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 around the district, the city or the country. Sidewalls take the brunt of curb damage, while treads are subject to scrubbing and punctures from nails and other debris. 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.
Construction
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.
Performance
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.
Benefits
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 its advanced technology compounding, which uses layers of rubber to deal with the many demands made on the tread area of the tire. 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. These characteristics are uncompromised by the need for tread rubber to be fuel-efficient as well as to provide greater fuel savings.
Additionally, the tread design can provide significant improvements in wear rate and avoid irregular wear, as well as fuel-efficient designs, which tend to be more rib-like, as opposed to block-type tread designs.
Protecting and Maintaining Tire Technology
Understanding and choosing tires with outstanding technology and fitting them to the fleet’s buses won’t amount to much if the tires aren’t also properly maintained. Every fleet should have a formal tire maintenance program and policy, to make sure its tires are properly cared for so they provide the most service to the fleet and make the most of the technology with which they were designed. An effective tire maintenance policy should stipulate and record several measurements:
- Established air pressure data
- Loads carried by vehicles
- Frequency of tire inspections
- Removal timelines for retreads, as well as the number of retreads expected
- Expected life of the tire casing within the fleet
- Inspection of failed or end-of-life tires
In order to establish a baseline, fleet managers should begin collecting tire data over a year. Then fleets may find that there are certain changes that can be made for greater efficiency or better cost savings. This will be the first step toward a written policy related to their tire business and retreading.
Training for those who handle tires is crucial. An untrained person can destroy good equipment without the proper training. In fact, OSHA supports this view by requiring anyone that touches a tire to have basic training. Several sources offer formal tire handling training, including tire manufacturers and the Tire Industry Association (TIA).
Training should also be provided to drivers. Drivers should check air pressure and the condition of the tires in all positions during the pre- and post-trip inspections. If repairs are needed, they should be handled immediately to avoid problems. If the tread has reached the specified retread pull point or DOT minimum tread depth, the tire should be pulled.
There are two approaches to establishing a tire maintenance program. Some fleets prefer to do it all in house, while other fleets choose to outsource all or part of the program. Regardless of the option chosen by the fleet, it is a good idea to work with a dealer that is close to assist the fleet in emergency situations and is willing to work with the fleet in reducing overall tire costs.
At the most basic level, Michelin recommends a “Top 10” list of tire maintenance practices:
- Check tires for correct air pressures. Every shop should have a master air gauge and every driver should have an accurate pressure gauge and be instructed to check the tires on their vehicle every day.
- Drivers should conduct a visual inspection of their vehicle’s tires prior to operating the vehicle. The drivers should look for signs of irregular wear in the tread or shoulder areas of their tires and examine the tires for bubbles or bumps which may be caused by air infiltration or foreign objects.
- Check the vehicle’s owner’s manual or the vehicle load and tire information placard to determine precise air pressures for the loads that the vehicle is designed to carry.
- Check tires for correct air pressures. A tire that is 20 percent below the optimal air pressure is considered a flat tire. A tire that is run under these conditions will experience casing fatigue that could lead to a catastrophic failure or a zipper rupture. If the tire has been run 20 percent underinflated, it should be removed from the vehicle and scrapped.
- Never weld or apply heat to the wheel when the tire is mounted on it. This can cause serious damage to the tire and can cause the tire to explode, causing personal injury.
- Store tires properly when they are not in use. Place them in a cool, dry place away from direct sunlight to avoid premature aging. Tires should be stored standing up on their tread, not stacked in a pile on their sidewalls.
- Check tires for correct air pressures. A tire that is run 10 percent underinflated will lose 10 percent in tread wear and will come out of service quicker.
- Beware mixing tires on your vehicle, especially across an axle. Try to match tires with the same tread depths, same tread patterns and same height (or diameter).
- Keep your tires clean! Wash them with warm soap and water. This will help prevent premature aging and deterioration of the rubber.
- Check tires for correct air pressures. Depending on how much your tires cost, you may be losing between $15 and $30 a tire due to underinflation (10 percent or more underinflated).
As the “Top 10” list above emphasizes through repetition, tire pressure maintenance is highly important and should be the focal point of any tire maintenance program.
So while the economy continues to struggle, creating a difficult environment to stay profitable and competitive, tires present an area for savings. Understanding the technology that goes into transit tires and then putting a maintenance plan in place to protect those tires will help a fleet save money on its tire costs — money that will go straight to the bottom line.
Doug Jones is a customer engineering support manager at Michelin Americas Truck Tires.
