Trailer sway on the coastal road
You are driving a D1 minibus with a 1,800 kg braked trailer on a coastal rural road. The trailer carries eight kayaks on a roof rack, so the centre of gravity is high. The measured nose weight is only 35 kg; the permitted range is 50–80 kg. Trailer tyre pressures are 2.0 bar against 2.8 bar recommended. You are descending a gentle hill in a crosswind at 72 km/h. The road has a 70 km/h limit for trailers. Without warning the trailer begins to yaw left and right; you feel the rear of the minibus being pushed. You have 14 passengers, all belted, and no load in the minibus. The trailer has overrun brakes and a working safety cable.
1. What are your first actions in the next few seconds, and why should you not brake hard or countersteer?
Read the worked answer
Ease off the accelerator smoothly and hold the steering wheel firmly in the direction you want to go. Do not brake hard and do not countersteer against each swing. Hard braking transfers weight forward, unloads the trailer axle and can make the yaw worse; countersteering adds a second steering input that can amplify the oscillation. Keep the steering as straight as the road allows and let the speed fall. If you must reduce speed more, use gentle, progressive braking after the sway has begun to settle, because the overrun brakes need a smooth deceleration to work without snatching. Switch on the hazard lights if following traffic is close. As soon as it is safe, pull completely off the carriageway, stop, and keep passengers in the vehicle until you have checked the coupling and load.
2. What in the loading and tyre setup made the sway likely?
Read the worked answer
Several factors combined. The nose weight is only 35 kg, below the permitted 50–80 kg, so the trailer is not tracking firmly behind the tow ball and is more likely to oscillate. The kayaks are on a roof rack, raising the centre of gravity and increasing the pendulum effect. The trailer tyres are at 2.0 bar instead of 2.8 bar, so the sidewalls flex more and the trailer is less stable. You are also at 72 km/h where the trailer limit is 70 km/h, and the crosswind and downhill section add lateral force. Any one of these reduces margin; together they made a small disturbance grow into a sustained yaw. The loading plan should have put heavier items low and over the axle, with the nose weight checked on a scale before departure.
3. After stopping, what would you check and change before continuing?
Read the worked answer
Check the coupling is still locked, the safety cable is attached, the jockey wheel is clear, the lights work and the trailer tyres are undamaged. Redistribute the kayaks so the centre of gravity is lower and further forward, and adjust the load until the nose weight is within 50–80 kg; do not simply add weight at the rear to increase nose weight. Inflate the trailer tyres to 2.8 bar when cold and inspect them for damage. Check the trailer brake linkage and the minibus tyres. Reduce speed to 70 km/h or lower, and drive more slowly still in crosswinds or on descents. If the nose weight cannot be corrected with the available load, do not continue with the kayaks on the roof rack; leave part of the load behind or use a different trailer.