Rubber and organic fiber that compose tires are viscoelastic materials and have such characteristics that absorb and release energy through heating by hysteresis loss accompanying with deformation. In these characteristics, grip is generated even on a slippery road surface and vibration and shock are lessened. On the other hand this hysteresis causes internal loss and hereby rolling resistance. To reduce rolling resistance, a reduction of this loss is required. However, the functions mentioned above are eliminated making these two properties incompatible.

  The largest contributing factor to the internal loss of the tire is the tread section that has a large mass and deformation. Since hysteresis of tread rubber is the source of grip, rolling resistance and grip create completely incompatible characteristics. Especially, since grip on a wet road is important for safety, no compromise is permitted and various technologies have been developed for making these factors compatible. The most important one is the evolution of tread compound. Since vibration absorption capability and noise decreases with reduced loss, vibration on the vehicle body is likely to increase.

  Both grip and rolling resistance depend on the hysteresis loss of the tread but related frequency and strain domain are different. Addressing these differences, the basic concept to provide compatibility is to increase the loss in grip range while decreasing the loss in rolling resistance range. More specifically, rolling resistance is related to the rolling deformation of the tire in a low strain and low frequency range while high strain and high frequency deformation occurs locally in the grounding range under slipping condition on a wet road if it is observed microscopically. Accordingly, technologies to control these areas individually are being developed.

  The compound is a mixture of rubber (polymer), reinforcement materials (carbon black and the like), a crosslinking agent (sulfur), oil, age resistor, etc and through a crosslinking reaction caused by heating molecules of the polymer are connected to network construction making strong rubber.

  Polymer and reinforcement materials are the largest contributing factor to compound loss. Polymers whose molecules are designed so that the loss in a high temperature zone (rolling resistance range) corresponding to low frequency is low while it is high in a low temperature zone (wet grip range) are being developed.

  Rolling resistance is increased if the amount of carbon, a reinforcement material, is increased. Technologies to form carbon in chain-like linked construction are being developed. This long chain carbon makes it possible to achieve necessary reinforcement effect with less carbon amount.

  In addition, silica (SiO2) has been used recently as a reinforcement material. Silica is dispersed in the compound in a form of micro coagulation. When the compound experiences large local deformation this coagulation generates loss and high wet grip is obtained. Since a coupling agent is used to unitize silica with polymer, mutual movement between polymer and silica is small thus reducing the loss generation. But if only silica is used such demerits as friction under high input condition are derived. Accordingly silica is combined with carbon in many cases.

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