Anyone who has ever watched a van or a heavy dump truck pass by has thought about the colossal weight that presses on the asphalt. However truck wheel load — it’s not just the weight of the car divided by the number of wheels. This is a complex physical process of interaction between an elastic tire, a metal disk and a multi-layer road surface, which determines the service life of highways and traffic safety.
Unlike passenger vehicles, where the pressure is distributed relatively evenly due to their lighter weight, heavy vehicles create extreme pinpoint impacts. It is these impacts that are the main cause of the appearance of ruts, cracks and destruction of the road base. Understanding the mechanics of this process is critical for logisticians, drivers and road network designers.
In this article, we will look in detail at exactly how the load is calculated, what factors influence its distribution, and why one overloaded axle can cause damage equivalent to the passage of thousands of passenger cars. You will learn about the role of tire pressure and suspension design in the formation of the contact patch.
Physics of the process: from the weight of the car to the contact patch
The basis for understanding the problem is the transition from the total mass of the vehicle to the force acting at a specific point. When the truck is stationary, the load is transferred through the suspension to the axles and then to the wheels. The key parameter here becomes contact patch - area of contact between the tire and the road. It is within this area that the vertical force is transferred to the asphalt concrete pavement.
The size of this spot directly depends on the air pressure inside the tire and the load on a particular wheel. If the tire pressure is low, the contact patch increases, which in theory should reduce the specific pressure, but in practice leads to overheating of the tire carcass and uneven tread wear. Conversely, an overinflated tire transfers the load through a smaller area, creating a “stamp” effect that pushes down the road surface.
It is important to note that the load is not distributed statically. In motion, dynamic coefficients caused by road unevenness, braking and acceleration are added to the vertical force. Dynamic load can significantly exceed the static one, especially when driving at high speed along a broken section of the highway.
When calculating the load, always take into account the dynamic factor, which for freight roads is usually taken to be 1.15–1.30, since static weight is only part of the equation.
Design parameters and standard values
There are strict formulas for engineers and designers to determine the permissible load. In the Russian Federation and CIS countries, the main regulatory document is SP 34.13330, which regulates the parameters of axial loads. The calculation is based on the permissible load on a single axle, which for roads of categories I-II is usually 10 tons, and for roads with increased requirements - up to 11.5 tons.
However, the load on one wheel is a derived quantity. For a dual-pitch tire (when there are two wheels on one side of the axle), the load is divided in half, but taking into account the coefficient of uneven distribution. If the axle is loaded with the maximum permissible 10 tons, then each of the four wheels theoretically accounts for 2.5 tons. But in reality, due to differences in tire diameter and suspension condition, the distribution may be asymmetrical.
Below is a table showing the dependence of contact pressure on the type of tire and the total axle load at standard inflation pressure:
| Axle type | Axle load (t) | Oshinovka | Tire pressure (kg/cm²) | Contact area (cm²) |
|---|---|---|---|---|
| Single | 10.0 | Single-pitch | 8.0 | 625 |
| Twin | 18.0 | Gable | 8.5 | 1058 (per wheel) |
| Lifting | 9.0 | Single-pitch | 7.5 | 600 |
| Tandem | 20.0 | Gable | 9.0 | 1111 (per wheel) |
When designing a roadway, engineers use this data to calculate the thickness of the asphalt and pad layers. An error in calculations leads to the road being destroyed in a couple of seasons instead of the required 10-15 years.
Effect of tire pressure on weight distribution
Truck tire air pressure is a variable that is often neglected by drivers, and it becomes fatal to the tire. There is a direct relationship: the higher the pressure, the stiffer the tire becomes and the smaller the area of its contact with the road. This leads to a sharp increase in specific pressure per square centimeter of asphalt.
On the other hand, insufficient pressure causes deformation of the tire sidewalls. In this case the load is redistributed on the shoulder areas of the tread, and the center of the wheel may not even touch the road. This not only kills the rubber itself, but also creates an uneven impact on the road surface, causing local depressions.
"Stamp" effect at high pressure
At a pressure above 9-10 atmospheres, the tire becomes almost rigid. In hot weather, when asphalt concrete softens, such a wheel works like a press, pressing crushed stone into the lower layers or causing plastic deformation of the bitumen mixture.
Modern pressure monitoring systems (TPMS) help monitor this parameter in real time. For mainline tractors, the range of 8-9 atmospheres is considered optimal, while for construction equipment working on the ground, the pressure can be reduced to 5-6 atmospheres to increase the contact area.
Dynamic loads and travel speed
Static weight is only half the problem. When the truck moves, inertial forces come into play. A wheel hitting an uneven surface, even a small one, causes a short-term load jump that can exceed the static value by 2-3 times. This phenomenon is called dynamic impact.
Speed plays a key role here. The higher the speed, the greater the amplitude of vibrations of the suspension and wheels. At speeds above 60 km/h the dynamic addition becomes significant. That is why speed limits and a ban on the movement of heavy vehicles are introduced on sections of roads with weakened surfaces or during spring thaw.
In addition, it is important to consider the nature of the movement. Frequent acceleration and deceleration, characteristic of an urban environment, creates additional horizontal shear stresses in the contact patch. Asphalt experiences not only compression, but also shear, which leads to the formation of waves and swells in front of traffic lights.
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Suspension types and their effect on the road
The design of a truck's suspension directly affects how load is transferred from the frame to the wheels. Leaf spring suspension, still common on many trucks, has a high coefficient of friction between the leaves. This makes it quite rigid, and it absorbs impacts worse, transferring a significant part of the dynamic load to the road surface.
In contrast, air suspension, which is equipped with modern mainline tractors and trawls, works much smoother. Air cushions effectively smooth out unevenness, reducing the dynamic coefficient. Trucks with air suspension cause less damage to roads, which is legislated in many countries through a system of permits for the transport of heavy loads.
⚠️ Attention: The use of trucks with faulty air suspension or “broken” springs is prohibited, as this leads to uncontrolled impacts of the wheels on the road, destroying the asphalt instantly.
It is also worth mentioning the balancing suspension, which is typical for construction equipment (dump trucks, concrete mixers). It allows the wheels to go around large irregularities while maintaining contact with the ground, but on asphalt such a scheme can create specific loads when the wheels slip or lock.
The problem of overload and its consequences
Axle overload is the scourge of highways. Drivers and logisticians often try to load the car to capacity, ignoring weight distribution. The law of the square and the fourth degree states that the destructive effect of a wheel on the road does not grow linearly, but to a degree depending on the load. An increase in axle load by just 10% reduces the service life of the coating by almost half.
In practice, this means that one overloaded dump truck can cause damage comparable to the passage of dozens of cars. Overload leads not only to the destruction of the top layer of asphalt, but also to deformation of the base (crushed stone cushion) and subgrade, which requires major rather than surface repairs.
The fight against overload is carried out using weight control. Stationary and mobile systems measure the load on each axle separately. It is important to understand that even if the total weight of the machine is normal, uneven distribution of the load (skewed to one side or to one axis) is a violation and creates a dangerous local load.
The destruction of the road does not occur from the total weight of the traffic flow, but from single extreme impacts of overloaded axles that exceed the calculated strength limit of the pavement.
In addition, an overloaded wheel has a smaller diameter due to flattening of the tire, which changes the rolling kinematics and can lead to skids or rollovers when cornering, creating emergency situations.
Seasonal restrictions and coverage conditions
The impact of truck wheel loads varies dramatically depending on the time of year and temperature. In winter, at subzero temperatures, asphalt concrete becomes brittle and hard. In this state, it resists vertical compression well, but is susceptible to impact loads that can cause chipping.
The spring period, known as "thaw", is the most critical. At this time, the roadbed is saturated with water, which does not have time to leave due to freezing of the lower layers or heavy rainfall. The soil under the road softens and loses its bearing capacity. During such periods, even a standard load of 6-10 tons per axle can become destructive.
That is why seasonal restrictions on weight parameters are being introduced. During this period, movement is allowed only with a load not exceeding 50-60% of the norm. Ignoring these restrictions leads to the fact that the wheels push through the asphalt along with the base, creating deep ruts that cannot be straightened by pothole repair.
⚠️ Attention: The movement of heavy vehicles during the spring restrictions without special permission is equivalent to causing damage to infrastructure and entails fines and reimbursement of the cost of repairs.
In summer, in hot weather, the bitumen binder softens. Asphalt becomes plastic. In this condition, the main danger comes from long-acting static load (traffic) and horizontal braking forces. A truck sitting in a traffic jam can leave a dent that will remain deformed.
How is equivalent axle load calculated?
The calculation is carried out using a formula that takes into account the actual load on the axle and the standard one. The reduction coefficient shows how many times the destructive effect of a given load is higher than the standard one. Typically, degree 4 is used (for flexible coatings), that is, if the load increases by 2 times, the destruction will increase by 16 times (2 to the 4th degree).
Why is a dual-pitch tire better for roads?
A dual-pitch tire (four wheels on an axle) allows you to distribute the weight of the axle over a large area. Although the total axle load may be higher, the specific pressure per square centimeter of road is reduced. In addition, the presence of four wheels ensures safety if one of them punctures.
Does tire width affect load?
Yes, directly. The use of wide-profile (“balloon”) tires allows you to increase the contact area without increasing the number of wheels. This reduces the specific ground pressure, which is especially important for off-road vehicles, but on hard surfaces such tires can create specific loads on the edges of the track.
What is axial weight and how to control it?
Axle weight is the sum of the loads transmitted to the road by the wheels of one axle. It can only be controlled on scales. It is difficult to visually determine axle overload, but it can be noticed by severe flattening of the tires or body roll. Portable weighing platforms are used for precise control.