Abstract
Keywords: tire, urban bus, service reliability
Tires are subjected to a diversity of use and operating conditions, depending on the vehicles they are mounted on, the use and maintenance practice of their users, the environment in which they operate, and random events that can cause damage or affect their operation. Over time, as science, engineering, and manufacturing technology have advanced, tires have offered an increasingly higher degree of utility, reliability, and value. The development of new car models is usually followed by the simultaneous development of the corresponding components also including the tires. Tire engineers and chemists optimize the tire structure for performance factors relating to traction, wear, ride comfort, handling, durability, rolling resistance, and more. Cost or economy is a factor as well. The result of this co-operation should be a good accordance of the new vehicle and its tires. However, tires can become damaged, or when pushed beyond their limits, they will fail. Over time, failure rates have decreased, but not vanished. Still more complicated is the issue of safety. Tire failure during operation, for instance on a highway at speeds safe for conditions, is the precursor to an inconvenience, i.e. pulling over and applying the spare, or, it is the first in a series of events that may lead to loss of control, and in some cases an accident.
You have to be very careful because the service conditions for city bus tires are quite different to the others and differ from city to city. That is the reason why we have to verify the selection of tires under conditions it will operate (Danon and Vasić, 2006).
Our aim has been to make the choice of the new tires as objective as possible and to utilize even better the data and experiences attained by Belgrade transport. When you are choosing tires for city buses economic reasons force you to select the tires from the available models on the market. This required the procedure to be developed which might assist the choice and acquisition of bus tires. From a durability perspective, the tire is designed to provide a balance of desired attributes, while relying on the end-user for basic care and maintenance. The objective is to design a product (tire) that meets industry, and government standards, and performs according to the consumer´s needs until the tire is worn out and replaced. However, external factors can affect the tire´s ability to endure and shorten or end its service life. Performing a failure analysis on a tire is a scientific process that begins with an understanding of the product, including its materials, mechanics, and operational characteristics. In the paper city bus tire service conditions, possible causes of failure and tire reliability-durability have been analyzed. The most frequent causes of the final failure of the city bus tires (in Belgrade) are mechanical damages (punctures, side damages, ruptures by its mounting part on the wheel...). The second important reason is uneven and excessive wear. The heat is the third reason for final failure.
Into the analysis are included nearly 3,000 written-off tires of different dimensions and origin. For each group of tires quoted the corresponding data have been calculated, necessary for later analyses.