Why Do Really Big Trucks Sometimes Have Wheels That Don't Even Touch the Ground?

Have you ever noticed those extra wheels on big trucks that seem to hover above the ground when the vehicle is empty? These are often referred to as donkey wheels - a term that stems from their appearance and their function. This article will explore the reasons behind this unique design.

Design and Fuel Efficiency

Trucks, especially those designed to carry heavy loads, often have extra axles. These axles are engineered to be raised for easier maneuverability in turns. They are typically only used when the truck is heavily loaded, but there are instances when you might see these axles raised with the truck running empty. This design became popular in the 1970s following the first major “oil shock,” highlighting the importance of fuel efficiency.

Every tire encounters rolling resistance, which consumes fuel. For a fully loaded truck, the number and surface area of the tires are optimized to support the load. However, when the truck is unladen, it requires less tire surface area for support. The extra rolling resistance is not necessary, leading to the installation of wheels that can be raised off the ground to reduce friction and rolling resistance. This is particularly common on dump trucks but less so on semi-trucks.

Economic Benefits

Another factor driving the use of these raised wheels is the cost savings on toll roads. In many countries, toll roads charge based on the number of wheels in contact with the ground, which aligns with the weight distribution of the vehicle. For example, if a 18-wheeler truck is empty, it can save a significant portion of its toll costs by raising some of its axles. Similarly, in Colombia, the cost of tolls depends on the number of “on-ground” wheels, and the cost can vary significantly between different axle configurations.

Practical Usage and Benefits

When transporting very heavy loads, trucks need to distribute weight across extra axles. Once the load has been delivered, these extra axles are no longer needed and can be raised to prevent unnecessary wear and tear on the tires, wheels, and brakes. This practice also leads to better fuel economy as there is less friction on the ground.

In some cases, the reduction in the number of axles on the ground can save money on tolls. For example, in South Africa, unloaded 18-wheelers can save around 20% of toll costs by raising an axle. Similarly, in Colombia and other regions, the cost differential between 5 and 6-axle trucks can be as high as 20%, making it economically advantageous to keep fewer axles on the ground when the truck is not fully loaded.

Conclusion

The concept of donkey wheels is a clever engineering solution that maximizes fuel efficiency and reduces costs associated with tolls and wear and tear. Understanding the underlying principles can help in optimizing truck operations and improving overall transportation efficiency.

Key Takeaways:

Trucks with extra axles can have these axles raised to reduce rolling resistance and fuel consumption. Raising axles on empty trucks saves money on toll roads in many countries. Donkey wheels are an effective way to distribute weight and reduce wear and tear.

By implementing such strategies, trucking companies can enhance their operational efficiency and reduce costs associated with fuel and tolls.