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My bike has recently started behaving strangely when I ride no-handed.
When I let go of the bars, they are stable, but quickly develop a wobble which becomes quite violent. I put my hands on the bars to stabilise them, release, and the same thing happens again.
The headset appears fine, and when I have my hands on the bars it is also fine.
What could be going on?
Looks very much like a resonance. A video of this on a smoother road would help.
If it really is resonance, then it does not necessarily mean a mechanical problem with any of your components. Though it surely may lead to it. Resonance occurs when the frequency of periodically applied forces is close to natural frequency of the bike. Periodically applied forces are most likely originating from the wheel rotation. Natural frequency of the bike depends on a lot of factors and is hardly practical to calculate.
What you need to know is that the natural frequency of the bike will to some extent be impacted by any changes you make to the bike. Literally any change in any area of the bike could make it better or worse. Try a few things, but don't go too crazy about it. Resonance (aka speed wobble) can be quite bad to the point where rider cannot control the handlebars but it does look that bad in your case. It's not worth spending a lot of money on solving this issue unless it really impacts the quality of your rides.
I would try things in this order:
Cheapest/Easiest
More complicated:
One of my bikes used to do exactly the same. Handlebar starts to osciallate even on the smoothest roads. But it only happens between 20 and 30 km/h and only when bike has panniers and front rack. It stopped doing so when I got rid of the panniers... Who could have guessed. I now use bikepacking bags
Some good reading on the subj: https://cyclingtips.com/2020/07/bicycle-speed-wobbles-how-they-start-and-how-to-stop-them/
Something's loose - you need to find it.
The headset is a prime candidate, so good work for checking that already. Anything in the fork/hub/wheel can be contributing.
Also work upward - Check both the stem mountings, headset bearing preload in the vertical direction as well as horizontal.
Basically push and pull and twist everything, looking for whatever's not right. Have a good go at the fork legs/tines too - could be in need of service. If you have lockout, turn that on and see if the problem goes away.
Also check your tyre pressures, and try the same diagnosis on a smooth road surface.
Other answers have said that there's something out of balance. That's likely true; but remember, that nothing is in perfect balance. One side of the bike is heavier than the other, the front is heavier than the back, and any bike that's been around a while has been beaten out of perfect alignment...
I have always traced front steering shimmey back to an underweighted front wheel. If the bike is weighted towards the back (i.e. the front wheel has less than half the weight on it), the front wheel can develop a shimmy from even normal imbalances. Do you have a rack on the back? Are you carrying cargo?
If the answer is 'no', then this answer probably isn't that helpful.
