It is critical for bus fleet managers to take a close look at their costs and determine where they can squeeze the most out of their operation and stay competitive in this challenging economic time. Many people within the transit industry don’t realize that bus tires can comprise a significant portion of their operating cost — often the third highest cost after labor and fuel.
Bus tires can take serious punishment as they transport passengers. Sidewalls take the brunt of curb damage, while treads are subject to scrubbing and punctures. Significant temperature changes can result in unanticipated pressure loss when a bus leaves its garage or depot and enters potentially harsh outside conditions. Extremely high temperatures caused by braking are also a major concern. In transit fleets, there is also a concern about the tires being overloaded.
Since transit tires face constant abuse, it’s important that fleets understand the technology behind bus tires and how to choose the right tires for their application — tires that will provide the best overall value for the fleet and the bottom line.
Understanding Tire Technology
While the vast majority of the commercial vehicle market has been running radial tires for several decades now, radial tire technology, which was first developed for commercial vehicles in 1953, provides the foundational basis for additional advancements allowing tires to better operate in difficult environments. Radial tires are fundamentally different than their bias-ply predecessors.
The radial tire is constructed in two parts. First, a single layer of rubber-coated steel cables arches from one bead to the other to form the tire casing. Second, numerous rubber-coated steel belts are placed in the crown, under the tread, to form a strong stabilizing unit.
To increase a radial tire’s strength, larger diameter steel cables can be used. These steel cables help reduce punctures, tears and flats. They also help distribute heat, resulting in a cooler running tire, improving fuel economy.
The combination of stabilizing steel belts and the single-layer radial casing allows the tread and sidewall to act independently. The sidewall flexes easily under the weight of the vehicle and its cargo, while the tread helps provide even contact with the ground. Greater vertical deflection is achieved with radial tires.
The stabilizing steel belts and radial construction help minimize tread distortion. As the sidewalls deflect, the belts hold the tread firmly on the ground, minimizing tread scrub and greatly increasing tread life. The independent action of the tread and sidewalls keeps the tread flat on the road when negotiating curves and encountering side forces. This allows the tire to hold to its path.
By keeping the two sections free from each other, the tire’s overall performance is enhanced, but the separation also allows tire designers to specifically enhance or customize the tread or sidewall area with specific rubber compounds, designs or other technologies to improve the performance of that portion of the tire. One area where radial tire technology benefits the transit application is in the sidewall.
Because of this foundational aspect of radial tire technology, tire designers can make additions to the structural support in the sidewall of tires without affecting the crown area. This allows for stronger, more damage-resistant sidewalls. Special rubber compounds are often used in the sidewall area as well, to further guard against cuts and abrasions from the debris or curbing typically associated with the urban application.
Other applications of tire technology can be found in the tread area, which provides several opportunities to increase the efficiency and effectiveness of a tire. First, in the compounding of the tread rubber, Michelin deploys advanced technology compounding, which uses layers of rubber.The bottom layer of rubber is more heat resistant to protect the casing from overheating and providing a greater opportunity that the casing could be retreaded. The top layers of tread rubber for transit tires are designed with compounds that are capable of withstanding the higher degree of scrubbing that comes with frequent starts, stops and turns.