Rear-heavy box trailer behind a family hatchback
Marko is preparing a rented box trailer for a weekend trip. The trailer has a maximum authorised mass (MAM) of 1,300 kg and an unladen mass of 320 kg. His car has a kerb mass of 1,420 kg, a gross vehicle mass (GVW) of 1,960 kg, a rear-axle limit of 920 kg, a towbar limit of 1,300 kg and a permitted nose weight of 75 kg. He loads two e-bikes (28 kg each) and 180 kg of camping equipment at the rear of the trailer because the tailgate is easier to close there. With a bathroom scale under the coupling he measures a nose weight of 24 kg. At a public weighbridge the car’s rear axle shows 900 kg with the trailer coupled, the driver inside and 90 kg of tools already in the boot. The car’s rear tyres are at the solo-pressure shown on the door pillar. On the first open-road section the outfit begins to feel unstable at about 75 km/h.
1. Which measured values are unacceptable or marginal, and what does each one tell you about this combination?
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The first hard point is the nose weight: 24 kg is far below a sensible towing figure for this outfit. The trailer’s actual mass is 320 + 56 + 180 = 556 kg, so it is well under its 1,300 kg MAM and the towbar’s 1,300 kg limit. But 24 kg is only about 4.3% of the trailer’s actual mass, below the usual 5–7% range and well below the car’s 75 kg maximum. A low nose weight lets the trailer pivot and oscillate, which matches the instability at 75 km/h.
The rear axle is not yet illegal, but it is marginal: 900 kg against a 920 kg limit leaves only 20 kg. That matters because correcting the nose weight will add load to the rear axle. The trailer total mass is legal, the towbar limit is respected and the nose-weight maximum is not exceeded. The problem is distribution and the small rear-axle reserve, not an overweight trailer. Solo tyre pressures are also wrong for towing.
2. Describe the loading changes and checks you would make before driving again.
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Move the e-bikes and heavy camping equipment forward so that the mass sits over or slightly ahead of the trailer axle, keeping it low. Aim for a nose weight of about 45–55 kg, not the 75 kg maximum. Then re-weigh the nose and the car’s rear axle, because adding roughly 25 kg at the coupling will push the rear axle close to or over 920 kg.
To create reserve, move about 20–30 kg of the tools from the car boot into the trailer over its axle, or leave them at home. Check that the car’s GVW is still respected and set the rear tyres to the towing pressure in the handbook. Confirm the coupling height, that the locking indicator is fully engaged, that the breakaway cable is attached to its dedicated anchorage, and that the load is strapped so it cannot move. Do a short low-speed test and re-check the nose weight and tyre temperatures before joining faster traffic.
3. If the outfit starts to snake again despite the corrections, what is the correct immediate response and why?
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Ease off the accelerator smoothly and keep the steering wheel steady. Do not brake hard and do not make a sharp steering correction, because both can amplify the yaw and cause a jack-knife. Let the speed fall gently; if the trailer has a stabiliser or overrun brake, it will help only if the coupling and linkage are correct. Once controlled, stop in a safe place and re-check nose weight, tyre pressures, load restraint and coupling engagement.
The reason is that snaking is a self-reinforcing oscillation: a sudden brake or steering input adds a lateral impulse at the coupling, and the trailer’s inertia pushes the car’s rear axle sideways. A gentle reduction in speed reduces the energy available to the oscillation. If it recurs at normal road speed after rechecking, the outfit should not continue until the cause is found.