I have a "bending stress" question, which is causing me "actual stress"... I have a solid 30m Aluminium shaft, which I need to replace with an equivalent hollow shaft, but I am having difficulty in calculating the internal and external diameters. The solid shaft diameter is 0.118m, and all I know about the hollow shaft is that the external diameter should be 2.5 times greater than the internal diameter. I worked out the second moment of area for the solid shaft (9.5*10^-6), but can't seem to find the right way of using this to find the hollow shaft dimensions. Any help with this would be much appreciated.