Tires flatspot when they remain loaded without rolling for a period of time. When the tires are rolled again, the initial flatspot decays and vanishes or stabilizes to a much smaller magnitude (called residual flatspot). Flatspot is a low harmonic phenomenon and primarily contributes to the first three or four harmonics of tire run out. The tire non uniformity caused by a flatspot may induce noticeable vibrations in some vehicles during operation in the decay process.
The magnitude and decay of a flatspot depends on many factors - tire construction, material creep properties, tire radial load and time duration, inflation, tire and ambient temperatures, tire mileage, etc. In this paper, the fundamental concepts involved in the formation and decay of flatspot are discussed. These characteristics can be objectively measured in a laboratory and flatspot trends can be explained.
Methods of experimental characterization and computer modeling of tire flatspotting are necessary for successful diagnostics and treatment of flatspot related issues. Details of a test to measure flatspot induced forces in the lab are described. Finite element based modeling approaches to flatspot analysis are presented and diagnostic and comparative analyses of tire component contributions to flatspot are shown. In addition, case studies are presented to illustrate the influence of various factors on the magnitude of flatspot. Some guidelines on the effects of various service conditions on flatspot will also be presented.