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I'm not sure if oscillation is the right way to describe this, but when I'm riding at speed (say around 35-40 km/h) and take one hand off of the bars, the bars shimmy or oscillate – kind of like they are resonating with something. It's not bad, but it's new – I'm pretty sure the bike didn't used to do this…

Does anybody know what would cause this? Or better yet, what can be done to eliminate it?

In case it matters the bike is a Trek 620. The front wheel is a pretty plain vanilla rim with Schwalbe Marathon tire (32-622) and a Sanyo dynamo hub.

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    Sounds like the wheel is out of true. If you pick the bike up and spin the wheel, can you see it wobble, sideways or up and down? – andy256 Aug 5 '15 at 5:16
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    Look at the tire also. Sometimes the wheel can be true but the tire has a wobble. – andy256 Aug 5 '15 at 5:25
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    The front wheel may be unbalanced (tyre with a heavy 'spot'). Or when you release one hand you lean backwards at the same time and remove weight from the front wheel which is the most likely cause. – Carel Aug 5 '15 at 7:46
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    I suspect that the tire is a little lop-sided, either because it's made that way or because it's not perfectly centered on the rim. Or it could be a resonance as suggested by Zwirbeltier. Or maybe you've just become more sensitive to your bike after you've gotten used to it. – Daniel R Hicks Aug 5 '15 at 12:34
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    Resonance around the steering axle is a common phenomenon. It happens at a certain speed depending on wheel load, steering angle and rake. The wobble is gone below or above the critical speed. Motorcycles and cars have steering dampers for this reason. On a bicycle the rider's arms have said function. And the load on the front wheel is easily altered by position changes. – Carel Aug 6 '15 at 9:57
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I have two theories that might explain the wobble you're experiencing. Perhaps both have their part in it.

Self stabilizing of the front-wheel (resonance)

Usually bikes are built in a way that stabilizes the front-wheel. The term for it is trail (called "Nachlauf" in the image). It's the horizontal distance between the steering axis and the wheel's axis. On bicycles this is often realized as a slight forward bend of the fork. enter image description here

So whenever the front wheel gets a bump there is a force trying to put it back to the straight position. Due to that there is bound to be some resonance frequency. With increasing speed and thus stronger bumps some of bumps are bound to be nearly in resonance with the front-wheel and to create your wobble.

Precession

Whenever the wheel isn't exactly in the direction of the driving direction it gets some force parallel to it's rotation axis. As every rotating object it reacts with a movement called precession (just like a gyroscope). Also this gets more intense with speed because of the higher angular momentum and the stronger force on the wheel.

So what would help?

  1. Increase the trail of the front wheel. See this article on the pros and cons of that approach.
  2. Sit in a more inclined position - thus putting more of your weight on the handle bar.
  3. Put your hands further apart on the handle bar (increasing your leverage). You can easily experiment with this: Usually the wobble begins at 20-25km/h if you're driving free-hand and at ~30km/h if you put your hands close together.
  4. Avoid elastic frames. Especially frames with a low trough step (originally for wearing skirts while driving) seem to add to the wobble.
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It seems like it could be:

  • Loose Hub
  • Loose Headset
  • Untrue Wheel
  • Tire messed up one way or another

From your description, it's hard to believe that the source is in the rear half of the bike (I'm assuming it also happens when you're not pedaling, correct me if I'm wrong).

It seems to be a fairly minor problem (for now at least), that could be very hard to pinpoint.

If you can't find the problem after a technical inspection, I would just keep riding. If you notice the problem getting worse, inspect it again (it'll be easier to find).

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    @dlu BSO raises an important point about whether you're pedaling when this occurs. Up till now I think we've all been assuming that you meant the shimmy rate matched the wheel rotation rate. If that's not the case then the problem could be something else entirely. Do yourself a favour and check that the forks and handlebars are firmly attached. Try pushing the front wheel sideways, at the top and at the bottom. I'm thinking of safety related failures here. – andy256 Aug 6 '15 at 1:30
  • Thank you both. I'll take a careful look at the whole front end. The times I've noticed this I'm not pedaling – usually it happens as I'm setting up for a turn and taking my left hand off the bars to signal. – dlu Aug 6 '15 at 4:57
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Another possibility that's not mentioned in the other answers is that the plastic reflectors that attach to the spokes can be enough to unbalance the wheel. On my road bike, I found that my wheels would vibrate alarmingly, though it was somewhat faster than you have: around 55km/h. Completely solved by taking the reflectors off; probably a more safety-conscious solution would be to install two reflectors on each wheel so they're balanced.

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