Interesting crash in today’s World Championship TT

[Stephen Cain]

I thought this would be a good place to see if anyone had any thoughts as to what may have happened to cause Chloe Dygert’s crash in today’s time trial.

Here’s an article about the crash: 

https://cyclingtips.com/2020/09/chloe-dygert-crashes-over-barrier-during-world-championship-time-trial-defense/

Here’s video of the crash: 

https://twitter.com/sporza_koers/status/1309136302474956804

One of the leading theories is speed wobble, and of course there is a lot of debate whether the equipment or rider was to blame. As expected, the comment sections for the articles covering the crash are awful. I was hoping we could have a more scientific discussion here!

Best,

Stephen Cain, Ph.D.

Assistant Research Scientist

Department of Mechanical Engineering

University of Michigan

[Curt Beloy]

The bike got disturbed while cornering and with just a video it's hard to know if that's due to a feature of the road, a mechanical issue with the bike, or an action by the rider. If they find something mechanical after inspecting the bike or what she felt, that would be important to know. Otherwise it looks a lot like a high speed instability that you see in some motorcycle crashes! She is cornering while in the aero bars and after the first few oscillations she is essentially out of the saddle, too. There may be little damping from the body in that position so the bike can oscillate underneath her once it gets disturbed.

Interested to hear people's thoughts, too.

Curt Beloy, PE

www.4motoengineers.com

[Andy Ruina]

I have little experience with these things, either personally or studying.

 

From the video it looks to be too high frequency to be dependent on rider actions or reflexes.

And it also looks like, but you'd really have to look closely to see, that the bicycle has substantial deformation.  So it looks to me like some kind of instability involving frame deformation, rolling contact, the mass of the bike and the passive mechanical impedences of the rider at the hands.

 

That is vague and general enough to be  a pretty empty theory.

 

   

 

— Andy Ruina,                     Mechanical Engineering, Cornell

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227 Bryant Ave, Ithaca, NY 14850;                    607 327-0013

 

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[Robert Rae]

Stephen,

The bike's geometry is at fault. Felt has made a fundamental flaw with a steering-head that is too steep.

Back in the 1990s I devised the missing theory regarding bike geometry, where a line of tilt is projected rearward at 90 degrees to the steering axis of the spinning front wheel that must go under the contact patch of the rear wheel to secure it to the ground, otherwise gyroscopic precession has the potential to lift the rear wheel off of the ground, as appears to have happen here. Seen on the first graphic here  https://raerdesign.wixsite.com/design

Regards Robert

https://raerdesign.wixsite.com/design 

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[Anthony Doyle]

Bike Accident

Bike crash

 A. J. R. Doyle, 

bicycle.tudelft.nl/schwab/bicycle/doyle1987.skill.pdf

It is obviously impossible to make an informed comment with only that video as a guide, however, the following might be of interest.

As a young man I had an ‘interesting’ crash on a bike.  I had never given any serious thought about how a bike worked and, as the young are inclined to do, just set off full of hope and excitement.  I went into a turn that gradually tightened up.  I was going too fast for my level of knowledge and skill and realized as I entered the start of the bend that I needed to turn tighter. As everyone knows, to increase the angle of lean one has to make a brief front wheel turn in the wrong direction to get the roll into the lean started. To my surprise and concern I felt the steering go suddenly very light and beyond that angle of lean the control movements I was used to were somehow working differently.  Just as I was worrying about that I became aware that the radius of turn I had selected was no longer matching the radius of the turn of the road and I was going to end up hitting the far curb unless I tightened the turn. However in order to tighten the turn I needed more lean and that meant slacking off a turn that was already too slack. Yes! I hit the curb.

There are a number of relevant observations:

1.    The maximum rate of turn for a bike is limited by the angle of lean. When the tyres slip you have exceeded the limit.

2.    Any turn requires a force at right angles to the direction of travel towards the centre of the turn. The tighter the turn and the faster the speed the greater this force must be. The action of the tyres on the road produce this force. Both front and back tyres contribute. Any slackening of this force leads immediately to an increase in the radius of turn.

3.    The force towards the centre of the turn forms a couple about the centre of gravity in the vertical plane tending to rotate the rider back to the upright position.  This couple must be balanced exactly by the angle of lean. At extreme angles of lean changing the angle becomes awkward. To increase the angle of lean, the radius of turn must be briefly increased, which may be enough to put the rider into the curb.  To decrease the turn the handle bar angle must be momentarily increased to provide the extra force needed to start the recovery from the lean. There are two problems here. The first is that in a really tight situation this may exceed the adhesive limit for the front tyre. The second is that at high angles of lean any added steering angle starts to move the point of contact of the front wheel round to the front of the wheel and the action becomes as much one of lifting as steering.

4.    Finally the lightning of the steering at high angles of lean. The front wheel geometry puts the contact patch of the front tyre behind the point where the steering axis meets the road. This ‘trail’ provides a degree of castoring tending to iron out random disturbances.  As the angle of lean increases so the trail distance reduces.  At some angle of lean, exactly where depends on the design detail, the trail distance reverses and the castoring effect now exaggerates disturbances instead of damping them. Up to that point the rider has been controlling the bike with pushes on the bars which balance out the castoring forces. To command a turn to the right he applies a torque to the steering by applying an angle independent force to the right handle bar. This overcomes the castoring torque and forces the bike to lean to the right. The more the bike leans to the right the stronger the castoring torque trying to restore upright running. The rider keeps on overpowering this torque until it is judged that the angle of lean is sufficient, at which point the push is reduced so that the two torques balance out and the angle of lean remains constant.  Doing this makes the bike run in a curve but control is exercised round the angle of lean not the angle of progress.  At high angles of lean the castoring couple gets less and less until at some critical point it starts working in the opposite direction. At the point where it is zero there is no longer any torque from the front wheel geometry and controlling the bike requires a similar skill to that used when riding very, very slowly, where the castoring couple is virtually nil.  Beyond that zero point controlling the bike gets more like trying to ride the kind of trick bike where the front wheel has been changed to work in reverse.  If all this happens just as you suddenly realize that you desperately need to turn tighter, loss of control is more or less inevitable.

Cheers,

Tony Doyle

 

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[Robert Rae]

Continued from above...

With the bike at around 1/2 meter from the edge of the road before crashing is the best shot of the shadow reduced at the contact point of the rear wheel indicating the rear wheel has left the road surface - the influence of the gyroscopic precession of the front wheel having a detrimental effect on the rear wheel due to faulty geometry.

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[Pierre Ethier]

Hello!

First time uploading a message here. Hope it's done correctly.

The video shows the wobble appearing as soon as the bicycle goes into the ‘ditch’ in the asphalt and the wobble frequency is much too high (as noted by Andy Ruina) for the rider to cause it directly. This seems a typical case of bicycle wobble illustrated for instance with a bicycle going down a hill at about 35 MPH (56 KMH) on this site:  Shimmy, or Speed Wobble

From 2001 to 2006, I owned and raced an  XC hardtail Specialized mountain bike with a Manitou front fork (that was never as stiff as today’s RockShox, with larger suspension tube diameters). This bike would go into a wobble at only 14-15 MPH (22-25 KMH) when sitting back hands OFF the handlebar. Very surprising the first time, but easily stopped as soon as my hands returned on the bar. I feared the worst when I hit 50 MPH (82 KMH) during a race down a long hill, but the wobble did not reappear. Oufff !

From a Torso-Arms-Handlebar 2-Wheeler Steering Theory standpoint, things look a little different.
As explained on this site at section 1.4.3 Which Torso-Arms-Handlebar geometries work best or worst, the Torso-Arms-Handlebar (TAH) mechanism of a race bicycle with drop handlebars is one of the worst geometry. When the rider leans a few degrees, the handlebar has to go many degrees to conform to this TAH mechanism. This means the rider cannot easily control the handlebar which is thus sort of let to itself. So Dygert’s bicycle was free to wobble. It did and the young lady went out into the ditch.

The bicycle could possibly be designed and built not to wobble. But another solution would be that when you understand better this TAH theory, you can practice voluntarily orienting the handlebar in opposite direction from where you want to go, by using this TAH mechanism consciently, instead of losing time trying to do a countersteering 'jerk' on the handlebar. This way you don’t let the handlebar go where it wants, neither do you push it too far or in the wrong direction.

Yours,
Pierre Ethier
Quebec City, Quebec

[Jason Moore]

Hi all,

Nice discussion!

The frequency is around 4 Hz, based on my analysis of the video. Wobble/shimmy frequencies of bicycles with a seated bicyclist have been shown to be 6-10 Hz for typical speeds. At her speed you'd expect wobble/shimmy at 8 Hz or so; double what is shown in the video. She seems to get bumped vertically if you watch the seat closely just before the action starts. She then disconnects from the seat a couple times and then fully. Does anyone know of any studies of bicycle wobble/shimmy when you are off the saddle? My initial thought would be that the frequency should be higher than the seated case.

Jason

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[Curt Beloy]

To me, this looks like the 'weave' mode of vehicle oscillation. I say that because of the amount the rear frame is moving along with the front end, as well as being at lower frequency than wobble. I mostly study motorcycles and am not aware of weave stability ever being discussed in bicycles- but they are dynamically similar, so it seems plausible. The rider's weight damps weave vibrations, but in this case the rider is not quite attached to the bike. 

Curt Beloy, PE

www.4motoengineers.com

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[Jason Moore]

Marco Reijne and I wrote up our take on this crash: https://mechmotum.github.io/blog/dygert-crash.html

Comments welcome!

Jason

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[Curt Beloy]

Jason,

very cool. I can see the time trials body position was likely a big factor for the damping (elbows over the stem and weight forward, on the pedals). I tend to look at the design of the bike as a constant (unless a mechanical failure occurs) and everything else as a variable. I wonder if Felt will study this and redesign so she feels comfortable taking a corner like that in the future. What are the industry standards for designing for and testing for stability if you know?

I learned a lot about bicycle dynamics from that article. thanks!

Curt Beloy, PE

www.4motoengineers.com

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[Jason Moore]

Curt,

Thanks! I agree that the bike design is mostly a constant. They really don't differ that much, dynamically speaking, between models. I don't think there are industry standards in bicycling for stability tests. Hopefully that is something we can develop with companies in the coming years. We've been in touch with some bicycle companies that are interested in adding these types of things to their toolboxes.

Jason

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[Robert Rae]

The article Jason and Marco wrote up on this misses the issue, which is a bit strange given the answer is written here; in 2 words it is a design fault as I said above.

Attached is a photo with the original diagram displayed concerning the missing theory with gyroscopic precession.

Regards

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[Jason Moore]

Robert,

What evidence do you have that demonstrates that this line from the front wheel center perpendicular to the steer axis has anything to do with an unstable 4 Hz oscillation? Please provide something besides a few paragraphs on your website that explains this in a scientifically rigorous manner. I read the page on your website, but it seems like it is anecdotal speculation. I'm happy to stand corrected, but I don't think your theory is anything that's generally accepted in the literature. I do think the one thing we've learned over the last 20 years is that there is no magic theory to bicycle stability, handling, or control and there isn't likely ever to be one!

Jason

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[Robert Rae]

Jason,

I devised this theory 29 years ago, it has never been proven wrong to date, and when I made it public long ago (more so for motorcycles) it had widespread acclaim from experts in the field.

Regards

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[Jason Moore]

Robert,

Ok, thank you for clarifying.

Jason

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[Damian Harty]

An interesting assertion about head angle, Robert. The assertion is apparently at odds with some work Tony Foale performed in the 1980s?

The real question is "what evidence would be needed to disprove the hypothesis?" We can probably use a modern multibody model to look for that evidence.

Regards,

Damian

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[Robert Rae]

Damian

I found Tony Foale to know little more than a few basic principles; he lacks in-depth knowledge of motorcycle dynamics.

Regarding his experiments, he states are 'limited by time and money', the use of a heavy slow BMW with the rider with his hands off of the handlebars taking body mass rearward all of which mitigates against the scenario I described. Think of a motorcycle as two large gyroscopes, which it basically is, and work from there for a clear understanding.

Regards 

Sent from my iPhone

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[Damian Harty]

Hello Robert, thanks for your comment.

Your opinions of Mr Foale are interesting, I have always found him to be charming, courteous and extremely knowledgeable. He has not shared an opinion of you with me. 

I looked at your web site. It has many interesting and testable hypotheses. It's also intriguingly dated 2023. Is there something else you have invented that you're not mentioning?

Damian

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[Robert Rae]

Hi Damian,

Unfortunately my opinion of Mr Foale is the opposite of yours. It was past correspondence with him on a forum that demonstrated his lack of knowledge, which I didn't initially object to, but his smarmy wrongful answers and the fact he was misleading others to the extent it was dangerous with what he was expressing through ignorance, stupidity or malice is wrong on every level. I can remember a couple of examples of which.

Back on track, I never knew about a wrongful date on that site. I put it up here solely because it was the first access I could find to demonstrate what I was saying - it was not my first choice when doing so. And, yes, there is a second and third layer of even more advanced technologies to be applied that are not shown on the site; so by default...

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[Jason Moore]

Hi folks,

Speaking as one of the moderators of this list, I just wanted to give a reminder that we should leave personal comments about people off the list. Please stick to the science.

Thanks,

Jason

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