Posting this as I have just had this occur on my TT bike during a high speed decent at over 40mph. This was extremely violent side to side and my first though was somehow I’d had a serious mechanical failure in my front wheel or forks.
As I was not in the bars I could gently ease the back brake on and although it took a few seconds the shaking of the bike stopped as the speed came down again, although it took the rest of the ride for me to calm myself down as my heart rate data can attest.
Hers some data and the moment you can see my heart rate turn around mid decent. I did manage to do a close to 40mph decent later in the ride.
Out of interest, this happened on the 70.3 Weymouth course, which we rode yesterday even though the event is pushed back a year. It’s a great course with nearly 4000ft of climbing for your 56 miles.
Possible causes are hard to pinpoint, this was likely my first time at over 40mph on this bike, but also I’d had back wheel issues rubbing my frame and had to stop to manually adjust the wheel. To complicate things further I was running latex tubes for the first time.
From what I’ve read I’m not inherently blaming any equipment here, rather that it is an oscillation at some sort at high speed in certain situations with heavier riders / high seat posts. For completeness sake, here’s my equipment list
Cervelo p3 2018 / Zipp 404 fire crest front and rear / conti GP5000 25mm / Vitoria latex tubes