Tech & Wearables

Technical FAQ: Chloé Dygert’s disastrous front-end wobble

Lennard Zinn analyzes how geometry, braking, and tire choice could have played a role in Dygert’s world championship crash.

Have a question for Lennard? Please email him at veloqna@comcast.net to be included in Technical FAQ.

Dear Lennard,
Any idea what happened with Chloé Dygert’s bike for it to shake so much that she crashed out of worlds like that?
– Brett

Dear Brett,
What a heart-breaking crash! Dygert had clearly put in the work through the pandemic and during the first half of the course required to win that race. She was so far ahead at the time split that it looked like she just had to roll it steady, and she would win, as Christian Vande Velde had just said on the live NBC coverage.

Team general manager Nicola Cranmer told me, “Yes, that was a bad day for sure. She is doing pretty well all things considered. There was no equipment or mechanical errors. Chloé is a fierce competitor and pushed the limits.”

When I first saw it, I thought it was a front tire problem—either a rolled tubular or a burped tubeless tire, and, after watching the crash in slow motion, I no longer think that. I wondered about a tubeless burp because Dygert’s team had sent out a pre-race press release showing her with a custom-painted Felt DA time trial bike with world champion stripes, seen in revolving photos on the team web site. it said the bike is equipped with 700c x 25mm Kenda Valkyrie TLR Pro tires. The bike she was riding and the one on the roof of the USA Cycling car were black, however, and I have no idea if she was riding tubeless tires; riders on teams sponsored by clincher tire companies often race on tubulars, anyway.

I’ve watched the crash in quarter-speed slow motion many times to evaluate what role her equipment played in this. Slowing the third (slo-mo) replay on this video by three quarters is really slow.

I think it’s abundantly clear that Dygert would have made the corner if her bike had not wobbled. Is it to be expected that a bike would wobble simply by cornering at high speed, hands on the aero bars, weight forward on the bike? I found two YouTube videos with thousands of views by guys “analyzing” the crash and who attribute it entirely to that; they claim a bike will always get a speed wobble at high speed in a corner when in the aero position. Well, having witnessed in person David Zabriskie rip around downhill traffic circles at incredible speed without deviating from the aero position when winning the 2005 Tour de France prologue from Fromentine to Noirmoutier-en-l’Île, I don’t believe that. His Cervelo time trial bike showed no signs of speed wobble. That’s not to say that some time trial bikes may shimmy under those conditions, but it’s not a given. Dygert has ridden a lot of miles at high speed on Felt time trial bikes and pursuit bikes without coming out of the aero position, and I doubt she would enter a corner like that at speed if she had any fears about the bike’s stability.

I endeavored to ascertain whether each one of the 10 wobbles I saw corresponds to one revolution of her wheel, thus ruling in or out the possibility of a dislodged tire. I attempted to discern the distance between the first wobble and the point of impact with the guardrail. Each revolution of her wheel with a 25C tire would be 2.12 meters, so 10 revolutions would be 21.2 meters, or 70 feet. I count 20 green pads attached to the guardrail. I don’t know the dimensions of each pad. They look similar to me to a 4’ X 8’ (48” X 96”) sheet of plywood, so, given the metric system in Italy, perhaps they are 1m X 2m (39” X 79”) in size.

She was riding a 54cm Felt DA time trial bike. Although the geometry detail numbers seem to be missing on that page, we can estimate the bike’s wheelbase to be 1000mm and the tire diameter (700 X 25C) to be 675mm. So, the total length of the bike is about 1675mm (66”). Due to the angle of the camera being behind her, it’s unfortunately hard to tell how long those pads are relative to the length of her bike.

I count 10 abrupt back-and-forth wobbles on her trajectory before she hits the guardrail, the first one being about even with the round blue sign with the angled white arrow on it. The wobbles appear to start at about the fourth pad, and she hits the guardrail perhaps four pad-lengths beyond the last pad, so she gets in 10 wobbles over the distance of perhaps 20 pads. Rather than being two meters long as I guessed above, if the pads are instead only one meter in length (about the length of her bike’s wheelbase), then that is about 20 meters, or very similar to the 21.2m her bike travels in 10 wheel revolutions, indicating that each wobble corresponds to one wheel revolution. On the other hand, if the pads are instead each 2m in length, then she traveled more like 40m (131 feet) over the 10 wobbles, so each wobble corresponds to around two revolutions of her wheel and eliminates the possibility of it being a tire issue. Another way to look at it is that she does all 10 wobbles within a span of 3 seconds. If she were going 30mph, then 3 sec x 30 miles/hr x 5280 feet/mi x 1hr/60min x 1min/60s = 132 feet.

She does not appear to have a front flat; each time the wobble re-centers her, she seems to be atop an inflated tire. Furthermore, a rolled tubular would have kept rolling off further with each jerk of the front wheel, and a burped tubeless tire or dislodged bead would have immediately deflated; in either case, she would have slid off into the pads, rather than fighting for control and riding past all of the pads. Both methods of determining the distance over which she was in the speed wobble indicate that it’s too far for it to be one wobble per revolution, so I don’t think it has anything to do with the tire.

Chad Haga has a theory that she hit the front brake, causing the speed wobble. I can’t be sure that she did that. Clearly, her right hand stayed on the aero bar (and away from the brake lever) when she hit the guardrail. When she starts going over the rail, it is clear that her left hand is not on the aero bar, and I cannot be certain at what point it came off of the aero bar.

Right when she is about to pass the round, blue sign with the white arrow on it, her left shoulder lifts, perhaps to move her left hand to the base bars. If it did go to the base bars (and the lever) in the corner, then, assuming that her bike is not set up “moto-style”, she might have indeed touched the front brake with it. That could explain some of why her weight is being alternately thrown forward and then back onto the saddle as well as the initial break in front-wheel traction.

An extreme example of a “tank-slapper” high-speed wobble.

I think it was a “tank-slapper”, where, once the front wheel breaks traction, a chain reaction happens, because the fork oversteers past the desired line, starting the process over and over again by repeatedly breaking front tire traction, causing the bike to jerk back and forth. This was exacerbated by her right arm being on the aero bar; the added momentum of her arm out there with so much of her weight concentrated on the bar added to the momentum already swinging the fork and wheel back and forth beyond the line of her turn, breaking front tire traction each time. And with her right arm out there, she had no way to slow the rear wheel. Especially in a turn at high speed, it is very difficult to regain control from this before running out of road, at the apex of the turn.

How the front tire initially lost traction remains a question. Combined with entering the turn too fast, it may have been by Dygert hitting the front brake too hard or by a bump at the beginning of the turn, or both. It looked in the live coverage as though both Victor Campenaerts and Tom Dumoulin both experienced wobbles in the same corner, and a bump might help explain that coincidence.

My guess is that, had Dygert sat up and put both hands on the base bars before that turn, she would have made it through just fine. First of all, the air braking caused by her body coming up might have scrubbed off enough speed that she would not have needed to brake. And, had she needed to slow down more to stay on her line, she could have applied the rear brake to do so.
― Lennard


Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.” He holds a bachelor’s in physics from Colorado College.

Follow @lennardzinn on Twitter.