Seoul 2000 FISITA World Automotive Congress F2000G427 June 12-15, 2000, Seoul, Korea Development in Sidewall Reinforced Run-Flat Tire Jin Kook Kim*, Yang Nam Lim, Nam-Jeon Kim Kumho Tire R&D Center , Kumho Industrial Co., Ltd. 555,Sochon-dong, Kwangsan-gu, Kwangju, 506-040, Korea Despite the disadvantages in aspects of ride comfort due to the thick sidewall, sidewall reinforced run-flat tires, compared to other types of run-flat tires, have more advantages in use of commercially available rim, easy assembly of tire and rim and manufacturing processibility. In this paper, the design concepts and problems encountered in the development of sidewall reinforced run-flat tires are studied. The condition of no internal air pressure under the vertical load induces the severe bending force on the sidewalls of the tire. To develop the sidewall reinforcing rubber compound and the sidewall construction to support this bending force efficiently are the core concerns for designing the sidewall reinforced run-flat tire. The bending force also causes a problem of bead unseating from the rim. In addition to that, another problem is how to solve the manufacturing process problems which come from thicker sidewall compared to the standard tires. Finite element method was widely used to understand the basic mechanism and to design the construction of the run-flat tire. Keywords: run-flat, tire, sidewall, tire which has reinforced sidewall to support vehicle INTRODUCTION weight Despite the disadvantages in aspects of ride comfort due Pneumatic tires appeared about 100 years ago. Only a to the thick sidewall, sidewall reinforced run-flat tires, few years were taken for pneumatic tires to replace the compared to other types of run-flat tires, have more wheel of a horse-drawn wagon which had been used in advantage in use of commercially available rim, easy many thousand years. Since then, the tires have allowed assembly of tire and rim and manufacturing processibility. dramatic improvements in ride comfort, handling, high In this paper, the design concepts and problems speed driving performance and fuel economy of the encountered in the development of sidewall reinforced vehicles. Pneumatic tires, however, have one decisive run-flat tires are studied. If it is not otherwise mentioned, shortcoming. That is, the vehicles can not move with no run-flat tire in this paper means a sidewall reinforced tire. internal air pressure. Through the review of literature and Finite element method was widely used to understand the patents published, we can easily recognize that a lot of basic mechanism and to design the construction of the run- efforts has been made to solve this shortcoming since flat tires. pneumatic tires have been used. Those efforts have resulted in the development of pneumatic tires that are able DESIGN CONCEPTS to function for a limited time and distance at zero or near- zero inflation pressure, or commonly referred to as “ run- Tread area of the tire is the contacting area to the road flat tires”. surface. Bead area of the tire is fitting area to the rim. And Run-flat tires have certain advantages over standard tires sidewall area of the tire is the middle area between tread not designed to allow the vehicle to continue running with and bead which flexes when the tire is rotating. The a loss of inflation pressure. It enhances the handling of the comparison of cut section layout between standard radial vehicle with sudden loss of inflation pressure, allows the tire and sidewall reinforced run-flat tire are shown in driver the opportunity to find a more convenient time and figure 1. location to change the flat tire, allows the driver the ability In the development of run-flat tires, there are certainly to avoid hazardous roadside situations and allows to several difficulties. Figure 2 shows the deformed shape of eliminate the spare tires. run-flat tires with zero internal pressure under the vertical Run-flat tires that have been proposed until now can be load. This condition induces the severe bending force on broadly categorized into the following five types: 1)solid the sidewalls of the tire. The core concerns for designing tire which has no space inside of the tire; 2)dual tire which the sidewall reinforced run-flat tire are to develop the has another chamber inside of the tire; 3)sealant tire which sidewall reinforcing rubber compound and sidewall has self-sealing fluid inside of tires ; 4)supporting ring tire construction to support this bending force efficiently. The bending force also causes a problem of bead unseating from the rim. In addition to that, another problem is how to * Corresponding author. e-mail : solve the manufacturing process problems which result [email protected] from thicker sidewall compared to the standard radial tires which has a supporting ring so that it can provide support to the tire under excessive air loss; 5)sidewall reinforced 1 Higher section height makes higher bending deformation which is the main challenges to be solved. Therefore the higher the tire series, the more difficult the run-flat tire development. In this paper, main focus is on the development of 60 series run-flat tires which have comparatively high section height. Usually for the 60 series run-flat tires, 80km running distance at 80km/h is required. SIDEWALL CONSTRUCTION As mentioned earlier, for a run-flat tire to support the Figure 1 Comparison between standard and run-flat tire vehicle weight without air pressure, severe bending forces occur in the sidewalls. A little bit special sidewall layout is required to efficiently deal with this bending force. The main design concept of sidewall layout is to minimize the bending deformation within a given material. That is, as shown in figure 3(a), the sidewall construction, which is composed of fabric cord rubber composite layers at the outside of sidewall which are capable of resisting tensile force and rubber layers at the inside of sidewall which are capable of resisting compressive force. This kind of design concept is well known with numerous patents. Comparison tests are performed for the three sidewall constructions as shown in figure 4. All three cases of tires A,B,C were passed the regulation test for the indoor inflation durability (ECE R-30) which is on the normal usage condition. Zero pressure durability tests were conducted on a 67-inch drum loaded with a vehicle which Figure 2 Deformed shape of a run-flat tire at zero pressure of the weight was adjusted. The test results showed that sidewall construction design could make 2 times difference in zero pressure durability performance. The sidewall bending deformation creates the compressive force in tread area. The force makes the tread buckling. Inversely speaking, the higher compressive stiffness in the tread area, the less deformation in sidewall as shown in figure 3(b). A long run with zero pressure causes damage in the sidewall, especially the inside sidewall. During tire rotation, a region under highly concentrated compressive stress can be easily damaged. Therefore, an efficient design of sidewall construction is needed. One more point which should be considered is that the major flexible point in sidewall is changed along with different vehicle weight. (a) (b) So it is difficult to decide on a more reinforced region in the sidewall. Figure 3 Sidewall & tread design concept Figure 5 Comparison of deflection Figure 4 Three cases of sidewall construction 2 SIDEWALL MATERIAL One of the main factors to be considered for the run-flat tires is the tire weight. With a sufficient mount of rubber commonly used, any run-flat tire can support the vehicle weight at zero pressure condition. This tire, however, will be useless in real world due to the undesirable weight. 60 series run-flat tires known today are usually 30-50% heavier than common radial tires. Since tire weight is directly related to the fuel efficiency of vehicle, the weight must be kept as light as possible. Modulus of a sidewall reinforced rubber has an effect on the tire deflection. Effect of modulus is shown in figure 5. The deflection causes compressive deformation in the inner sidewall area. The compressive strain change during rotation is shown in figure. 6. Continuous running causes a cyclic loading resulting in hysterisis loss and a temperature rise. A typical S-N curve is shown in figure 7. As shown in this figure, strain amplitude and temperature rise affects the number of cycles hence tire life. In order to increase the number of cycles, strain amplitude and Figure 6 Compressive strain variation per a rotation temperature must be reduced. In other word, high modulus on sidewall inside element and low hysterisis rubber compound is needed. The normalized zero pressure durability test results of run-flat tires at zero pressure are shown in figure 8 and figure 9. Low vehicle weight and low speed give a dramatic increase in zero pressure running distance. High Modulus rubber usually shows an inferior flexing capability compared to conventional modulus rubber used in tires. At inflation condition, the cyclic strain amplitude per rotation of a run-flat tire is larger than that of a conventional tire's. Consequently a superior capability of flexing is required for relatively high strain amplitude. During most life of their lives, tires are run at inflation condition. So an effort to satisfy an inflation durability rather than zero pressure durability may be more difficult. A tire internal pressure monitoring sensor is needed to indicate low air pressure which will cause high compressive strain amplitude in the sidewall. Figure 7 S-N curve BEAD UNSEATING Figure 9 Normalized running distance at zero pressure Figure 8 Normalized running distance at zero pressure at given speed at given load 3 The design parameters of the tire bead are tire bead toe diameter, bead base angle, bead wire diameter, tire bead flange shape, as shown in Figure 10.