricated? Vehicle condition
- You're driving an articulated lorry. What could happen if you change to a lower gear while you're going too fast? Braking systems
- What would you secure with a dog clip? Vehicle condition
- How frequently should the components of a fifth-wheel coupling be inspected? Vehicle condition
- What should you check for wear or damage on a drawbar unit? Vehicle condition
- You're using a lorry-mounted crane to unload building materials. What safety features should you use? Vehicle loading