The stiffer the front end of the bike, the less likely it is to happen.

So - have you checked that your preload on the headset is good? Maybe tighten that up or have a shop check it?

Do you have a lot of spacers? Maybe you can get an up-rise stem to remove some spacers, or use a single taller headset cap to improve stiffness there?

Other than that, relax, touch your knees to the top tube, or stand up and unweight the saddle a bit.

Also you might try a less deep front wheel for courses with big descents.

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

Jack nailed it. Do one of these (not both):

"touch your knees to the top tube, or stand up and unweight the saddle a bit"

These two suggestions work by changing the natural frequency of the system. The natural frequency is where the system is most likely to resonate, i.e. oscillate with increasing amplitude. That's a shimmy. The natural frequency of any system is proportional to the square root of the system's stiffness divided by its mass. You're not going to change your mass quickly while riding, so change the system stiffness instead.

1. Clamping your knees on the top tube stiffens the system, increasing the natural frequency to one that's higher than your current speed. The shimmy stops.

2. Unweighting the saddle a bit makes the system more flexible, decreasing the natural frequency to one that's lower than your current speed. The shimmy stops.

All bike+rider systems have a natural frequency. And there's always some speed that matches it. So plan ahead for these simple reactions and you can tame the beast.

Cheers,

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Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Cannondale & GT Bicycles
(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)

So in theory I can take down my opposition by humming at the right frequency. I have a cunning plan.....

Hi HowMuchLonger,

Great idea! Can you hum at a pitch of about 9 Hertz? ;-)

Cheers,

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Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Cannondale & GT Bicycles
(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)

Knees to the top tube worked for me. I did the same descent 3 times with a Flo 90 front wheel. First time I thought the shimmy was wind. Scary. Second time I realized it was a shimmy. Third time I was read and clamped the top tube with my knees and no shimmy.

Wife was with me riding a similar bike w/ different wheels. No shimmy for her.

Rode the same descent last weekend on a road bike w/ a shallower wheel. No shimmy. Math is hard.

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/kj

http://kjmcawesome.tumblr.com/

At what speed did the wobble occur? I understand everyone is different, bikes, mass, wheels. Just trying to understand where it might become an issue.

I have a end of season race that will have a fast downhill section and I will try and recreate it before to see what happens for me...

For a lot of bikes/riders it never occurs.
Could be any speed.

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

It is possible that deep wheels could help induce the shimmy, when a gust wiggles the front end a bit.

Watch how damon sets it off by tapping the back of the stem:

https://www.youtube.com/watch?v=xODNzyUbIHo

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

Cool video. Today it happened while I was riding a Cervelo R3, 56 cm, not many spacers, 170 lb rider, and the front wheel is an ENVE 8. Didn't happen with a Flo 30 on it. And I was probably going about 40 mph, with mild wind. I was a bit nervous on the deeper rim -- hadn't ridden it since last season -- so I may have been a bit stiff.

Yeah, I've never had a high speed wobble on any of my bikes. I know people that have had issues and have read about it plenty of times. About the only thing I've seen in common on many of the stories is that they are usually smaller bikes and shorter riders that have the issue.

Mine was at ~47mph. I think 45 is just about "fast enough" for me.

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/kj

http://kjmcawesome.tumblr.com/

As some of the posts above state correctly, this can happen to any bike, any rider, any speed. The correct physics (scientific) term for this is resonance, and its defined as vibrational motion at an object's natural frequency when a person hits, strikes, strums, plucks or somehow disturbs the object. Each natural frequency of the object is associated with one of the many standing wave patterns by which that object could vibrate. It is the same phenomenon that caused the collapse of the Tacoma Narrows bridge. This happened to me once during a choral performance, when all 120 or so of us stood up on the risers at the same time (and apparently at just the correct "wobble" frequency) - the risers started swaying larger and larger amplitude. We then all moved at different frequencies upon the risers, and it did dampen and stopped after several agonizing seconds. But it could have brought us down easily. Those unaware of the laws of physics were unfazed. I was terrified.


https://www.youtube.com/watch?v=j-zczJXSxnw

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Two wheels good. Four wheels bad.

HS wobble = change out the wheels (front) to one with better aerodynamics.

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Training Tweets: https://twitter.com/Jagersport_com
FM Sports: http://fluidmotionsports.com

Just so everyone does not get some aeroelastic engineer upset, the Tacoma Narrows bridge failure is not an example of resonance. It is an example of flutter. High speed wobble is a similar phenomenon, in that it is not due to resonance of natural frequency, if it was the magnitude would be reduced as speed increase or that it would not always occur at the same speed. Just think a road with bumps that are spaced at half distance to the road you experienced high speed wobble, if it was resonance on this road you would experience high speed wobble at half of the speed. http://velonews.competitor.com/...-speed-shimmy_309601. The real take away is the high speed wobble is self exciting.

The good news is that solution is similar to a resonance problem, change the stiffness.

 

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Two wheels good. Four wheels bad.

Thank you for clearing that up. Was the chorus riser thing flutter or resonance? Was I panicking appropriately (being in the top/back row)?

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Two wheels good. Four wheels bad.

What a great explanation, thanks Damon! I'm nerding out here, remembering film footage of the Tacoma Narrows bridge failure...

All great advice.

You have to allow the bike to move a little with the bumps and corner by leaning and balancing the bike not steering it. Above about 5mph you should never be "steering" a 2 wheel vehicle. You goal is to make it turn into a corner. It fundamentally different from cornering in a car. You don't even need to be touching the bars to corner. You can turn a bicycle 100% by shifting the weight. This is actually easier to do at speeds above 20mph. IT's actually a really cool feeling going down a long twisty maybe 4-6% grade and riding no handed just leaning into turns. All you are doing it countering the gyroscopic forces of htethe rotating mass of the wheels. Put a 600lb motorcycle in neutral and you can do the same thing, but it takes a lot more leaning to counter the rotating mass of the heavier wheels.

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TrainingBible Coaching
http://www.trainingbible.com

I have never really had a front end wobble on my road bikes despite hitting speeds over 55 mph on a steel bike on a downhill in San Diego a long time ago. I have had one when riding my motorcycle, a CB-750K back in the early 80s when that bike was pretty new. A wobble on either is an E ticket ride, and if you don't get a good adrenaline rush when one happens you might just be dead and not know it.

I hope you don't ever get one - got it twice last summer and it was very frightening. I'm 6'4" and 175, so not just a smaller guy phenomenon.

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Aaron Bales
Lansing Triathlon Team

I got horrible wobble on my Felt B2 with a SRAM S80 front wheel and disc cover in back on the Keene descent in Lake Placid a couple days before the race in 2010. I was going about 45 and had no idea what to do. I thought I was going to die. I did the race with my regular wheel and took off the disc cover and was fine. I didn't read about clamping the top tube with your knees or unweighting the saddle until after I had sacrificed my race time to the aero gods.

Its a rare event, but Damon and Jack are incorrect on this. Its not resonance, its Hopf bifurcation.

http://velonews.competitor.com/...-speed-shimmy_309601

I am guessing resonance. But panic is appropriate for either case.

Man, thanks for posing this question and for a lot of good responses. I'm 5'10, 160 and ride a Felt DA2 with Zipp FC 800's. Two weeks ago I'm dropping down the hill from my house and for the first time experienced the wobble. Scared the holy crap out of me. At 59, a 40+ mph bike crash is likely to bring especially bad news. Last week when I dropped down I was completely paranoid so I never even went to the aero bars...at about 35 I believed I could feel the wobble coming so I just hit the brakes and decended the rest of the way with my tail between my legs.

Not sure I have the courage to try it again clamping my legs to the top tube as a preventive measure.

Does tightening your sphincter help cause I'm sure doing that!

As a side issue, I have a hill where I can relatively safely hit 50 MPH. (8%, half a mile, straight, good surface) Close to home and I fly down this thing at the end of nearly every ride. It is good to know what this speed feels like before a race. Everyone should do the same sort of thing if possible. You don't want your first encounter with high speed to be in a race with other riders around. Amazing how many people do not follow this advice.

For example, Boise 70.3 has a decent downhill right out of transition. Maybe 3/4 mile at -5%. Both times I did that race I had to sit up and ride my brakes down the hill at around 30 mph because some tool in front of me was riding his brakes scared in the middle of the road and I couldn't pass safely.

There is an ideal speed hill 20 miles from me. 12-15% over a half mile of so. Perfectlty straight, pristine pavement, no side streets and the only people driving around there are Amish buggys. I hit 57 or so before I decided that was fast enough. Might be wrong but I bet I could hit 65 on that road with my race wheels, trisuit, helmet etc. Don't intend to test that theory. http://www.strava.com/segments/1539994

I never said resonance :)

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

Tomorrow as part of a multi sport mountain relay (ski, XC, mt bike, road bike, run, kayak) race i'll be descending 3,600ft over 27 miles... the 'shimmy' is my biggest fear...this was good timing for this post, but I did hear all these ideas before. I've a wobble once, and hope it doesn't come back tomorrow... Here's to clamping the top tube.

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________________________________________________
Proud member of FISHTWITCH: beating you to T1 for over a decade, and working on beating you to T2...

Have yet to have it happen (that I remember at least) on a bicycle, but have been there many times on a motorcycle. In general on the bike, it's kinda like driving on ice, let out a little throttle, but not drop off entirely, and keep yourself centered and press the bars forward evenly (i.e. locking your knees to the top tube).

I'm prone to agree that it's not induced by resonance, but the wobble generally is an oscillation (as opposed to wobble induced by locking up the front tire temporarily then letting it out). Keeping yourself centered and the bars even will encourage the oscillation to remain stable and not put you on the pavement.

Worst high speed wobble I ever had was about 8-9yrs ago at 80mph on my bike, long banked curve on I-135, semi-tractor didn't see me and started to change lanes on top of me. I had to commit deeper to the curve, drug my peg HARD and broke my back tire loose. I still remember that split second where I had to decide whether I wanted to let off the gas and end up under the semi, or take my chances with the skid going forward. My reflex was that rolling down the highway had better odds than getting squished, so I kept on the throttle, and since the wheel was loose.... WOBBLE WOBBLE WOBBLE... FML that was close... Haven't spent much time on the lowside of semi's since then. Damn glad I was on a Liter-bike and could power away at 80mph, instead of a smaller engine without the juice.

Haha, yeah, when it happened to me yesterday I was definitely tight on my "seat" out of surprise and fear. I actually think that can make it worse since it puts more weight on the back end of the bike. The wobble subsided when I unclipped

In my situation, I also have a zero-offset seatpost -- wonder if that mades it worse. Anyway, I am glad it happened now -- I still have some time to regain my confidence on fast descents before my next race, and to put a little more thought into a wobble-free setup.

I've got a recurrent problem with wobble. Its been consistent across three bikes (all properly set up and maintained carbon frame road bikes; Look, Cervelo & Pinarello) and several different wheel sets so its not the bike. I've had it at low speed - albeit high cross wind on a bridge, on the flats and even while going uphill. I've learned to do most of the things which are outlined above but I'm clearly just a crap bike handler in some way and it persists. Seemingly no rhyme nor reason. I can descend at 70km/h one day and then get a shimmy starting at 35 the next. The three stand-out events were more like tank-slappers than shimmy. I don't even worry about shimmy any more. Friends riding behind me did not know how I stayed on. MTB friends have given some good advice like keeping soft hands and riding with my weight through my feet, not on the saddle. I've just learned to live with it, although haven't given up on resolving it.