Axle overload after adding a spare part
Your 7.5 t C1 box lorry has a permissible gross mass of 7,500 kg and a kerb mass of 3,300 kg. The front axle is rated at 2,800 kg and the rear axle at 5,000 kg. With 3,600 kg of pallets loaded at the rear, a weighbridge shows 2,250 kg on the front axle and 4,650 kg on the rear axle. You now want to add a 500 kg spare part at the very back. The tyre placard gives 4.75 bar for the rear axle at maximum load; your cold pressures are 4.3 bar. Rear tread is 3.0 mm, the ABS lamp is off, and the sidewalls show no cracks. The dispatcher says the gross mass is still under 7,500 kg, so it should be fine. You must decide whether to depart, how to redistribute the load, and what to do about tyre pressures.
1. Can you depart with the 500 kg part placed at the rear? Show the mass and axle-load calculation.
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Not as loaded. The gross mass would be 3,300 + 3,600 + 500 = 7,400 kg, which is under the 7,500 kg permissible gross mass. But the rear axle would be about 4,650 + 500 = 5,150 kg, exceeding its 5,000 kg rating by 150 kg. The front axle would remain around 2,250 kg, still below 2,800 kg. The dispatcher is looking only at gross mass; axle limits are separate and legally binding. Departing would overload the rear axle, reduce steering stability, increase tyre heat, and risk an enforcement penalty. The part must be left behind or the load redistributed.
2. How should you redistribute the load to make the vehicle legal?
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Move at least 150–250 kg from the rear to a position further forward, or leave the part behind. If you move 250 kg forward, the rear axle falls to about 4,900 kg and the front rises to about 2,500 kg, both within their limits. Secure the pallets so they cannot slide rearward on the first bend or brake application. Re-weigh the vehicle after redistribution rather than relying on estimates, because the exact effect depends on the distance from the axles. Check that the gross mass, front axle, rear axle, and tyre loads are all within limits before departure.
3. What should you do about the rear tyre pressures?
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Set them to the placard value of 4.75 bar cold when the rear axle is at or near maximum load. The current 4.3 bar is 0.45 bar low; underinflation reduces load capacity, increases sidewall flexing and heat, and worsens handling. Check for leaks or damaged valves before inflating. Do not reduce pressure to soften the ride, and do not exceed the placard pressure. After adjustment, confirm the pressures when cold and monitor them during the journey.