Road Cycling - Occasional uncontrollable wobble at speed

One of my buddies has a disturbing problem.

Occasionally on downhills at speeds above 55km his bike goes into this uncontrollable wobble.

I searched a week ago and found some discussion but today I can't find anything.

His second hand bike must be one of the earlier carbon frames as it has 6 speed cassette.

One reader suggeted griping less hard and on next ride he didn't have problem but then it isn't everyday.

Any ideas?

Could be a few things.Loose headset, wheels etc.
Check the head set first.Hold front brake and push the bike back and forth.If you hear clicking its the headset.If there is tighten it up.

If not check the wheels for play by moving them side to side to see if there is any play, if so get out the cone spanner and tighten them up.

A lot of bikes develop a "wobble" at certain speeds. The cause is the subject of debate, but one of the contributing factors is that a resonance is set up in the frame. It usually isn't really a problem except when coasting, and especially when trying to ride no-hands. Sometimes, pressing a knee against the top tube will dampen the resonance and reduce the wobble.

Or getting out of the saddle. Seems counterintuitive, butit works.

A

A very real possibility is that of a poorly matched fork. If the amount of trail is too little (like, say, less than 40mm), then it will become very prone to lateral inputs, and will develope a serious vibration at higher speeds.

Originally posted by roadbuzz
A lot of bikes develop a "wobble" at certain speeds. The cause is the subject of debate, but one of the contributing factors is that a resonance is set up in the frame. It usually isn't really a problem except when coasting, and especially when trying to ride no-hands. Sometimes, pressing a knee against the top tube will dampen the resonance and reduce the wobble. There may be mechanical things that could be the cause.But otherwise,Roadbuzz nailed it.