Adding gussets to a sfbd frame?

If its a straight joint, I usually make them about 2.25x4 out of 1/8" plate. I did these start ones too, because the bags where mounted right behind it, but it was mostly for looks. It will work, but not technically ass effective as a diamond shape.

Seth, if the are 2.5x4 then your are saying to run them longer in the side to side position instead of up and down?

Yes, longer sideways, assuming its a straight joint. I only make them about 2.25" tall though, this keeps the edge off the rounded portion of the tube.

But if its an angle, I will cut and shape them to fit the angle better, but they generally are not 4" long then.

Dose the diamond shape have a big difference over say a 2 1/2 in tall 4 inch long gusset with rounded ends?

I understand what you are trying to do here, but you are just asking the same question over and over. There is so much more about stresses than can be addressed in a forum such as this and since most of us are not true engineers a lot of it is more of a feeling than an absolute. I think that you need to go out and get Carroll Smith's Engineer to Win and read the chapter about stress risers and make your own deductions. Most of this will make a lot more sense at that point. Then if you still have some questions, let us know.