I've rarely/never experienced a definite shimmy, but the closest I've come to it seems to have been on a Pugsley with 26x3.8 Black Floyd tires (think fatter Fat Franks). The huge contact patch of this tire serves as a large lever against the road surface. The effect is that when turning, the leverage against the road makes tire want to turn even more, and it take some effort to pull out of a turn. At speed, especially no-handed, the minor steering corrections got amplified, while the steering geometry wants to keep the bike stable and moving in a straight line. It's not terribly scary, but it's easy to reproduce and observe. I think, but can't prove, that this interplay between contact patch leverage and steering geometry is one potential reason why fatter tires tend to shimmy more. On the other hand, I've come to be acquainted with a lot of 1980s Treks with skinny tires that seem to have a shimmy (according to their owners).
On Wednesday, April 24, 2013 9:45:02 AM UTC-5, john wrote: > > @Cyclofiend: > > Thanks for the suggestions. I am in fact light (150#), rather tall, @5,11" > on a largish frame (60cm Sam), which has a tall head tube. I will try > testing with concentration on COG and posture changes. > > @Will: > > Thank for that bit rom Jan H. I know that running fat tires on my Surly > Big Dummy had the immediate effect of causing pronounced shimmy. I can't > help but think that fat tires on a flexy frame, ridden by a light rider is > the culprit. Nothing else changed in the equation! Just heavy, fat tires. > > It's interesting that now, I'm on skinnier, lighter tires (albeit with a > heavier wheel). > > 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.
