JohnJ80

Sure, I think I can.

I should be clearer on the frame thing - it could also be an imperfection in the layup but the geometry is ok. At any rate, I doubt the Felt engineers did any actual testing of frequency response of the frame design in general but certainly not of her frame in particular. This turn didn't look all that special or difficult to me as a amateur rider but certainly not that special to a world class cyclists like Chloe. Bottom line here, I think Felt owns this problem. If I were her, I wouldn't get on that design again unless they could show me they found and fixed the problem and then I'd want them to do some frequency testing of the exact frames/bikes they were planning on giving me. It's either in the design or in the layup.

To your bolded points:

1. "has her weight down in a tuck and the saddle is relatively lightly loaded". She's on her aero bars and the very front of her saddle as she enters the turn. Her outside leg is straight, pressing down hard on the pedal, and her knee is pointed into the turn to allow her to lean the bike. Her tires aren't sliding, they're anchored to the pavement by sufficient friction. So the seat has less weight on it than it normally would if she were, say, riding with her hands in the drops or on the shifters. What that does, in my opinion, is free up the seat end of the seat tube and rear triangle to oscillate. Had she been sitting on the seat with more weight and farther back, she would have anchored that end. The lower end is the stiffest part of the bike in the bottom bracket and I doubt that can oscillate much.

2. "as she was carving the turn she was loading up the rear triangle". Even though she's not sitting on the seat, her weight is carried by the frame and ultimately the tires. She's pressing down hard on the outside pedal to put as much weight as she can as low between the two wheels as she can. The bike wants to straighten up but she's holding it down and by controlling the front wheel and her CG, she's forcing the bike around the radius. That puts stress through the back triangle and loads it up. But, because she's not really pressuring the seat or even not at all briefly, all that energy is available to "spring" out of the frame and provide the energy to oscillate the top of the seat tube and, I would presume, the top tube. Because of her right knee being used to help her turn, she can't clamp it to the top tube or the bike would straighten up somewhat and she'd miss the turn and run off the road. So between the two, it looks to me like she was either going to crash where she did or slightly farther through the turn but it looks like she was kind of cooked when she hit that dip/thing and she had no room or time to recover.

3. "You can see her stand up a bit from the seat as she hits the thing/dip in the road probably pitching her slightly forward. That unloads the seat more and removes any damping she might have been able to provide. " I can't see what she hit from the video or the imperfection in the road, but there was something there. You can see if you step through it, she gets pitched slightly forward suddenly, and from my viewing, it looks like she comes off the seat even more (i.e. nose of saddle). This would make the lightly loaded end of that "tuning fork" even more able to oscillate. That's when the oscillations rapidly grow in amplitude if you watch the side to side movement of the saddle - it gets wider and wider until she goes over the edge.

to the weight on the seat issue - of she had been sitting on the seat more solidly so in essence she had equal weight on her pedal and her seat, that effectively changes the frequency of oscillation of that seat tube to be probably twice as high. That’s important because that means it is likely out of the resonant frequency range and because it’s going to be harder to get something as gross as a CF seat tube with a 120lb damper on it to oscillate at that higher frequency. But if one end is unweighted and all the weight is on one end, it’s kind of like you holding a tuning fork by the end - it will still oscillate when you whack it.

I'm an electrical engineer so I'm a little out of my league here on mechanical resonance, but the signs all seem to be there. The math is the same whether it's electrical or mechanical.

J.