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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?

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    Your video shows bumpy trail, so I suspect the small bumps are regular enough that they resonate with your fork/steerer setup. Check out this site: desolutions.com/blog/2015/06/what-is-a-vibration-resonance A famous instance is the old Tacoma Narrows Bridge -- cool video! Have you actually checked your headset and wheel hub as mounted to make sure there is no play?
    – Armand
    Aug 16 at 11:16
  • Any headset bearing play? Any hub bearing play? How’s the fork play?
    – Michael
    Aug 16 at 12:21
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    there's no play in the headset bearing. Also the bumps on the track are random bumps, not with a regular distance between them (which could induce resonance). Aug 16 at 13:07
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    Oddly, it could be due to something loose on the rear rack, or elsewhere on the frame. Aug 16 at 14:38
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    It seems pretty clear to me that it's resonance. Remember, the bumps don't have to be regular, just enough of them that your bike will be getting a nudge at the right point in all or most resonance cycles. Of course, the resonance is there almost certainly because of some play.
    – Armand
    Aug 16 at 17:22
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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

  • Change tire pressure. Very first thing to try. Costs nothing. Do it.
  • Change handlebar position either by flipping the stem or removing/adding spacers
  • Change your front shock pre-load setting
  • Remove your bar-ends. See if this makes difference. If it does, but you still want to ride with bar-ends, look at different model.

More complicated:

  • Get another pair of tires (or maybe you already have it).
  • See if you can borrow a fork from someone you know.
  • See if you can get hold of a different handlebar (wider, narrower or different backsweep / rise)
  • Get a different diameter wheels (ideally borrow from a friend rather than buy)

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/

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  • Could be something to do with the fact that I had one of these hanging off the back. No child or bike attached, but it adds weight, and as you say would change the natural frequency of the bike Sep 23 at 8:27
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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.

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    I had wobbles like this. Turned out to be loose spokes on my rear wheel. Like you said, very likely SOMETHING is loose. Aug 16 at 19:28
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    The head set could be too loose but also too tight. When caused by the head set it happens in very narrow band of speed. Nothing below and nothing above.
    – Carel
    Aug 16 at 20:14
  • I checked the whole bike. Nothing is loose. Tyre pressures are fine. Sep 23 at 8:23
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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.

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