Bike wobbles with no hands: Why?

Good deed of the day done.

He giveth, then he taketh away.

Could be bike geo (e.g. track bike is harder to ride hands free compared to tourer)

Actually I found track bikes easier to ride hands free than my more relaxed road bike... weird isn't it? even my 79 degrees RiGi is easy to ride hands free..

Is no one going to mention fork rake and trail?

wheels out of balance/true.

Whilst rolling down a long hill today doing mid to high 20's on my geared bike the front end started wobbling, slightly at first but it got progressively fiercer over a very short space of time to the point where I really expected to come off the bike. Although this happened very quickly and my brain was going 100mph in self preservation mode I remembered reading an article about speed shimmy and vaguely recalled a solution of gripping the top tube with both knees. This did the trick and the wobbling stopped but it seriously scared the crap out of me.
A quick trawl round the web gave the sheldon article reffered to above and this interesting article.
Read it, it may save your arse one day.
I've previously noticed a hint of a shimmy on a couple of occassions, both on a downhill roll and both on cold mornings. Today's wobble happened on a long downhill decent minutes after a cafe stop so I was also a bit on the cold side.

Sheldon's article here with a referenced to cold descents.

i remember reading an article about this once, something about a strange concoction of the resonant frequencies of the materials in the frame being reached i seem to recall.

Glad you survived intact!

It could be caused by a loose headset, or just a flexy frame.

i get this on my touring bike, i think due to the flexy rims i have on there.
Like you say is oscillates progressively worse and worse on it's own.

Knowing mr tester I doubt it was a loose headset.

It's about the frequency of harmonic flex in the frame if my skim read of Brandt is correct

Be afraid, be very afraid!

YouTube- Bicycle Shimmy or Speed Wobble

Also happens to motor bikes

YouTube- motorbike speed wobble ending in extreme crash

Where was the wobble coming from, front or back...?
Could be a dink or slight bend in your seat stay if you've took a tumble recently, either that or maybe your headset needs a tweek.

Where was the wobble coming from, front or back...?
Could be a dink or slight bend in your seat stay if you've took a tumble recently, either that or maybe your headset needs a tweek.

The wobble was from the front and identical to the first video above. There's nothing wrong with the bike which is a quality make full carbon fibre framed machine under a year old. The wheels flex slightly from side to side but then they do on my other bikes. Everything else is as new. The 'science' in the Sheldon (Jobst Brandt) article explains the causes nicely.

This is boring ... just deal with it and STFU.

blimey! it took you 2 days to read the thread and type that.
lol wtf etc.

It needed to be said.

Yeah, get the bus looser.