Pickup converted to a refrigerated box: axle loads, payload and category
A 2020 double-cab pickup is booked in for a body conversion. The plate gives kerb mass 1,980 kg, GVW 3,200 kg, front axle rating 1,450 kg and rear axle rating 1,800 kg. On our weighbridge, empty with a full tank, it shows 1,120 kg on the front axle and 860 kg on the rear. Wheelbase is 3,220 mm and rear overhang is 1,180 mm. The customer wants an insulated cargo box of 480 kg whose centre of gravity sits 620 mm behind the rear axle, plus a refrigeration unit of 60 kg mounted 380 mm behind the rear axle. The tyres are 225/70R17C, load index 115 single, rated 1,215 kg each. He insists the conversion is straightforward because the truck 'stays under 3.5 tonnes, so the licence and the plate are fine', and he expects to keep carrying his usual 600 kg of chilled goods on the same route.
1. Calculate the axle loads after the conversion and state the maximum payload the vehicle can carry with the load placed at the box centre of gravity.
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Step 1 – combine the added masses. Total added = 480 + 60 = 540 kg. Combined centre of gravity behind the rear axle: b = (480 × 620 + 60 × 380) / 540 = 320,400 / 540 = 593 mm.
Step 2 – load transfer. With L = 3,220 mm, the rear axle takes m(L + b)/L = 540 × 3,813 / 3,220 = 540 × 1.184 = 639 kg. The front axle is relieved by 99 kg.
Step 3 – new kerb loads. Front 1,120 − 99 = 1,021 kg; rear 860 + 639 = 1,499 kg; total 2,520 kg.
Step 4 – payload. Rear-axle headroom is 1,800 − 1,499 = 301 kg. Payload at the box CoG adds 1.184 kg to the rear for every kg carried, so the maximum is 301 / 1.184 ≈ 254 kg. GVW would then be only about 2,774 kg.
Conclusion: the vehicle is axle-limited long before it is weight-limited. The 600 kg the customer expects is not achievable as specified. The tyres (1,215 kg each, so 2,430 kg per axle) are not the constraint; the axle rating is.
2. The customer insists on keeping a 600 kg payload. Give the engineering options and the check that decides whether each is acceptable.
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Every option must be judged against both axle ratings, not GVW alone.
- Move the load. Reposition the box so its CoG sits over the rear axle (b = 0). Rear share falls to 540 kg, rear axle becomes 1,400 kg, headroom 400 kg, so about 400 kg of payload with the front unchanged at 1,021 kg. Still short of 600 kg, and every 100 mm you move ahead of the axle spends front-axle headroom, of which only 429 kg exists.
- Lighter body plus repositioning. Dropping the box to roughly 300 kg and mounting it over the axle gives a rear load near 360 kg (rear axle ~1,220 kg) and around 550–560 kg of payload. This is the realistic route to the customer's figure.
- Re-rate the vehicle. Manufacturer-approved uprated springs and a second-stage plate raise the rear rating, but the front axle still caps the payload, and above 3,500 kg GVW the vehicle becomes N2 with different licensing, inspection and speed-limit rules.
Deciding check: load to the proposed payload with fuel and driver aboard, weigh each axle on a bridge, and confirm both are within rating. A calculation is only a prediction; the weighbridge is the evidence you record.
3. What must be recorded and told to the customer about classification, licensing and documentation?
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The 'under 3.5 tonnes' argument is only half right. At 3,200 kg GVW and five seats the finished vehicle stays in the light category, so an ordinary car licence still covers it, provided the conversion does not push GVW past 3,500 kg. That argument does not make the rear axle legal.
Record: the new kerb mass of 2,520 kg; the permitted payload and where it may be placed; the second-stage plate and body-builder documentation; the tyre ratings; and a notification to the insurer that the vehicle has been modified. If the unladen mass on the registration document changes materially, the customer must update it.
Give the customer a written loading chart of payload against position. Overloading an axle is an offence and a handling and braking risk even when total weight is legal, and an assessor will check the plate and the load after a claim. Do not release the vehicle on a verbal 'I'll keep it light' - the driver who overloads it will be the one who says nobody told him.