Originally Posted by
DangerousDanR
Ouch! Hope you heal up quickly.
That was the drill for those flexible flyers of the 1980's. John gave a very good explanation of the forces involved. Basically anything that can create a vibration can cause this, so most of the suggestions may help.
I have a Peugeot CFX-10 from the early 1980s with a 61 cm top tube. It seems to wobble if anything is wrong. One item which has changed the wobble point on that bike is tire balance. Patch a tube, by the side of the road, wobble at 30 MPH. Good wheel balance, wobble at 35, so I would add wheel balance to the list of wobble causes.
Very good point about wheel balance. If the system is underdamped or resonant and that wheel imbalance hits the right frequency, it would probably set off the wobble behavior. Silca makes some wheel weights so you can actually balance that out.