I've had speed wobble, shimmy, with a lightweight steel frame. Road surface or wind can make the frame go into resonance and induce speed wobbles. Shivering can also induce it. Also, weight behind the saddle such as a large seatbag, or a rack.
When I have to carry a large bikepacking style seat bag for brevets, I purposely over tighten the headset so there is a bit of resistance. Probably not good for bearing life but effective in my case.
There is a headset available to prevent speed wobble, viscoset by cane creek.
Enviado desde mi SM-A536B mediante Tapatalk