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Preventing High Speed Wobble
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Hi, I know there are some related threads about High Speed Wobble, but I am looking specifically for some techniques to prevent it from happening or stop it when it begins.

I have had it happen twice, including today while riding a deep front wheel -- I'm amazed I didn't crash. I have a tri coming up where I know there'll be some descending.

I have heard:
- clamp legs around top tube.
- keep pedaling.
- avoid brakes.

Anything else in terms of bike set up (balancing a valve stem?) or body relaxation and posture on bike would be great. Thanks in advance.
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Re: Preventing High Speed Wobble [AG Tri Newbie] [ In reply to ]
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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.



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
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Re: Preventing High Speed Wobble [jackmott] [ In reply to ]
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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,

Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Cannondale & GT Bicycles
(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)
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Re: Preventing High Speed Wobble [damon_rinard] [ In reply to ]
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damon_rinard wrote:

All bike+rider systems have a natural frequency.

Cheers,

So in theory I can take down my opposition by humming at the right frequency. I have a cunning plan.....
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Re: Preventing High Speed Wobble [HowMuchLonger] [ In reply to ]
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Hi HowMuchLonger,

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

Cheers,

Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Cannondale & GT Bicycles
(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)
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Re: Preventing High Speed Wobble [AG Tri Newbie] [ In reply to ]
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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.

/kj

http://kjmcawesome.tumblr.com/
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Re: Preventing High Speed Wobble [AG Tri Newbie] [ In reply to ]
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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...
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Re: Preventing High Speed Wobble [brilittle] [ In reply to ]
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For a lot of bikes/riders it never occurs.
Could be any speed.

brilittle wrote:
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...



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
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Re: Preventing High Speed Wobble [kjmcawesome] [ In reply to ]
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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


kjmcawesome wrote:
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.



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
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Re: Preventing High Speed Wobble [jackmott] [ In reply to ]
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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.
Last edited by: AG Tri Newbie: May 29, 14 12:52
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Re: Preventing High Speed Wobble [jackmott] [ In reply to ]
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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.
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Re: Preventing High Speed Wobble [brilittle] [ In reply to ]
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Mine was at ~47mph. I think 45 is just about "fast enough" for me.

/kj

http://kjmcawesome.tumblr.com/
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Re: Preventing High Speed Wobble [AG Tri Newbie] [ In reply to ]
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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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Re: Preventing High Speed Wobble [AG Tri Newbie] [ In reply to ]
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HS wobble = change out the wheels (front) to one with better aerodynamics.

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Re: Preventing High Speed Wobble [DrTriKat] [ In reply to ]
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DrTriKat wrote:
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

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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Re: Preventing High Speed Wobble [chaparral] [ In reply to ]
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Re: Preventing High Speed Wobble [chaparral] [ In reply to ]
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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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Re: Preventing High Speed Wobble [damon_rinard] [ In reply to ]
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What a great explanation, thanks Damon! I'm nerding out here, remembering film footage of the Tacoma Narrows bridge failure...

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Re: Preventing High Speed Wobble [jackmott] [ In reply to ]
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jackmott wrote:
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.

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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Re: Preventing High Speed Wobble [AG Tri Newbie] [ In reply to ]
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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.
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Re: Preventing High Speed Wobble [vecchia capra] [ In reply to ]
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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.

Aaron Bales
Lansing Triathlon Team
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Re: Preventing High Speed Wobble [AG Tri Newbie] [ In reply to ]
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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.
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Re: Preventing High Speed Wobble [damon_rinard] [ In reply to ]
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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
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Re: Preventing High Speed Wobble [DrTriKat] [ In reply to ]
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DrTriKat wrote:
Thank you for clearing that up. Was the chorus riser thing flutter or resonance? Was I panicking appropriately (being in the top/back row)?

I am guessing resonance. But panic is appropriate for either case.
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Re: Preventing High Speed Wobble [chaparral] [ In reply to ]
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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!
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