Originally Posted by
genejockey
I got that once years ago my my Ritchey, coming down a short but fast non-technical descent. I hit 40 and the bike started wobbling and scared the hell out of me. I think it was the first time I'd gotten over 40 on it. I had only had the "5 Minute Fit". I took it in for a professional fit, where he dropped the bars, raised the saddle and moved it forward, which balanced my weight better, and it never recurred. I always credited the fit for solving the problem, but years later I read that wobble can be more of a random thing.
I think there are any number of factors that can
initiate a speed wobble - road surface ruts, loose headset, unbalanced front wheel, etc - however, whether or not this relatively small input
propagates/amplifies into a full-scale wobble is due to different factors, like F/R balance, "death-grip" on the bars, frame flex etc - factors that promote system resonance.. Some frames and rider positions are just more prone to wobble, some just aren't, even if they're traveling on the same roads and descending the same hills. Rider setup can be modified, but some frames will always be wobble-prone, IMO.