On Mon, Aug 9, 2010 at 10:32, Bob Cooper <robertcoo...@frontiernet.net> wrote: > Jobst is the only person who has shed enough light on this problem of > mine that, when I actually apply his advise, it has an effect. All > praise to Jobst for that post to the FAQ all those years ago. > http://sheldonbrown.com/brandt/shimmy.html
We have discussed shimmy at much length over on the iBOB list. My opinion is that there is a distinction between what I term "speed wobble", which is what you get coasting down a hill at very high speeds, and "shimmy", which is what you get when you ride along at 16 mph no-hands. In terms of physics, they might be the same phenomenon, but it seems like bikes are often prone to one or the other, so I think there is good basis for the distinction. I think low-trail bikes are more prone to shimmy, and high-trail bikes relatively more prone to speed wobble. I would expect Rivendells to be more prone to speed wobble - the high trail geometries that started to supplant lower trail geometries a few decades ago seem to have been motivated in part by a desire to eliminate garden-variety shimmy. Pretty much all my bikes exhibit shimmy under some circumstances, and most of my road bikes can shimmy if I ride along no-hands, even when coasting. It's hard to imagine that it's something I am doing as a rider if I am not even pedaling. My low-trail bikes all shimmy riding no-hands, particularly when loaded. Flexible racks contribute to this phenomenon. My mid-trail Centurion hasn't shimmied, but I also don't have any racks on it. I did find recently a slight speed wobble while coasting down a steep hill, though. My 20"-wheeled Swift Folder is the only bike I have that has never shimmied, but it's a fixed gear, so I have never tried going 40 mph down a hill on it. My 20"-wheeled cargo bike shimmies ferociously, so I know it's not the wheel size, but the frame flex characteristics. I think Jobst's description is lacking, because it only addresses the speed wobble aspect, and not the shimmy aspect of this phenomenon. James Black -- 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-bu...@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.
