Even if the owner of ordinary driving how gentle, road conditions how good, the maintenance of the tire how in place, tire use a long time, it will gradually aging. Tires usually last about five years, after which they begin to age.
The main performance of tire aging is the surface hardening, and then the appearance of crack. Aging tires lose their elasticity, and if they continue to be used, the tread will deform and there is a risk of a flat tire.
