I am trying to make a 34"dia ring out of hot rolled 8mm dia bar. My method is to make plywood disc (3/4" thick), and then wind the 8mm bar around the disc (about a turn and half or so), and then release it, and end up with just over one turn of 34" dia, then I'll cut off the excess and fit the ring into a circular groove of 34" dia (again the groove will be routed 3/4" thick plywood sheet) and weld the ends together. You've guessed it, I'm using plywood tooling - for cost reasons, and because I want to do everything myself. I made some earlier rings using this method those were 48" dia, again using 8mm dia hot rolled bar. After endless trial and error, I found that for a 48" ring, I needed a 30" dia plywood disc as the former to bend the rod around. I now need to make some 34" rings, but I don't want to have to go through all that trial and error again to find out what dia plywood disc I need as a former. Yet I don't believe I can extrapolate anything from my earlier rings. Is there any rule of thumb or formula to work out what dia disc of plywood (former) I need to make? Don't forget, I need the ring to be 34" dia after I release the force used to wrap the rod around the former (hence it is clear the disc must be significantly smaller dia than the finished ring).