Hi ridenfish39,

Sorry you had that experience.

Wobble is related to system stiffness, so bigger bikes are (all else equal) more susceptible. We've engineered increased stiffness with increasing sizes so the bigger frames are stiffer in a proportional response, but any structure still has a natural frequency so can resonate under the right (wrong?) conditions. Even in the medium size, the SystemSix is stiffer than last generation aero bikes so is less susceptible to wobbles, and the larger sizes only get stiffer, but that said, the higher stiffness just raises the natural frequency so that if/when a wobble starts, you're going at a higher speed!

In the old days nearly everyone's bike would wobble, so we all knew what to do: "touch your knees to the top tube, or stand up and unweight the saddle a bit."

This good advice is from a thread you might find helpful: https://forum.slowtwitch.com/...ost=5103061#p5103061

More background that helped me understand shimmy or speed wobble by Jobst Brandt, still true today: https://www.sheldonbrown.com/brandt/shimmy.html

Cheers,
Damon

Edited to add: While I was typing several folks also posted.
1. The idea to ditch the bike works *if you're sure your new bike is stiffer.* We've measured and the SystemSix and it's among the stiffest, so in your case, changing is unlikely to help. In neal's case, the Tarmac of a generation or two ago was among the stiffer bikes (according to data measured by TOUR Magazine), so that may help explain his success at avoiding speed wobble.
2. Sure, check skewers (or through axles) or headset or wheels, but unless they affect stiffness they are unlikely to help. Because wobble requires a combination of conditions, by chance some riders have appeared to solve wobble by adjusting things, but the natural frequency only depends on mass and stiffness so it's unlikely the various mechanical things people have checked really have changed anything.
-DGR

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Damon Rinard
Engineering Manager,
CSG Road Engineering Department
Cannondale & GT Bicycles
(ex-Cervelo, ex-Trek, ex-Velomax, ex-Kestrel)