The drinard video is very good. That's exactly the thing I'm looking at when running my experiments.
On Dec 21, 9:34 pm, Tim McNamara <tim...@bitstream.net> wrote: > Try the simple things first: move the saddle slightly forward or > back, maybe 1/2", to change the weight distribution of you on the > bike. Change the distribution of stuff you carry on the bike (or > consider carrying less stuff). Raise or lower the bars to change > your position. Make sure the headset is adjusted. Make sure the > wheels are true and properly aligned in the frame. Swap the front > tire to the back and vice versa; look for casing defects in the > tires. Try less inflation pressure (or more) in the tires. Try a > different size tire. If your bike starts to shimmy with your hands > on the bars, stand up slightly and ease your weight off the saddle- > this will usually stop a shimmy instantly. > > The cause of shimmy is not exactly known. Since it tends to be speed- > dependent, it is likely that vertical and gyroscopic forces play a > part. There appear to be at least two fulcrum points, the rear tire > contact patch and the saddle. The bike frame appears to act as a > spring, whipping back and forth; nutation can be observed with the > steerer turning slightly in the head bearings as the front part of > the frame travels laterally. Interestingly the front tire contact > patch usually continues to track in a straight line but, as the > shimmy worsens, the contact patch can begin to arc back and forth on > the road. This is what is often meant when someone talks about a > death wobble, because the vehicle becomes unstable and will often > crash unless the wobble can be stopped. Wobbles can be started by > road surface irregularities > > *Great* video by the much-missed-in-rec.bikes.tech Damon Rinard of a > deliberately induced no-handed shimmy: > > http://www.youtube.com/watch?v=xODNzyUbIHo > > I wish that was in slo-mo. > > This is Rob English crashing at the Battle Mountain IHPVA event on > level terrain at about 70 mph (I bet he appreciated his fairing very > much). It appears to me that his pedaling effort caused the problem > and that as the bike started to wobble, his pedaling amplified it. > > http://www.youtube.com/watch?v=i5Dapy1xUq0 > > My observations with my bikes (a 1996 Riv A/R [26 x 1.25], a 1995 > Ritchey Road custom [700 x 25], a 1998 Gunnar Crosshairs [700 x 28] > and a home-built road bike [700 x 25] all with Panaracer Pasela tires > at the rated maximum pressure) is that: > > 1. none of them currently shimmy with my hands on the bars. > > 2. the Gunnar shimmied when I mounted a Nitto mini-rack on the front > fork, even with no load on the rack, and stopped as soon as I removed > the rack- I could feel the pulsation in my hands when they were on > the bars. The Gunnar sometimes shimmies if I ride no handed and lean > back. Reynolds 853 with Waterford fork upgrade. Oddly it did not > shimmy with a really old Eclipse bar bag (one of the ones from the > late 70s with a rack that slipped over the bars and under the stem > and with elastic cords going down to the dropouts). > > 3. the Riv doesn't, with or without the same Nitto mini rack, with > or without a load. Reynolds 753 frame with 531 fork. No shimmy that > I can ever recall having on this bike. > > 4. the Ritchey shimmies at certain speeds no-handed (19-20 mph and > up) if coasting but not if pedaling; worse if I lean back. Tange > Prestige Ritchey Logic tubes and fork tubes. > > 5. the home built bike shimmies occasionally no-handed. Reynolds > 501 frame and fork tubes. > > 5. the scariest shimmy I ever had was descending on a Bianchi > Reparto Corsa-built bike when I tried the aero trick of sliding off > the back of the saddle. It felt like the rear wheel was shimmying, > very disconcerting. The Ritchey replaced that bike and handles so > much better than the Italian job. -- 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.
