Category B with a heavy trailer on a mountain pass
You hold a category B licence (issued 2015, no additional codes). You are moving house and want to tow a twin-axle box trailer behind your 1,480 kg kerb-weight estate car. The trailer's unladen mass is 400 kg and its maximum permissible mass (technically permissible maximum laden mass) is 1,600 kg; the car's permissible maximum mass is 2,050 kg and its permissible towing capacity is 1,800 kg. The route crosses a pass with a long 9% descent and a signposted 60 km/h limit for vehicles with trailers. Your friend says: 'With B you can tow anything up to 750 kg, and above that it just depends on the car's towing capacity, so you're fine.' You are unsure whether your combination is legal and how to handle the descent.
1. Assess whether your licence category and vehicle combination are legal for this trip, and state what you would do before loading.
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The friend's rule of thumb is wrong on both counts. Category B does allow a light trailer up to 750 kg permissible maximum mass without further arithmetic, but heavier trailers are only covered when the combination stays within the B limits: the trailer's permissible maximum mass must not exceed the unladen mass of the towing vehicle, and the combined permissible maximum masses of car and trailer must not exceed 3,500 kg. Here the trailer's 1,600 kg permissible maximum mass far exceeds the car's 1,480 kg kerb weight, and the sum 2,050 + 1,600 = 3,650 kg exceeds 3,500 kg. So this combination is not covered by plain B, even though the car's 1,800 kg towing capacity would physically permit it. Before loading I would either choose a lighter trailer (permissible maximum mass at or below 1,480 kg, keeping the total under 3,500 kg) or obtain the appropriate extended entitlement (B96 or BE) and have the licence updated. I would also confirm the actual laden masses on a weighbridge, because overloading the trailer or exceeding the car's permissible tow load is a separate offence regardless of licence category.
2. On the 9% descent with the loaded trailer, what driving technique and gear choice do you use, and why is sustained braking risky?
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On a long 9% descent the engine must do most of the braking. I select a low gear before the gradient starts - typically second or even first in a manual, or manual mode with a low ratio in an automatic - and let engine braking hold the speed near the signposted 60 km/h. I brake firmly for short periods to shed speed, then release, rather than riding the brakes continuously. Sustained light braking overheats the discs and fluid; the pads can glaze (fade) and, in the worst case, the fluid can boil and the pedal goes to the floor. The trailer also pushes from behind, so the car's rear can feel light and the combination can jack-knife if the trailer brakes first or the car brakes too sharply mid-corner. I keep the trailer's overrun brake and breakaway cable checked, leave a much longer gap than solo (the total combination mass roughly doubles the stopping distance), avoid sudden steering inputs, and use a low gear plus engine braking so the trailer's brakes are only assisting. If I smell hot brakes or the pedal feels long, I stop in a safe lay-by and let everything cool before continuing.