Hi Waser (and a wave to AndyE
),
The sources of shimmy are complex indeed and can be very difficult to resolve.
Shimmy can occur at one time or under certain conditions and not at others. The same bike can shimmy uncontrollably under one rider while another rider of nearly identical size and weight cannot provoke it even with effort.
I would suggest you contact Thorn; they really can be enormously helpful in offering suggestions and they want their customers to be happy. Thorn and sister company SJS Cycles are sometimes really busy and they may simply have been swamped the times you called. Keep trying.
I had a problem with shimmy on my own Sherpa that proved intractable to all my efforts, crowdsourcing the problem to the Forum, and even for Thorn. I carefully documented in print and on video the problem, every attempted solution, and results of every failed effort so we both had a record of what had been tried so no steps were missed. Thorn made great efforts to help me and were consistently pleasant to deal with; a special nod here to Thorn's designer Andy Blance, who is a really nice fellow. In the end, the same bike that came to shimmy for me under almost all conditions was wholly stable even under load when ridden by Andy. Nevertheless, they did an exemplary job honoring their warranty and offered a solution that made me happy.
Your shimmy problem differs as mine had no discernible cause while you've identified a clear precipitating cause -- riding with no hands and with a minimal front load. "No hands" generally means sitting with a more upright posture on the bike at those times, unweighting the front wheel and transferring weight rearward. Unfortunately, in my survey of the literature and forums online, this is where shimmy is most apt to manifest and it is most easily damped by weighting the handlebars or by pressing a knee against the top tube.
If you find you really need to ride no-hands, then there are other possibilities to try, ranging from carrying a "dummy load" to weight the front to changing tire pressures or even mixing tire sizes to affect contact patch and effective trail. A more extreme solution might be to fit another Thorn fork with different rake as this can affect trail and stability, but you would want to consult Thorn first to confirm the effect on handling.
An evergreen cause of shimmy can be a worn or pitted headset or one that is merely loose and out of adjustment, so that is always worth checking first. I have fitted headsets with tapered rollerbearings to all my other bikes for greater longevity. These did not shimmy before, but others have reported the greater friction of tapered rollerbearings can damp some cases of shimmy in low-trail randonneur bikes.
I don't think a spring-type stabilizer of the type you are considering will have much effect because the spring acts on such a short moment-arm (bracket) at the fork crown. I don't think it will do much to resolve shimmy, but it might be worth a try. A true hydraulic or friction damper of the kind used on motorcycles and downhill MTBs would likely be more effective, but the results of the greater damping would be felt across the board, not just when riding no-hands. Having tried one, I found they can make deliberate quick steering inputs too languid in some conditions.
All the best,
Dan.
