on 8/9/10 9:15 AM, Rene Sterental at orthie...@gmail.com wrote: > Does this only happen when you brake and goes away if you stop braking? > > Does I happen in bikes with canti brakes but not on bikes with sidepull > brakes? > > I've had this problem only on a Gunnar with canti brakes. Have yet to > descend fast on the Atlantis to see if it will happen. Front > cantilever brakes when applied hard at high speeds produce this effect > due to fork flex and forces applied when braking as the fork flexes > and extends back repeatedly (my crude explanation). > > The first time this happened to me on a long fast descent it scared me > badly as I thought the fork was broken (carbon). > > This doesn't seem to happen with sidepulls as they are bolted where > there is no fork flex. I'm pretty sure brake type, leverage, pads, rim > quality are some additional variables that play a role here. A highly > modulating brake setup should help. There was very little modulation > on the Gunnar setup compared to how I've now learned to set up high > profile cantis after doing some research.
Rene - It sounds like what you are describing is canti-induced fork "chatter", as opposed to a shimmy. They are certainly similar in causing significant vibration, but quite different with respect to cause. My old (non-riv) CX bike did this heroically. To paraphrase other posts on other lists, basically, what you have is a flexing - either at the posts, through the legs or through the hanger - that hanger flex probably more than anything. This causes a micro-release not dissimilar to an automotive anti-lock brake. So, you are in effect releasing and stopping the rim at high frequency. It's more a fore-aft motion. Shimmy is well described in the Jobst posts that Steve cited. The vibration is different - more of an oscillation, and doesn't institute by braking. http://www.faqs.org/faqs/bicycles-faq/part4/section-25.html I've ridden alongside JimG when it occurs. He could forecast quite accurately when it was about to happen. Also, he could move above that speed and have it level out again, IIRC. - Jim - -- Jim Edgar cyclofi...@earthlink.net Cyclofiend Bicycle Photo Galleries - http://www.cyclofiend.com Current Classics - Cross Bikes Singlespeed - Working Bikes Gallery updates now appear here - http://cyclofiend.blogspot.com "There were messengers who named their bikes, but Chevette never would have done that, and somehow because she did think about it like it was something alive." William Gibson - "Virtual Light" -- 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.
