Driving technique, stability and handling of a lorry with heavy trailer
The exam tests whether you can predict how a long, heavy, articulated vehicle behaves before it does something you did not intend. Physics, not opinion: mass high up, long overhangs and an articulated joint all amplify small inputs.
The geometry of a turn
- Off-tracking (sporing): the rear axle of a long combination cuts inside the path of the tractor. A 16.5 m articulated outfit can sweep a metre or more inside.
- Tail swing (udhæng): the rear of the tractor or the front corner of the trailer swings outward in the opposite direction to the turn. This is what hits cyclists, bollards and parked cars.
- Swept path: on tight city corners, plan the turn so the trailer stays clear of kerbs and cycle lanes, without swinging into oncoming traffic.
Stability at speed
- Roll-over risk rises with centre-of-gravity height, speed and steering input. Enter bends already at the right speed; braking or steering hard mid-bend loads the outer tyres and can roll the trailer.
- Snaking (slangegang) is oscillation of the trailer, most common with a light tractor, an empty high-sided trailer and speed. The correction is to ease off the accelerator smoothly, keep the steering still, and let the combination settle. Hard braking or counter-steering amplifies it.
- Crosswind and passing high-sided vehicles push a lightly loaded trailer sideways; reduce speed rather than fight the steering.
Downhills and braking technique
- Use the engine brake or retarder and a low gear; continuous service-brake application causes fade.
- Never coast in neutral. The retarder works best at moderate engine speed, so choose the gear before the descent.
Worked example - the snaking trailer. An empty curtain-sided semi-trailer starts to sway at 78 km/h on a windy motorway. Reasoning: the oscillation is being fed by speed and by steering corrections. Actions: release the accelerator progressively, hold the steering wheel steady, do not brake sharply, allow the speed to fall until the sway stops, then continue at a lower speed with the load or the route reconsidered.
Traps that cost marks
- Braking hard when a trailer starts to sway.
- Turning late and sharp on a city corner, causing tail swing into a cycle lane.
- Believing the trailer follows exactly in the tractor's tracks.
- Choosing a high gear for a long descent to save fuel.