I think it would be very interesting to take a touring bike that shimmies under load, and build a version of it that is exactly the same in all respects (as far as possible), but stiffer with oversized tubing, and see if this version still shimmies. It would be important to keep all the other variables the same - tires, components, racks, load distribution and rider. This would be good experiment, has anybody done it?

Shimmy is very complicated, it has many variables that figure into it, but my intuition is that the stiffer frame will help to stop the inherent flex in the frame setting up the resonant wobble that eventually becomes shimmy. I may be wrong, of course, but I chose my Americano because of its extremely oversized tubing and strong design, and my theory seems to have been borne out so far. That bike has the biggest chainstays I've ever seen on a bike, and the rest of the tubing is also larger diameter than any other steel bike I've seen. I can stand up while pedaling the Americano up a hill, with a full load, and there is not a trace of frame flex. It's just rock solid, the way a fully loaded touring bike should be... and no trace of shimmy. That said, I think I have read maybe one or two comments from people talking about getting slight shimmy on their Americano, so maybe it ain't that simple.

I don't think it's fully understood yet. Ask any bike builder if they can guarantee that their bike won't shimmy and they will likely demur, because it's impossible to predict with any certainty (at least with our current level of knowledge).

Neil