String Flex Dacron Classic Recurve Bow String. The Classics are the most popular bowstring. Flex manufacture the Classics with a high modulus, low strech polyester, formely named Dacron. For use on entry, traditional or old models recurve bows. Filament material : High Modulus, low strech polyester (former Dacron) Serving material : propietary 4MAX EVO Loops : Over served.