It is well known that rolling resistance is different by more than 20% between bias construction and steel belt radial construction. Since the steel belt radial tire is the current standard such approaches as simpler construction and weight reduction are being used. Improvement of materials and construction such as the adoption of one-ply cord instead of two-ply cord using high strength cord materials and the adoption of high strength organic fibers such as Kevlar and PEN for belt is being made. Recently a method to optimize tire shape and construction through finite element analysis has been put into practice.

  Reduction of rolling resistance through construction brings about little decrease in grip. However, vibration and noise transmission caused by lower energy absorption increases. For example, reduction of tread thickness has a direct effect to decrease weight and rolling resistance, but it increases noise and ride harshness due to reduced cushioning effect. Although various methods are being applied to resolve these contradicting requirements, there is no quick-acting technology. Thus it is important to prioritize functions in combination with vehicles. For example, it is necessary to optimize the combination by understanding the sensitivities of vehicle suspension and body to input at different frequencies and input directions to take measures against weak directions and frequencies while allowing week points in the part to have enough allowance.

  Reduction of weight and rolling resistance is the most important theme in the technological development of tires but there seems to be a limit in development based on the current tire. In the future, it is believed necessary to incorporate new concept of system that includes tire and suspension in the whole vehicle.

   For example, it is currently a general rule to equip vehicles with a spare tire but elimination of a spare tire brings a substantial advantage with EVs that requires space for battery. In the case of Nissan Hypermini, a city commuter, a run-flat tire that ensures minimum running capacity even when punctured, was adopted to eliminate the need for a spare tire. Run-flat tire is an emerging technology and various methods are being proposed including one to change the wheel design, but there are also various problems such as weight increase, spring constant rise, and insufficient general versatility. However, run-flat tire system is believed a promising choice especially when the usage environment of EVs is taken into consideration.

Reference
Book title: EV Handbook
Written by: EV Handbook Publisher's Group
Published by: Maruzen Co., Ltd. (URL http://www.maruzen.co.jp)