Elastically extensible poly-V transmission belt for driving accessories of an internal combustion engine

A poly-V belt, particularly adapted for driving accessories of an internal combustion engine, comprising a body of elastomer material, a plurality of resistant filiform inserts longitudinally sunk into the body, defined by a cord wound in a spiral, and a coupling portion integrally connected to said body and comprising a plurality of V shaped ribs adjacent to one another and alternating with V shaped races, said cord being produced from a yarn comprising a plurality of filaments by means of a first twisting in which are formed at least two plied yarns each composed of at least one said yarn and a second twisting in which said plied yarns are twisted together to form said cord, said first twisting and said second twisting being made in opposite directions, wherein in the finished cord, said filaments form an angle smaller than 3.3° with an axis of the cord, and the material constituting said yarn is selected from the group comprising polyamide 6.6 and polyamide 4.6, and the belt has a tensile modulus of elasticity lower than 9N/rib Mm and a tension decay of less than 30% after 24 hours' use.