Axle load is the share of the total weight of the vehicle and cargo that falls on each axle or axle group. It is calculated by moment balance: each weight is multiplied by the distance of its centre of gravity from the support point and divided by the distance between the supports. In Türkiye, the result is compared with limits of 10–11.5 tonnes for a single axle and at most 24 tonnes for a tridem axle group.
Why is total weight not enough?
Roads and bridges do not feel a vehicle as a single weight but as separate loads coming from the wheels. Even if a 40-tonne combination looks legal, the tractor's drive axle can exceed 11.5 tonnes if the cargo has shifted forward. Article 128 of the Highway Traffic Regulation (Karayolları Trafik Yönetmeliği) therefore sets axle and axle-group limits separately from total weight. In permitted heavy-cargo transport, the General Directorate of Highways (KGM) also examines the axle loads in the application.
How is the calculation done step by step?
- Define the cargo: The weight and longitudinal centre of gravity are taken from the manufacturer's drawing. The importance of the centre of gravity is explained in the cargo centre of gravity entry.
- Determine the support points: A semi-trailer has two supports: the king pin at the front and the centre of the axle group at the rear.
- Distribute the cargo to the supports: Share on the axle group = weight × (distance from king pin) ÷ (distance between king pin and group centre). The remaining share sits on the king pin.
- Add the trailer tare: The trailer's own weight is distributed in the same way.
- Transfer the king pin load to the tractor: Using the distance of the fifth wheel from the drive group and the wheelbase, the shares of the front axle and drive group are found, and the tractor tare is added.
- Compare with the limits: For each axle and group, the result is compared with the regulatory value or the permit condition.
Worked example
On a three-axle trailer, let the distance between the king pin and the centre of the axle group be 9.0 m. The centre of gravity of the 30-tonne cargo is 5.4 m behind the king pin, and that of the 10-tonne trailer tare is 6.3 m behind it.
| Item | To axle group | To king pin |
|---|---|---|
| Cargo: 30 × 5.4 ÷ 9.0 | 18.0 t | 12.0 t |
| Trailer tare: 10 × 6.3 ÷ 9.0 | 7.0 t | 3.0 t |
| Total | 25.0 t | 15.0 t |
For a tridem group with an axle spacing of 1.3–1.4 m, the limit is 24 tonnes (21 tonnes if the spacing is 1.3 m or less); the group is 1 tonne over. If the cargo is moved 0.5 m forward, 30 × 4.9 ÷ 9.0 = 16.3 tonnes falls on the group, and the group total drops to 23.3 tonnes. The king pin load, however, rises to 16.7 tonnes. If the fifth wheel is 0.4 m in front of the drive group and the wheelbase is 3.8 m, about 16.7 × 0.4 ÷ 3.8 ≈ 1.8 tonnes is transferred to the front axle and about 14.9 tonnes to the drive group; the tractor tare is added to these. Since the cargo and trailer tare alone reach 40 tonnes, once the tractor tare is added the combination exceeds the 40-tonne limit set for five-axle vehicles; the transport can only be carried out with a permit, and this distribution is also the basis of the permit file.
How does hydraulic axle compensation work?
On modular hydraulic trailers, the suspension cylinder of each axle line is hydraulically connected to the others. The cylinders are usually divided into three or four groups. Because the pressure within a group is equal, the load is distributed evenly between the axle lines even on an uneven road. Three-point support maintains even distribution; four-point support is more stable against lateral tipping. The system also adjusts the platform height to keep the cargo level on sloping roads. Details are in the modular hydraulic trailer entry.
What happens if the limit is exceeded?
- Pavement damage: A heavy axle causes rutting and cracking in the asphalt.
- Bridges and culverts: Concentrated load strains the capacity of short-span structures; for this reason a bridge and culvert check is carried out.
- Vehicle safety: If the front axle becomes too light, steering control suffers; if it is overloaded, tyre and brake balance is disturbed.
- Sanctions: Article 65 of Law No. 2918 provides for an administrative fine on the operator for exceeding axle weight; when total weight is exceeded, the fine is applied separately to the operator and the consignor according to the percentage of the excess.
All weight limits are listed in the heavy cargo and permit thresholds entry.
How does Voltim Global support this?
In heavy cargo transport, Voltim Global calculates the axle distribution together with the vehicle selection and enters it into the permit file. The results of weighing carried out after loading is completed are compared with the values in this file before the vehicle sets off.