While driving along the highway, you've probably noticed a strange phenomenon: some trucks' rear wheels float in the air, barely touching the asphalt or even coming off it. This sight often causes bewilderment among inexperienced drivers of passenger cars, who may think that the truck’s suspension has burst or a breakdown has occurred. In fact, this is the normal operation of a complex mechanical system, without which modern cargo transportation would be impossible.
The answer to the question is Why do trucks lift their rear wheels?, lies in the combination of economic efficiency and preservation of the road surface. Drivers do this not for fun, but for the sake of strict cost optimization. Raising the axles can significantly extend the life of expensive tires and reduce fuel consumption when the vehicle's full load capacity is not used.
In this article we will analyze the device in detail lifting axles, their impact on traffic dynamics and legal aspects of operation. Understanding these processes is important not only for logisticians, but also for all road users in order to correctly assess the maneuvers of heavy trucks.
Economic feasibility of lifting axles
The main reason why drivers raise axles is simple savings. Truck tires are very expensive, and a set of 18 wheels for a tractor-trailer costs a hefty sum. When the truck is traveling empty or with a light load, the load on each axle is minimal. If all the wheels are running, they will wear out evenly, but without any real need.
By lifting one or two axles, the driver eliminates them from the rolling process. This allows increase tire life on the drive and front axles, which bear the main load. In addition, fewer wheels in contact with the road means reduced rolling resistance. The engine requires less effort to pull an empty trailer, which directly affects reduction in diesel fuel consumption up to 3-5% over long distances.
It is also worth considering the wear of other chassis components. The brake pads, wheel bearings and shock absorbers on the raised axle do not work. This reduces maintenance costs and increases the vehicle's service life as a whole. For transport companies that own a fleet of hundreds of vehicles, this difference in costs amounts to millions of rubles per year.
Design and principle of operation of the lifting axle
The axle lift mechanism is a complex system integrated into the frame of the truck or semi-trailer. They are based on pneumatic cushions, which, when compressed air is supplied, straighten and push the axle down, pressing the wheels to the ground. To lift, the air is released from the cushions, and special levers or additional pneumatic cylinders lift the axle up.
This process is controlled from the driver's cabin. Switches with axle position indicators are usually located on the dashboard. Modern systems such as EBS or ECAS, can automatically raise or lower the axle depending on the load detected by pressure sensors in the air suspension.
It is important to note that not all axes are created equal. There are:
- 🚛 Liftable axles — can work both in a lowered and raised state, have their own drive or be passive.
- 🚛 Jack-up axles — rise automatically when the vehicle is unloaded, without driver intervention.
- 🚛 Lifting axles — capable of not only lifting, but also turning to improve maneuverability (relevant for long trailers).
Structurally, the axis is fixed in the upper position with special latches or stops to prevent spontaneous lowering while moving. This is critical for safety, as sudden contact of the wheels with the road at high speed can result in a skid.
When buying a used truck, be sure to check the tightness of the pneumatic system of the lifting axles - air leaks lead to constant activation of the compressor and excessive fuel consumption.
Impact on maneuverability and maneuverability
Another important aspect is improved maneuverability. When the truck is moving with the rear axle raised, it efficient wheelbase decreases. This is especially true in city conditions or in narrow warehouse areas. Fewer points of contact with the road reduces resistance when turning, resulting in a tighter turning radius.
In addition, raising the axle is often necessary to overcome difficult road sections. If the truck is stuck or climbing a steep hill with poor surfaces, transferring weight to the drive wheels (usually the two axles at the rear of the tractor) greatly improves traction. Additional clamping force helps prevent slipping.
However, there is also a flip side to the coin. When the axle is raised, the vehicle's stability decreases, especially when cornering. The center of gravity shifts and the risk of rollover increases. That's why on highways with steep exits, drivers are advised to lower all axles for maximum stability.
⚠️ Attention: It is prohibited to lift the axle while driving, unless this is provided for by the design of the specific vehicle and is not safe in the current road conditions. Sudden changes in suspension geometry can result in loss of control.
Legal regulations and axis restrictions
The issue of lifting axles is strictly regulated by law, as it is directly related to the safety of roads. There are maximum permissible axle loads. For public roads in Russia and many CIS countries, this limit is often 6, 8 or 10 tons depending on the type of road and vehicle design.
If the truck is partially loaded, the load is distributed between all axles. If you lower all the wheels, the load on each axle will be low, but in total they will “rub” excess. If you raise the extra axles, the load on the remaining contacting wheels will increase. Main rule: the load on one axle should not exceed the established limit.
To control this, weighing systems have been installed on the roads. If the driver raises the axle to save tires, but at the same time the load on the remaining wheels exceeds the norm, he will receive a large fine. Fines for overloading amount to hundreds of thousands of rubles, which is incommensurate with savings on fuel.
Below is a table of approximate permissible loads (values may vary depending on the season and type of road):
| Axle type | Permissible load (summer) | Permissible load (winter/off-season) | Risk of fine |
|---|---|---|---|
| Single axis | up to 10 t | up to 6 t | High |
| Double trolley | up to 18 t | up to 14 t | Medium |
| Triple trolley | up to 24 t | up to 20 t | Medium |
| Lift axle (raised position) | 0 t | 0 t | Low (unless others are overloaded) |
How do scales detect overload?
Weighing complexes "Platon" and stationary scales measure the load while moving or at a stop. The system automatically calculates the load on each axle. If an axle is raised, it is not taken into account, but the entire weight is distributed to the others, which can cause them to exceed the limit.
Dangers and risks of operation
Despite the obvious advantages, operation with raised axles carries risks. The first one is uneven wear. If the driver forgets to lower the axle before fully loading, the entire weight of the load will fall on the main wheels and frame. This may lead to frame destruction, spring breakage or even wheels coming off due to overload.
The second risk is poor braking. The raised axle does not participate in braking. If the truck is fully loaded but one of the trailer axles is raised, the braking distance increases significantly. In an emergency, this can cause an accident.
The third factor is sustainability. A truck with a raised rear axle becomes more "wobbly". In a crosswind or when overtaking large equipment, such a car is more difficult to keep in the lane. Drivers should be especially careful and slow down.
⚠️ Attention: It is strictly forbidden to lift the axle if the truck is fully loaded. This will lead to critical overload of the remaining axles and destruction of the chassis.
☑️ Check before lifting the axle
Seasonal features and road surface
There is another nuance related to the time of year. In the spring, during the period of snow melting, axle load restrictions are introduced on the roads (usually up to 6 tons) in order not to damage the wet asphalt and the base of the road. During this period, drivers are often forced to lower all axles, even if driving with an incomplete load, in order to distribute the weight and pass weight control.
In winter the situation is reversed: the roads are strong, restrictions are lifted (up to 10 tons per axle). Here, drivers often lift extra wheels to save money. Also in winter, a raised axle helps avoid getting stuck in slush, as fewer wheels slip.
However, on dirt roads or rough areas, a raised axle can play a cruel joke. If the road is too bad, the driver can lower the axle so as not to damage the frame from vibration, or vice versa, raise it so as not to dig into the mud. The decision is always made based on the specific situation and experience of the driver.
Proper management of lift axles is a balance between saving resources and maintaining road safety.
Frequently asked questions (FAQ)
Is it possible to drive around the city with a raised axle?
Yes, it is possible if the load on the remaining axles does not exceed the permissible limits (usually 6-10 tons). In the city this is even preferable to reduce the turning radius and noise.
What happens if you forget to lower the axle before loading?
This can lead to serious damage: the air bags will burst, the trailer frame will become deformed, or the tire on the remaining axles will burst due to overload. A fine for exceeding the axle load is also possible.
Why is the front axle of the trailer raised on some trucks, while on others the rear axle is raised?
This depends on the design of the semi-trailer and the weight distribution of the load. Most often, the last axle is lifted, since when unloading (for example, with a crane or unloading backwards), the load moves, and the last axle is easier to unload for lifting.
Does a raised axle affect speed?
Indirectly yes. Lower rolling resistance makes it easier to pick up speed and saves fuel, but for safety reasons it is not recommended to reach maximum speed with the axle raised.