Axle Loads and Strap Capacity
You are loading a two-axle rigid truck with a 5.0 m wheelbase. The tare weight is 3,500 kg on the front axle and 4,500 kg on the rear axle. The front axle limit is 7,500 kg, the rear axle limit is 11,500 kg, and the gross vehicle weight limit is 18,000 kg. Nine pallets each weigh 1,000 kg and occupy 1 m of load-bed length. The load bed is 8 m long. Your planned load centre is 3.5 m behind the front axle. Company securement policy requires an aggregate lashing capacity of at least 1.5 times the load weight; each strap has a lashing capacity of 2,000 kg. The load is 2.4 m high and the pallets are stable but not shrink-wrapped together. You must check axle loads, decide whether the load can be moved 1 m rearward, and specify the minimum number of straps.
1. Calculate the axle loads with the load centre at 3.5 m and state whether they are legal.
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Use the wheelbase as the moment arm. Added front load = payload × (wheelbase − load centre) ÷ wheelbase = 9,000 × (5.0 − 3.5) ÷ 5.0 = 2,700 kg. Added rear load = 9,000 × 3.5 ÷ 5.0 = 6,300 kg. Total front axle = 3,500 + 2,700 = 6,200 kg, which is below the 7,500 kg limit. Total rear axle = 4,500 + 6,300 = 10,800 kg, which is below the 11,500 kg limit. Gross weight = 6,200 + 10,800 = 17,000 kg, below the 18,000 kg limit. The load is legal at this position, but the rear axle has only 700 kg of margin.
2. What happens if the load centre moves to 4.5 m, and what should you do?
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At 4.5 m: added front load = 9,000 × (5.0 − 4.5) ÷ 5.0 = 900 kg, so front axle = 4,400 kg. Added rear load = 9,000 × 4.5 ÷ 5.0 = 8,100 kg, so rear axle = 12,600 kg. That exceeds the 11,500 kg rear limit by 1,100 kg, so the vehicle would be overloaded on the rear axle even though gross weight is still 17,000 kg. Move the load forward or reduce the payload. The maximum load centre for the rear limit is found from 4,500 + 9,000x ÷ 5.0 ≤ 11,500, giving x ≤ 3.89 m. Keep the load centre at or ahead of about 3.89 m, then recheck both axles.
3. How many straps are needed and how should they be arranged?
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Required aggregate lashing capacity = 1.5 × 9,000 = 13,500 kg. With 2,000 kg per strap, the mathematical minimum is 7 straps because 6 straps give only 12,000 kg. Use at least 7, but arrange them symmetrically and practically: at least two across the front pair of pallets, two across the rear pair and the remainder distributed along the load. If straps run at an angle rather than straight down, their effective lashing capacity is reduced, so use more straps. Tension them against the load, protect sharp edges, and check that the pallets cannot slide or tip as a group. Seven is the minimum, not a target to beat.