Originally Posted by
3alarmer
...with respect, take a look at the wall thickness of the aluminum tubing in the original pictures. Rad Bikes doesn't care about frame weight savings, because their products are all electric bikes.
The portion of the seat tube above the top tube is not an especially high stress area if properly designed and the seat post fit is a proper slip fit the length of the seat tube. Ideally, all it does is provide someplace to hang the clamp that keeps the seat post from slipping down or up in the tube once you have proper positioning. The stresses ought to be, properly, taken mainly by the seat post, and distributed along the length of the seat tube that are in contact with it.
Which is why you have so many of us questioning what is going on mechanically. It doesn't much matter how well you heat treat the frame if there is a constant rocking back and forth of the post and saddle, and the bulk of those forces are absorbed by that poor little aluminum alloy collar...it's gonna break off in the manner seen in the pictures.
3alarmer, I think I understand your point. We are both wondering what happened, with one fact that is clear being that the frame broke. And it was in an area where a thick tube was used. And it was right at the weld. And the member above who does aerospace welding thought that the bead looks good. That leaves me to believe that it was a heat treating issue, or a poor quality material issue, or (possibly but unlikely in a factory environment) the wrong choice of welding rod.
Your point about distributing the force along the seatpost has some merit, however, wouldn't this make such a break that was observed less likely?
Your point about rocking back and forth is what engineers refer to as fatigue stress. That's why the collar should be so thick: the same forces spread over a thicker tube mean lower local stress and less fatigue effect. Speaking as an engineer (BS/MS/PhD, Registered Professional Mechanical Engineer), I suppose that the design engineer could have underestimated the forces so that even that thick collar wasn't enough. So the failure mode could have been design-related. But that tube looks honkin' thick, man! One other data point has been others saying that their bike from that mfr had no problems. Which leads me to suspect that the true issue was likely the post weld heat treat. What is certain is that if it was a weld that hadn't been heat treated, welding it again with no heat-treat is almost certain to lead to disaster.
My main point (in case I rambled, which I do) has been that a weld fix that is not too costly is likely to be ineffective and will break again or (put another way) that a weld fix that will work will involve proper heat treating and refinishing and will be less cost effective than purchasing a new frame.