Twitchy handling at higher speeds

[j beede]

My road bike gets a little unstable feeling at higher speeds during downhill runs... starts around 30-35+ MPH. The gyro effect from the front wheel is not increasing enough to over come "the shakes". Tire pressure, front/rear alignment, front hub play, headset play, fork rake and trail... anything else to consider? It bugs me to see mountain bikers on the same roads doing 40+ comfortably! Your thoughts?

[cyclerUK]

Some bikes are prone to shake at high speeds. Some say grip the top bar with your knees when on the decent?
What wheels are you using?
I have a light bike with Ksyriums and that feels a bit jittery on downhills. But if I fit my Open Pro's on, it feels O.K.

[Joe_W]

Well, the shaking is a resonance effect. Gripping the top tube with the knees is an old trick, it works, I know cause I had to do it. Sometimes the shaking will disappear if you loosen that steel hard grip on the handlebar as the resonance frequency is changed. You might also want to read up on the subject on http://www.sheldonbrown.com/brandt/shimmy.html