Re: Frame Building questions
I agree with everyone else that TIG welding is the best choice, but the equipment can be expensive. I would also like to offer acetylene/oxygen torch and brazing as a valid means of fabrication. It can be a more economical way to go.
I modified my bike frame using brazing and it has worked out well. Every joint I welded had an internal sleeve inside the main tubing to strengthen the joint. I did end up developing a crack in the front loop near the bottom bracket. I rebuilt this area with two gussets attached to the bottom bracket and welded to either side of the tube.
I also agree that the frame is the most important component, strength wise, and that extra attention must be given to stress points, like head tube, rear chain stays, dropouts, and bottom bracket.