Axle load is the share of a truck's weight and cargo carried by each axle. You can calculate it with nothing more than the lever rule: cargo sitting close to the rear axle is carried almost entirely by that axle, while cargo halfway along the wheelbase is split evenly. This guide explains the formula, the difference between JBB, JBI and payload, Indonesian MST limits by road class, and a worked example for a CDD truck.
JBB, JBI, payload and axle load
These four terms are easy to mix up.
| Term | Meaning | Where the number comes from |
|---|---|---|
| JBB | Maximum gross vehicle weight (GVW) under design and type approval | KIR book, manufacturer data |
| JBI | Maximum gross weight permitted on the road class being used | Road class, signage |
| Payload | Net cargo the truck may carry | JBB or JBI (whichever is lower) − kerb weight − crew |
| Axle load | Weight carried by one axle or axle group | Calculation or axle weighing |
Each axle has two limits: the technical rating of axle and tyres from the manufacturer, and the road's MST. See the glossary entries for JBB and axle load.
MST by road class
Under Law 22/2009, the heaviest single-axle load (MST, muatan sumbu terberat) a road may take is:
| Road class | MST |
|---|---|
| Class I | 10 t |
| Class II | 8 t |
| Class III | 8 t |
| Special class | more than 10 t |
Tandem axle groups on a tronton are assessed differently from single axles; check the official technical rules and manufacturer data. Signs on a specific road section always take precedence.
The physics: the lever rule
Think of the truck as a beam resting on two supports: the front axle and the rear axle (or the centre of a tandem group). If a load of weight W has its centre of gravity a distance d behind the front axle, and the wheelbase is L, then:
- rear-axle share = W × d / L;
- front-axle share = W − rear-axle share.
For several cargo groups, add up every W × d (the moments) and divide by L. Each axle's total load is its kerb weight plus its share of the cargo. If d is greater than L, meaning the centre of gravity lies behind the rear axle, the rear axle carries more than 100% of the cargo and the front axle is partly lifted. That is exactly why tail-heavy loads are dangerous.
The same idea applies inside containers; see our guide to centre of gravity and weight distribution.
Worked example: a CDD box truck
Assumptions (catalogue figures, not a specific unit): wheelbase 3,350 mm; the deck starts 700 mm behind the front axle; deck length 4,300 mm; kerb weight 1,850 kg on the front axle and 1,400 kg on the rear; axle ratings 3,300 kg front and 6,000 kg rear; JBB 8,250 kg. Crew weight is left out to keep the example simple.
Case 1: 5,000 kg spread evenly over the deck.
- Cargo centre of gravity: 700 + 4,300 / 2 = 2,850 mm behind the front axle.
- Rear-axle share: 5,000 × 2,850 / 3,350 ≈ 4,254 kg. Front-axle share ≈ 746 kg.
- Front axle: 1,850 + 746 ≈ 2,596 kg (limit 3,300 kg, fine).
- Rear axle: 1,400 + 4,254 ≈ 5,654 kg (limit 6,000 kg, fine, and under the 8 t MST).
- Gross 8,250 kg, exactly the JBB. The front axle carries about 31% of the total.
Case 2: 4,000 kg stacked in the rear 2.4 m of the deck.
- Cargo centre of gravity: 1,200 mm from the rear of the deck, i.e. 3,100 mm from the deck front, or 700 + 3,100 = 3,800 mm behind the front axle. That is 450 mm behind the rear axle.
- Rear-axle share: 4,000 × 3,800 / 3,350 ≈ 4,537 kg, more than the cargo itself.
- Front axle: 1,850 − 537 ≈ 1,313 kg; rear axle: 1,400 + 4,537 ≈ 5,937 kg.
- The gross is only 7,250 kg (below JBB), yet the rear axle is almost at its 6,000 kg rating and the front axle carries only about 18% of the total. Steering goes light and braking is less stable.
The centre-of-gravity window. For 5,000 kg of cargo, keeping both axles within their ratings requires the centre of gravity to sit between about 2,379 and 3,082 mm behind the front axle, which is 1,679–2,382 mm from the front of the deck. The deck midpoint (2,150 mm) falls inside it.
Tronton and trailers: same rule, two steps
On a tronton the rear axle is a tandem group. For a rough calculation, measure the wheelbase from the front axle to the centre of the tandem group and apply the same lever rule. Catalogue data for a tronton box, for instance, lists kerb weights of 4,200 kg front and 5,300 kg rear with a JBB of 26,000 kg. JBB minus kerb weight is 16,500 kg, while the catalogue payload is only 15,000 kg; the difference is a margin for crew, fuel and equipment.
For a tractor head with a semi-trailer, work in two steps:
- Split the trailer's load between the kingpin (the coupling at the fifth wheel) and the trailer axle group, using the lever rule with the kingpin-to-trailer-axle distance.
- Treat the kingpin load as cargo on the tractor and split it again between the tractor's front and rear axles.
That is why the position of a 20' box on its chassis matters so much: slide it forward and the tractor takes more; slide it back and the trailer axles do. Chassis types are compared in our guide to wingbox and container trailers.
Load placement tips
- Put the heaviest items around the middle of the centre-of-gravity window, usually slightly forward of the deck midpoint, and on the bottom layer.
- Spread concentrated loads such as machinery over timber beams so the deck floor is not damaged.
- On multi-drop runs, plan the unloading sequence so the remaining cargo is not left at the tail after the first few stops.
- Secure the load so it cannot surge forward under braking; movement changes axle loads and can damage the cab. See cargo lashing and securing.
Common mistakes
- Checking only the gross weight. Case 2 shows a safe total with a dangerous distribution.
- Measuring from the deck edge instead of the front axle. The distance from the front axle to the start of the deck must be added.
- Forgetting kerb weight per axle. The engine and cab make the front axle heavy before any cargo goes on.
- Ignoring a light front axle. It is as dangerous as an overloaded one.
- Treating a calculation as a weighbridge reading. Bodies, tanks and equipment differ by unit.
Calculate before you load
Enter the truck and cargo data into the axle load calculator to get each axle's load and the centre-of-gravity window, then move on to the ODOL check for dimensions and JBB. In the Load Planner axle loads update automatically as you build the load in 3D. For the legal background, read our ODOL rules guide. The results are a simulation and a preliminary check, not a substitute for official weighing or the KIR book data.