Overloaded drive bogie and a tunnel posted at 4.20 m
You are driving a 25.25 metre modular combination, a three-axle tractor unit with a semi-trailer and a link trailer, carrying 33 pallets of bagged fertiliser, 24.8 tonnes of payload, to a farm supply depot in Gudbrandsdalen. At the Statens vegvesen control station on the E6 the weighbridge printout reads: total 49,200 kg against a permitted 50,000 kg for this section; steer axle 7,850 kg against 8,000 kg permitted; tractor drive bogie 19,600 kg against 19,000 kg permitted; semi-trailer axles 11,300 kg against 18,000 kg permitted; link trailer axles 10,450 kg against 18,000 kg permitted. The card in the cab states a height of 4.10 m, measured a week ago with the combination empty on new tyres. Ahead lies a mountain pass where a tunnel is posted at 4.20 m, and the forecast is wet snow above 400 m. Every pallet was loaded tight against the front bulkhead of the semi-trailer, and nothing has been restrapped since the depot.
1. The control officer says the combination is overloaded. Which axle group is the problem, and how do you correct it without taking freight off?
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What the figures show Only one axle group is over: the tractor drive bogie at 19,600 kg against 19,000 kg, so 600 kg too much. The steer axle has 150 kg of headroom, both trailer groups have thousands of kilos spare, and the total is 800 kg under the permitted 50,000 kg. Nothing needs to be unloaded.
Why moving pallets aft works A pallet sitting near the kingpin passes most of its weight down through the fifth wheel into the tractor. Move the pallets rearward on the semi-trailer deck and that weight is carried by the trailer's own axles instead. Roughly four fifths of the weight you take off the kingpin comes off the drive bogie, which is exactly what is needed; two pallets of about 750 kg each is plenty of margin.
What to watch The same transfer also unloads the steer axle, because the fifth wheel load is shared between the tractor's two axle groups. Only 150 kg of headroom exists there, so check the minimum steer axle load on the vehicle card before you accept the result. Less load on the drive bogie also means less traction on a snowy pass. Re-weigh on a public weighbridge after moving the pallets, and re-tension the straps.
2. The tunnel is posted at 4.20 m and your card says 4.10 m. Do you drive through?
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No, not on that figure. A round blue/red height prohibition is an absolute limit, not advice, and 10 cm is not a real margin.
Why the 4.10 m is not the laden height
- It was measured empty. With 24.8 tonnes on the deck the suspension sits differently, and air suspension does not necessarily sit lower under load.
- It was measured on new tyres. Worn tread can change the height by well over a centimetre.
- The forecast is wet snow above 400 m. Snow and ice build up on the trailer roof and on any roof hatches, easily adding several centimetres.
- Any measurement carries tolerance, and the sign has no obligation to give you spare room.
What to do Before the pass, stop somewhere level and safe and measure the highest point of the loaded combination, which is usually the trailer roof, a roof hatch or the load itself, with a rigid rod or a height gauge. Proceed only if the measured height plus a sensible margin clears 4.20 m with the roof hatch closed. If not, take the alternative route or ring the operator. If you cannot measure it, you do not drive through.
3. The officer also questions your load securing. Explain why the tarpaulin and the load's own weight are not securing the pallets, and what you check before departure.
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Weight is not securing Bagged fertiliser on pallets is a deformable, sliding load. Friction alone carries only a fraction of its weight, and on a wet or dusty deck that fraction is small. The rules assume the load must withstand 0.8 g forwards and 0.5 g sideways and rearwards, so 24.8 tonnes generates a forward force of roughly 19.8 kN that friction will not absorb. A tarpaulin is weather protection only and secures nothing.
What to check
- The rated strength of the headboard; it takes only the forward force it is designed for, and the pallets must actually bear on it.
- Direct lashing over each stack, with the number of straps calculated from the lashing capacity on the strap label, not from habit.
- Edge protectors wherever a strap crosses a sharp corner, and straps pressing on the rigid part of the pallet rather than cutting into the bags.
- Anti-slip mats under the pallets, which raise friction and cut the number of straps needed.
- Re-tensioning after the first 10 to 20 km, and again after any load move at the control station.