Kevlar is a synthetic fiber that is about five times stronger than steel. Kevlar is used in tires in one of two ways: as a replacement for the steel coils that form a tire's edges (known as beads) or as sub-tread (the protective layer resting between the external rubber and the internal fabric casing forming a tire's structure). 

Tires with beads made of Kevlar are lighter by around 2 ounces (56 grams) each. Unlike a regular tire, these tires can be folded up easily. Casings or sidewalls including Kevlar have some added weight and a somewhat more rigid ride, but these tires can often go a year or more without a flat because of Kevlar's ability to deflect glass and other sharp objects, defending the tube against puncture.