Why after a period of time the bicycle tire become less stiff?

Over time, a bicycle tire can become less stiff due to several factors, including the breakdown of materials, loss of air pressure, and exposure to environmental conditions. The rubber compounds in the tire may deteriorate from UV light, ozone, and temperature fluctuations, leading to a loss of elasticity. Additionally, if the tire is not regularly inflated, it can lose air pressure, resulting in a softer feel. Regular maintenance and proper inflation can help prolong the stiffness and overall performance of the tire.