I understand it well enough on my Hillborne that I can avoid shimmy when I want to, so I don't think I have a problem to solve. We've never read a convincing description for why bikes shimmy, that's true. But I'd argue we've never read a convincing description for why any INDIVIDUAL bike shimmies. At least I want to get to that point.
The thing I'm doing that I think differs from the 'experts' is that I'm deliberately making it happen to see if I can develop a more general understanding. The thing I've not seen in any of the commonly cited articles (Heine, Brandt, Moulton, Sheldon(RIP)) is that in none of them has the author deliberately made shimmy happen in order to study it. They all report their own experiences and second hand rumors of poking around trying to fix it on somebody's bike (only sometimes their own) and guessing at what the causes were and what worked after the fact. My hypothesis is that it's a tuned resonance problem. When multiple structures have matching resonant properties they get to ringing at max amplitude. If that's what you have, changing the resonant frequency of one, so that it doesn't match the other will make the ringing subside. The Taipei 101 skyscraper has a relatively tiny pendulum near its roof to cancel out resonance of the building from earthquakes. Several, maybe all the authors above would agree it's a tuned resonance problem, but they don't agree on what the resonating subsystems are. If you don't know what they are, you certainly can't deliberately and precisely tune or de-tune them. Jan suspects that the top tube and the down tube are separate resonators, and that bikes that have the same diameter tubing in the top tube and the downtube are tuned to each other and are much more likely to shimmy. Moulton guesses that the chainstays are in some way a semi-isolated subsystem and his use of Columbus SP tubing in chainstays gave his bikes anti- shimmy properties. Others say that frame flex has nothing to do with any of this. On Dec 21, 2:37 pm, Steve Palincsar <palin...@his.com> wrote: > On Tue, 2010-12-21 at 14:29 -0800, William wrote: > > Nobody conclusively says that such things are > > causes or enablers of shimmy, or will bet their life on removing such > > loads being a cure to shimmy > > They'd be very foolish to do so, since there are many things that can > cause shimmy including rider behavior. > > > Another near-term experiment will be to load the same Saddlesack large > > to a much stiffer rear rack. The Hillborne now has the rack that came > > with my Burley Piccolo. It's a beast. Fat cro-mo tubes welded, > > bomber. If the shimmy comes from rear load floppiness, and if the > > critical flopiness comes from the rack, then that Burley rack should > > change the shimmy properties substantially. If the critical flopiness > > comes from the Saddlesack large, then the rack won't change > > anything. > > I hope you solve your problem, but when you've done it you still won't > be able to make any kind of sweeping, definitive statement about what > causes and what cures shimmy. At most you'll be able to say "I did this > to cure mine." -- 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.
