Thank you all for your really insightful comments! I tried a few more things, including some your suggestions -- made sure the wheels were aligned, checked tire pressure, etc.
I noticed that the problem was much worse when I was on smooth asphalt, but would almost disappear when I was riding on gravel, cobblestones, or bricked sidewalk, so it occurred to me that the problem was this "harmonic" vibration -- not that the bike was out of alignment, but that it was too perfectly aligned. So on a whim, I *loosened* the bolts to the rack -- not enough that the rack wobbles or is in danger of coming unscrewed, but just enough that there can be some miniscule "give" between the rack and the frame. My idea was that if I could disrupt the perfect transmission of this vibration, then it wouldn't radiate to the front wheel, but would cancel itself out. It was the exact opposite of DougP's suggestion and the opposite of my original inclination, but it worked! It went from violent shimmy to completely smooth. No shaking at any speed. Is this perhaps another version of the "self-restoring force" Ken was talking about? (The maths are also beyond me.) In any case, if the problem happens again in another context, I'll be sure to look into "tire damping" (nice term) as a first resort! -- Thanks again, everyone! I'll finally be posting pictures of my AHH soon, now that I've got it properly scratched up and broken in. Five months in Berlin, and besides the daily commute, a few wonderful day rides already, and and now some inn-to-inn rides planned for fall. Nicole P.S. in answer to Ken's question -- neither me nor my mechanic are well-skilled. -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To post to this group, send email to rbw-owners-bunch@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
