Recent trends in vehicle light-weighting and tire design requirements have created an increased awareness to tire flat-spotting. Tire flat-spotting occurs when tires remain in a loaded condition without rolling for an extended period of time. Tire flat-spotting can either be temporary or permanent depending on the length of storage and other environmental factors. Tire non-uniformity caused due to flat-spots often induces shake and shimmy (back and forth oscillation of steering wheel) vibration in vehicles due to increased tire-wheel force variation input into the chassis. This can result in increased warranty costs for OEMs (Original Equipment Manufacturers) as well as customer dissatisfaction exhibited in third party quality surveys like the annual J. D. Power IQS (Initial Quality Survey). While tire flat-spotting can also occur during the vehicle shipping process, it is particularly of concern for slow moving vehicle inventory that remains at manufacturing plants and/or dealership parking lots for extended periods of time.
While the effects and mechanism of tire flat-spotting are understood, current literature mostly documents the effect of flat-spotting using only tire uniformity measurements from simulated tire loading conditions. The study in this paper documents the case of tire flat-spotting under real-world conditions and the resulting effect on the vehicle on-road shake and shimmy performance. Flat-spotting effects are known to be more pronounced at elevated temperatures. A mid-size sedan vehicle was tested in Arizona for varying storage durations over the course of the summer months. Data showing the flat-spotting effect at the tire level as well as the effect of temporary and permanent flat-spots at the vehicle level was acquired. The data clearly shows the detrimental effects of tire flat-spotting on vehicle shake and shimmy performance. The measured increase in steering and floor vibration levels demonstrate the risk for warranty claims and customer dissatisfaction. Improving vehicle sensitivity to protect against such large increases in wheel-end force variation is both mass and cost prohibitive for OEMs and can interfere with other dynamic performance requirements.
The results of this study highlight the importance of proper OEM vehicle inventory management and vehicle maintenance (mainly movement management) on plant and/or dealership parking lots, especially for slow moving products. This can minimize tire-related warranty costs and the resulting customer dissatisfaction. It is important that OEMs work with tire manufacturers to include tire flat-spotting characteristics as one of the key considerations in the multi-attribute set used during the tire design and selection process.
