Modern automotive tyre design and performance has advanced greatly due in part to extensive fibre reinforcement research. PEN (polyethylene naphthalate) fibres, with twice the dimensional stability of the most advanced polyester fibres, are being evaluated for lyre reinforcement These fibres are undergoing extensive tests to determine their ability to replace conventional reinforcements such as rayon, nylon, PET and steel in many lyre areas including passenger radials, light truck tyres and possibly other heavy duty agricultural and truck tyres.