How do we know the shaft stiffness? is it measured by how many tonnes the material used to make the shaft?
is it measured by the shaft in and out diameter? i think this is most likely to determine the flexibility of the shaft?
many of the questions you asked have been answered before. searching will help you greatly. don't just continually post....
This is how Apacs determine the FLEX of the shaft http://www.apacscanada.com/information.php/info_id/7