Tubular tyre challenge tubs top casings are made of silk or top quality cotton weave called corespun the next level down being super poly corespun weave allows them to obtain both supple and strong but lighter casings at the same time these very top level casings can reach a tpi thread per inch count of over 320tpi at the next super poly level the tpi count can easily reach over 300tpi achieving also very strong and in fact less delicate casings than the higher cotton level the casings