There's a lot of ways of doing AFs.
I look at how the Head Stay is done on the donor bike. And emulate that , in terms of force vectors / vibration etc.
An AF frame that has a head stay from the shock tower - well that's where I take it from.
Conversely, with later 4t frames - with plates from the side ( which allow the frame spars to flex as they were designed to do ) either a curved cross tube ( emulating the front, curved, cross tube, to , once again allow the spars to flex as intended) OR plates going back up and in. OR, much like Gregrobos, but with the plate mount landing reinforced across the shock tower - mainly because I'm not sure of the gauge/internal shape of the forward section that I'm welding to. Boxing in Gregrobos head stay plates Would Not be a good idea. He's made them out of a appropriate gauge, and, with his knowledge and facilities, I'm sure an appropriate material, for the way he has done it.
A head stay is a head stay, especially on a twin spar alloy frame - making it much stronger / stiffer than it should be, can be just as dangerous (perhaps a hell of a lot more dangerous) as if it's too flimsy. See how minimalistic the later KTM head stays are made - very much a study in force / stress / flex / vibration directing and control. And head stays on conventional back bone frames do tend to be Much more of the frames integrity than twin spars.
With regards to the original posters build / repair:
The sheer (bad pun) rigidity of the head stay could be a major factor in the down tube / cradle problems. Even with how far away from the failure sites it is.
Another point is being absolutely sure of the material you Did use - make damned sure you are getting what you paid for - I've 38.10 x 17.45 x 3.125 in both 6060 ( well about 500mm of it, after sending the shit back about 12 years ago) and 6061 that, with the naked eye, you can't tell the difference.
The down tube repair - of concern now is the (std, ex factory?) 'join' above the plating, together with the welding of the plate across the downtube. I would have taken the plate up onto the down tube further, welding to the forward corners, that could be regarded as neutral axis ( far from correct, but used as a simplistic explanation here), with No weld across the down tube at the top.
And, of course, others advice on vibration issues, shimming mounts etc are all things to consider.
I've had no problems whatsoever with bending 6061T651, in 25.4mm x 3.125mm, in radius' as tight as 80mm. But it was done very carefully, on the best machines available. I've also annealed material, formed it, and heat treated it again, both as the insert, or in the full fabrication. No problems there either.
I've made full frames, swing arms, frame modifications, with and without heat treating after fabrication. And with heat treatment ranging from basic aging, to full solution heat treatment.
A lot of RULES are broken, with regards to materials and fabrication process', by a lot of us, a lot of engineers, companies etc. You research various methods / approaches, think carefully about how you are going to do something, then do it to the best of your ability. But always with thought to how you are taking your, or, more importantly, some one else's safety in your hands.
I am a qualified Mech Eng, a fully accredited welder, and have been designing and fabricating since my early teens. And I've made mistakes, I even do now. But, thankfully very few. I now herd small furry creatures for 1/2 of my living, and attend TAFE and UNI whilst getting my Dip Ed.
What I love to see are the various approaches, ideas and builds on this and other sites. I see some beautiful work and clever ideas, from so many blokes - and very little shit.
It reinforces my faith in the 'can do' part of human beings - though, admittedly, this site and others, do attract that sort of person, and not the drones that are such a worry at times.
