Originally Posted by
Steve_sr
Here is an interesting data point:
https://www.ticycles.com/news/servic...-fat-beat-yoke
Moots apparently had a MTB design that used the Ti chain stays as part of the suspension. Apparently they weren't concerned about titanium fatigue or perhaps they use a special Ti allow that was more fatigue resistant?
Metal fatigue is a complicated process. It occurs after repeated stress up to near the V yield limit. Note that metals vave various allowable distortion within limits, and repeated stresses well within the working range are OK, and when I consider about risk of fatigue, it's because of concern about uncontrolled stresses near the limit.
A cantilevered seat tube extension is not strong enough on its own, and relies on a rigidly constrained seat post to take a share of the load. This is why bikes of this design require that posts are inserted to 2-3 inches below the top tube.
Also note that the constant umrelieved stress of the distortion involved in clamping to an illfitted post acts a a dead load (imagine parking cement trucks on a bridge) on top of which the normal riding loads would be added.
I'm not saying this frame is doomed to premature failure. What I'm saying is that the OP address the issues and implements a fix consistent with basic design principles.
He has a number of viable approaches, and IMO should decide on one of integrity consistent with what he spent serious dough to buy.