Join Date: Jun 2009
Bikes: Reynolds 531P road bike, Rocky Mountain Metropolis, Rocky Mountain Sherpa 10, Look 566
The only time I've ever experienced severe oscillation on downhills was when I had a Vitus 979 years ago. I only weighed 160 then, so I'm not sure if it happened with even smaller people or not. It was the primary reason I sold it after only one season and went back to a steel bike, which was really the only other choice at that time. (Well, maybe titanium, but they were astronomically expensive.)
I've never experienced it since, although I'm (cough, cough) pounds heavier now. The steel bike that replaced the Vitus was a little squirrelly on fast downhills, but didn't vibrate or oscillate. The carbon bike that replaced the steel bike rides like it's on rails and is extremely stable, possibly due to the large diameter of the down tube and head tube.