ETA. I wasn't really sure what had happened (this was a pre-Tahoe, 2013 course recon ride) so I rode back up to the top and tried it again and the exact same thing happened. This makes me believe in the oscillation/harmonics of the bike theory at a certain speed. I was very glad to experience this before race day. On race day I preemptively put my knee against the frame on the descent and despite hitting similar speeds I never felt the bike get wobbly.

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Jason
None of the secrets of success will work unless you do.

2008 Felt DA, 54 cm
Short stem, flat
HED vantage 8 aeorbars, bottle between the extensions
zipp 808 FC CC with a disc cover on the rear

around 40 mph coming down Brockway in IM Tahoe

If I stayed off the front brakes, and kept the speed below 45 I seemed to be ok.

No bottle behind the saddle, just a small repair kit under the seat.

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Ryan

1. 2012 Specialized Shiv Comp Rival -- XL
2. Stock alloy bars sitting on one inch of spacers between the bars and head tube; Flo 60 front wheel with Continental Attack tire.
3. I was descending from Spooner Lake during America's Most Beautiful Ride in 2013. I was going 40+ in my aerobars and almost lost control of my bike. It felt like a cross wind gust caused my front wheel to lose contact with the ground and I'm still not sure how I managed to get back into control.

I have also had the pleasure of dealing with bike shimmy or wobble at high speed. The physics is very interesting. Many mistakenly attribute the phenomenon to resonance frequency but that is not correct. It is actually due to the Hopf Bifurcation. Apparently all bikes experience the Hopf bifurcation at a particular speed. For most bikes and riders the speed is much higher than they ever attain so they never experience it. The classic Tacoma Narrows bridge collapse which we were taught as an example of resonance frequency was also an example of Hopf Bifurcation. There is a very detailed explanation under Lenn Zinn's column at VeloNews. Just search on Hopf bifurcation to see a couple of threads. Also some cool video as I recall. Carl's experience with the rear weight is rather an interesting story. I suspect the weight impacted the torsional stiffness of the frame leading to wobble at speed. Generally the bifurcation from a stable to an unstable front end will occur at a particular speed. Get above that speed, usually on a downhill, and away she goes!
Joe

Great videos and article. Very Interesting Thank You.

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Dan Kennison

facebook: @triPremierBike
http://www.PremierBike.com
http://www.PositionOneSports.com

Only once in about 10 years of riding.

1. 2012 Orbea Ordu size 54
2. Stem maybe 110, 2 spacers, Profile cobra wing(?) with s-bends
3. Riding in Tuscon at Sanguro. Had just started the descent heading back into town from one of the overlooks. Happened within 50m. Shot me hard left into the other lane. My only thought was go down before you hit a catcus. I ended up having a rear blowout prior to coming to an abrupt stop in the sand.

Not a new bike, but my Ocotillo59DE twice going down Donner. This is why you will kill me in the race, I will be braking for dear life trying to make sure I make it down that hill alive.
Have only seen at high speed so I just try to stay under 30 MPH.

.

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Dave Campbell | Facebook | @DaveECampbell | h2ofun@h2ofun.net

Boom Nutrition code 19F4Y3 $5 off 24 pack box | Bionic Runner | PowerCranks | Velotron | Spruzzamist

Lions don't lose sleep worrying about the sheep

1st:-

Scott CR1
110mm stem
10mm spacers
Kysirium elite whels
Ritchey headset
23mm michelin pro 2 clinchers

Downhill corse chip seal with intermitent rougher patches..
Hit rough patch at 55kmh approx, tightened at the controls and the bike erupted into a voilet speed wobble..
Unable to steer effectively, rad bears left, i moved toward center of lane before gaining control.
Oncoming vehicle and myself very glad i missed them..

Total strip and rebuild revealed a slightly worn headset bearing and a very mild 0.8mm, bucle in front wheel..

Root cause was my reaction to the bump ie I CAUSED THE SPEED WOBBLE BY CLAMPING UP AND REMAINING SO AFTER IT INITIATED.

This happened to a far lesser degree 2 more times before I changed out the headset ie adjustment didn't lessen the propensity.

In short the conditions and biek can initiate it, the escalation is the result of the rider..

Second time;

LOOK 695
HED stinger 4
110mm stem
10mm spacer
Again downhill, at 87kmh.. passing through a ravine the hillside shading the road on one side ceased creating a funnel of wind...
This caught the bike side on severely causing a swerve, and almost simultaneously a rough section of tarmac.... the correction by myself and again tightening up caused a very severe speed wobble until it was slowed to 40kmh... then it ceased.... no oncoming taffic, but the cyclists i was passing about crapped them selves when they saw what happened... i felt a similar way...
In this case i found nothing untoward with the bike..
Root cause was again my reaction to a stimulus, in this case a massively strong and directed crosswind.....

BMC TM01, Medium Large
I would have to measure equivalent stem length, it's a bunch of BMC parts you put together to get the right length & drop.
Profile Design ProSvet aerobar
Descending at 68km/h.

Had everything checked immediately after, no problems found

It's a bit silly to say that an instability has nothing to do with natural frequencies and is due to a bifurcation.

A bifurcation is math-speak for a 'forking' of the solutions of a system as a function of certain parameters, and within this function lies some natural frequncies inherent to the system. The complex conjugate pair comes from underdamped solutions of a system, since the discriminant is less than 0.

In airplanes you get coupling between roll and yaw resulting in certain instabilities of various magnitudes, and you can also have what's called pilot induced oscillations, where some delay in reaction and overshooting response creates the instability. In cycling, there may be both contributions in any given case of 'speed wobbles'. I would suspect rider delays in balance in response to some lateral road disturbance as the main culprit though.

Bifurcation does appear to be a better description of the phenomon than resonance. If speed wobble is a resonance, then it would happen at a variety of speeds if the bumps in the road were at a certain frequency. And it would also go away when you went even faster. But there are people that experience consistently at a certain speed no matter the road surface and also it just gets worse and worse as speed increases.

Now just like aerodymanic flutter, the stiffness and mass affect when it occurs even though it is not resonance. Also the riders inputs affect it, since a a large input near the critical speed will cause it to appear at a lower speed, once again showing the difference from resonance.

As my bike has carbon forks can I be involved to try and understand... like a few others since it has happened (twice on the same hill) i've lost all confidence on that section. On other descents i'm generally fine but naturally more wary. I did have one wobble before but that was pre fit and since then i've tanked it down that specific hill and been fine.

Bike is:

1. 2014 Trek Madone 2.1 H2
2. Bars are 40 cm Zipp Service Course 70 (replaced at the fit)
Stem is standard 10cm, i have an additional 3mm spacer in from standard but the stem was flipped to +7 degrees.
3. Descending a long steady hill. It's about 800m length total, shallowing out in the last third. On both occasions I was about mid-way down when the whole front end went wobbly on me, moving from hoods to drops, out of saddle etc seemed to make little difference. Controlled braking and the hill stopping bought things under control. The road surface was a little broken up but the second descent was during a triathlon and several people flew passed me without issue. I had pedalled at the beginning but had moved the pedals to level on the horizontal and tucked forward.

As I said above, all confidence lost on that descent now :(

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http://www.tritriagain.uk

it's often a combination of parts, I would not lose sleep over it. You can stop it by standing slightly to disconnect your butt from the saddle or stabilizing the top tube with your knees. Getting all tense and sighting the bike just makes it worse. It seems like some bike-wheel combinations just wobble for some riders. Sometimes.

That said I have had carbon, steel and aluminum bikes that wobbled, or didn't

I tried the bum off saddle bit but didn't quite have the mindset at the moment it was happening to try the knee bit.... i shall if i can pluck up the courage to try it again!!!

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http://www.tritriagain.uk

it's not uncommon to look around the group in a race on a fast downhill and see people contending with wobbling bikes. I even have a full suspension mountain bike that wobbles violently at speed if i take my hands off the bars, it's kind of amusing to do when riding with others

I wish there was something to blame like new technology, but alas. I think the light wheels we put on for races may have something to do with it.

My 23 pound Merckx MX Leader goes downhill like a coal truck

1. Spez Shiv Med
2. Zipp Vuka Bull, 1 spacer
3. sprint race down short step hill going 40+ and 2x at IMMT going 40+, scary as hell, and to pull out of TT position and move onto base bar and hold on for dear life with some light braking (I HATE braking going downhill). Both races was cold weather and legs were felling like noodles, not sure if that played a part or not?
-- have wondered if i went faster would it have gotten worse or better?
-- have Trek speed concept 9.9 size large now and have never gotten anything close to speed wobble on it.

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2024: Bevoman, Galveston, Alcatraz, Marble Falls, Santa Cruz

1. what kind and size of bike?
2012 Felt DA4
2. what's the front end config? stem length, # of spacers, and if a tri bike than the aerobar type
Fixed Length Bayonet Stem that came with bike
3. what were you doing when it happened?
Anytime I got the bike over 35mph on a descent the front end just became unstable and started to vibrate and oscillate. I thought at first it might be a wheel issue but after a few weeks of riding the bike I had to replace the frame with a DA1 frame (I crushed the DA4 frame into my garage when I forgot to stop and remove the bike off the roof rack....sigh). The DA1 frame was completely the opposite situation, it felt (and still feels) super stable on descents and I can stay aero up to 50mph without feeling any vibration or loss of stability. All the parts from the DA4 were transferred to the DA1 so it had to have been the frame.

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Underground Race Team
http://www.OURtri.org | Twitter | Facebook

2009 Felt B2R with stock bayonet stem and bars

Front wheel was a SRAM S80.

The only time I ever experienced it was on a practice run on the Keane descent the day before IMLP in 2010. It was shaking so hard I thought I was going to die--violent oscillations back and forth at about 43 mph. I've never experienced it before or since, whether with that wheel or with the stock wheel. I took of the S80 wheel for race day because I was so terrified.

Not the natural frequency of the bike frame. The natural frequency of the lean-steer oscillation mode of the bike-rider combination.

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AndyF
bike geek

I have heard that improper wheel dish can be a culprit with these oscillations as well. Example being, a properly dished front wheel matched to an improperly dished rear wheel.

Cheers,
G

2009 Kestrel Airfoil 47, stock front end setup (90mm stem, no spacers, profile T2 cobra aerobars)
Riding Recovery Loop with the womens, 34 mph on I think it was 121st street or wherever we turned off Ft Tejon. Whole fork vibrated, I hit the brakes, and the womens behind me also slowed up because it was really easy to see the wobble. Not so fun.

This is true. A bike frame would have a really high first mode by any practical set of constraints. The "lean-steer" oscillation is the roll-yaw (more like steering angle of front wheel) coupling that I mentioned, which is a property of the bike-rider system.

I'd love to know how long the stem, and the total quantity of spacers under the stem, including headset top cap.

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Dan Empfield
aka Slowman

54CM Cervelo P2C
Stock front end with a ZIpp 404 (non-FC), 6 deg 90mm stem flipped up, 20mm spacer
Bombing down a hill at 50+MPH on the bullhorns

Clamped the top tube with my knees and it steadied.

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Rodney
TrainingPeaks | Altra Running | RAD Roller
http://www.goinglong.ca

1 - quintana roo caliente, size M(54), carbon frame and fork
2 - stock cockpit for a 2008, vision aluminum basebar, aluminum extensions, 80mm vision sizemore stem, maybe 20mm of spacers
3 - going downhill, wobble started to increase in severity as i got close to 35mph, past 35 i had to sit up and use both brakes while bracing the top tube with my knees. at its peak, the wobble would have wrecked me if i had stayed in the extensions.

2008 QR Caliente, with forward seat clamp(i ride steep), two large water bottles in the main triangle, small saddle bag,
Real Design Supersonic 60(Zipp flashpoint rims - the hybrid toroidal aluminum rim/brake track, structural carbon fairing, Real hubs, bladed spokes), Continental attack/force combo, conti tubes, Shimano D/A 7800 drivetrain, FSA Team Issue Carbon crankset 170mm-50/34x12-28, pd-7800 pedals, selle SMP evo saddle

During the Solvang Century, there is one fairly long steepish hill where the wobble occurred. i was accelerating due to the steepness, so not pedaling. It really felt more like the interaction of the air coming off the front wheel and the downtube. i have come close to this at a slightly lower speed with these wheels on my s1(a 51) recently, and will see if the same happens when i build up my p2sl(also a 51).

i am about to rebuild the Caliente as a fixed gear.

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"...I try not to ever ride as slow as 20mph. ;) ... And even more than that, I don't race with a speedometer. My computer is set up to show Power // Cadence // Time. I don't actually ever know how fast I'm going. I only know that if I'm in 53/11, and it takes more than 100rpm to hit my target watts, it's time to coast." - Jordan Rapp on '09 IMC