One of my road bikes is particularly susceptible to speed wobbles. I attribute it mostly to the 50mm wheels, as it will only start to pick up a shimmy at speeds above 40mph, with battling winds present. In dead calm conditions, I've confidently had that bike nearly to 60 w/o issue. I've tried changing my contact points when it starts to happen and keep a light touch on the bars, but nothing seems to fix it other than dropping below 40. Oddly enough, my other road bike with different 50mm rims, doesn't have the same issues. I can feel the wind pushing the bike, yet it doesn't develop the shimmy.