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. René On 8/9/10, Bob Cooper <robertcoo...@frontiernet.net> wrote: > Hello Angus, et al, > > The scenario is this: > > I’m riding Angus’ Quickbeam down Italy Turnpike outside Naples, New > York. > > Your Quickbeam has just been serviced by a great mechanic. The frame > is straight; the wheels are perfect; the tires are perfect; this > headset is perfect. By coincidence, the fit is perfect. > > I have my spare tube and a set of allen wrenches in a little bag under > my saddle. > > It’s a cold, sunny spring morning, and I’m shivering slightly. > > The fast guys are pulling away from me at 53 mph. > > I hit a bump in the road hidden by a shadow from a tree. > > I tense up and apply the brakes a little. I sit down reflexively to > lower my center of gravity, which we all do when it looks like we are > going to fall. To not do this requires extraordinary will power and > confidence. > > Instantly, your Quickbeam starts to shake violently. > > My arms go stiff, and I grip the brakes harder to make it stop, but of > course all this makes it shake harder. > > No one has been able to satisfactorily explain this phenomenon. Many > have tried; all have failed. > > I watched the video that Jim G made of his bike shimmy. I couldn’t > make a video of this on a big descent, because I can’t do anything > when it starts. It’s progressive. It’s violent. It’s profoundly > frightening. > > My 55 cm Raleigh with oversize aluminum tubing does this. My 59 cm > Peugeot PX-10 with one-inch top tube does this. > > All bikes do this, when I am the rider. > > All of my bikes do this at almost any speed, even 15 mph, but the > story ends differently at lower speeds, because I’m not frightened out > of my wits at 15 mph. To stay relaxed on a bike that going fast and > going out of control is very difficult. When the gradient ahead is > INCREASING, it’s even more difficult. > > Experience, skill, confidence, will power. > > Sometimes I have these at my disposal, sometimes not. > > Regards, > > Bob Cooper > > -- > 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. > > -- Sent from my mobile device -- 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.
