Shimmy is a seemingly mysterious thing, except it isn't really. With shimmy the bike is acting as a vibrating spring, anchored at the wheel contact patches with the ground and the rider's contact points. The headset allows the front end mass (bars, stem, fork, wheel, fender, bar bag, rack, lights, whatever is attached to the fork) to swing in the opposite direction of the mass of the rest of the bike. That's why a headset with more friction, such as a roller bearing headset, tends to reduce shimmy. I think, but don't know for sure, that lateral flexibility in the rear wheel also contributes (suggesting that a high-dish wheel may increase the risk for shimmy). Shimmy is usually most pronounced when coasting because the various loads on the bike are consistent and allows the sine wave to develop and amplify; shimmy is less pronounced when pedaling because of the shifting mass of the rider and the effects of pedaling causing frame flex, etc. I've seen one exception to that, a Lightspeed titanium frame one of the master's racers hereabouts used to ride. Shimmied like the dickens when he pedaled which is unnerving in a peloton...
Something has to start the shimmy. You have perhaps seen the shimmy videos of the coasting rider whacking the handlebar to start the shimmy. Road surface irregularities, an imperfectly mounted tire, a cold and shivering rider, a death grip on the bars (which transmits our body's natural tremor to the bike- the same tremor that blurs hand-held photos with slow shutter speeds), etc. If your tire was out of round on the rim, that might have been enough to start the shimmy. Back in my racing days, a teammate of mine had terrible problems with a shimmy. None of the bike shops in town could find the problem and fix it, and even when he had the opportunity to consult with pro team mechanics about it they were unable to diagnose or fix it either. The problem was so bad that he couldn't descend hills at more than than about 20 miles an hour. This was a handicap in races. One day he changed the front tire and the shimmy went away. I don't know whether it was something about that particular tire or about how the tire was mounted to the rim, but that solved the problem. By the way, if your bike is shimmying, standing on the pedals and rising a tiny bit from the saddle will almost always stop it. Usually just reducing your weight on the saddle is enough. Your weight is one of the things driving a shimmy. Tim On Apr 22, 2013, at 6:48 PM, john <johnco...@comcast.net> wrote: > Well, here's an interesting situation! > > I decided to take on the wheel issue first. I rode a very shallow hill at > perhaps 10 mph, no handed, sitting up. The shimmy was pronounced. Enough so > that I turned around, went back up the gentle grade, and tried again. Once > again, as the bike picked up a bit of speed, the shimmy returned. I turned > around, rode back up, did it again. And the shimmy returned. > > So I rode home, took the new tire off the new wheel, and installed it on my > old wheel. I went back to the same spot and tried again. The first time down, > at the same pace, on the same road, same lack of wind: no shimmy. I tried it > again: no shimmy. And again: no shimmy. > > Eureka! I thought. It must be the wheel. Well, this being my day off, I took > a good long flat ride on mixed terrain with that old wheel. Fantastic few > hours out on the bike. I'm really loving the Jack Browns - surprisingly, as > much or more so off pavement as on. > > This late afternoon, the wind had picked up considerably. Just to make sure I > had solved the riddle, I removed the tire, installed it on the new wheel, > went back to the same spot, and tried the test again: no shimmy. I did it > once, twice, three times. No shimmy. Went to different areas which were a bit > less windy: no shimmy. > > So now I can't reproduce the shimmy. Maybe the tire wasn't seated as well as > it could have been on the first trial? Maybe riding in wind has something to > do with it? > > I'll try again tomorrow morning. Hopefully the wind will have abated. > > On Monday, April 22, 2013 10:55:24 AM UTC-7, Dave Rivbike wrote: >> >> Hi John, If it's only happening when going no hands... the problem might be >> not ENOUGH weight (you) on the front of the bike. Your body is the engine, >> but it's also part of the steering and suspension. It's like taking the >> shock absorbers off your '64 Impala lowrider and bouncing around at >> inersections >> >> On Sunday, April 21, 2013 7:46:47 PM UTC-7, john wrote: >>> >>> Hello All. >>> >>> I thought I'd get some ideas on a recent shimmy development. Here is the >>> scenario: >>> >>> Rider specs: >>> John >>> Height: 5'11" >>> Weight: 150 soaking wet >>> PBH: 88.9 >>> SH: apprx 78.5 cm >>> >>> I ride a 60 cm. Sam HIll. >>> >>> I Never experienced shimmy with the stock build: Shimano deore front and >>> rear hubs, Velocity synergy wheels, (35 cm panaracer pasella tires), noodle >>> handlebars, SKS Longboard fenders, etc. >>> I ride with a nitto mini front rack, nitro rear rack (the one for >>> saddlebag support, not panniers). I ride with a Baggins "Adam" saddlebag. >>> No front bag as yet because I haven't found one to fit the distance between >>> the front rack and handlebars (I'm not particularly interested in buying a >>> nitro F-15 rack, but instead covet a Berthoud rando bag - yet I don't know >>> if the 28cm size will fill the gap. ) >>> >>> Nothing unusual for a Rivendell. >>> >>> Now, for the recent developments: >>> Swapped the old wheel (Velocity Synergy) for a newly built Velocity Synergy >>> with dynohub (Shimano Alfine). Have a lightweight B&M Cyo light attatched >>> to the right (rider's right while riding) of the rack. Switched from 35cm >>> Panaracers to 33.3 Jack Brown Greens. Got the JBs to fit under Hono >>> because I had issues with the Longboards on two different sets: (rubbing, >>> chattering, due to inherent fleibility) . I also shortened the stem just a >>> hair, to see if doing so would help alleviate some forearm discomfort I'd >>> been having. (The old setup was stem height equidistant to saddle height.) >>> Now the stem height is slightly below saddle height. That's it. >>> >>> >>> Now: experiencing some shimmy (and less overall feeling of stability) while >>> riding no hands sitting upright. This does not occur all the time, but it's >>> noticeable. The "response" doesn't happen all the time, but still, it's >>> there, and unnerving when it does. >>> >>> This "response" almost never happened with my old, original setup. >>> Frustrating! Because I much prefer dyno lighting for any number of reasons, >>> and wanted to run narrower tires to better fit underneath Honjo fenders, >>> which I much prefer to Longboards. My "upgrades" were plenty expensive, >>> too! And a pain in the neck, and time away from the bike. Still, I want >>> what I want: the best Riv ride possible. Is there an inherent compromise >>> here? Am I missing something? And if it is the new developments, where is >>> the problem? Stem height? Wheel weight? Tire width? >>> >>> Any comments? Clues? Suggestions? I'll continue to ride and see how it >>> goes, but honestly, as much as I like the changes I've made (except for the >>> need to run thinner tires with is not optimal for me), I'd go back to the >>> original set up if it meant the end to shimmy. I do not like to shimmy! >>> >>> Yes, obviously I will start with the easy stuff first. As in, I'll raise >>> the stem and see what happens (or doesn't). After that, I'm frustrated. >>> Very frustrated. >>> >>> Suggestions would be appreciated. Thanks. >>> >>> John > > -- > You received this message because you are subscribed to the Google Groups > "RBW Owners Bunch" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to rbw-owners-bunch+unsubscr...@googlegroups.com. > To post to this group, send email to rbw-owners-bunch@googlegroups.com. > Visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en-US. > For more options, visit https://groups.google.com/groups/opt_out. > > -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To unsubscribe from this group and stop receiving emails from it, send an email to rbw-owners-bunch+unsubscr...@googlegroups.com. To post to this group, send email to rbw-owners-bunch@googlegroups.com. Visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en-US. 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