I’m an electrical engineer with a background in control system and semiconductor, but the concepts in a mechanical system are the same (and so is the math). Basically, any time you have an underdamped system and you hit that system with an impulse at the right (i.e. resonant) frequency you are going to get oscillation that (probably) decays over time. When the wobble gets persistent it means that either the frequency you are hitting the system with is persisting (i.e. washboard road) or the bike is highly susceptible to that frequency. You change that by damping the frequency/oscillation, by changing the “length” (from a wavelength perspective) of tubes or changing the mass of the vibrating thing. It’s obviously a lot more complex than that with something like a bike, but it still offers clues about what to do.

First off, I’d make sure that your headset is tight, your hubs are properly adjusted and you brake rotors are tight to spec. I’d also make sure your rotors are clean and have no tendency to squeal - all of these could amplify the effects of an frequency input at the right frequency or produce the frequency themselves (i.e. brake rotors).

If it still persists, I’d change to a wider tire and I’d lower the tire inflation to get a more plush ride. That is going to damp any frequency input from the road (you mentioned rough road). I’d also be extremely careful about hitting big road defects like pot holes since those are an impulse that is compromised of many frequencies to get the sharp edge hit you feel. So even though it’s not a continuing frequency input like the washboard road might be, it can have sufficient energy in the frequency that sets your bike off. The wider tires/plusher ride will help here too. Silca has a great tire inflation calculator which will help. Most riders have their tire pressure too high anyhow. If that’s you, then this could be an easy fix.

If it happens a lot when braking, then I’d look at the rotors. If your rotors are getting worn they can get thinner and set up an oscillation. Resonant frequencies are one of the big worries in designing braking system so I’d even look at changing to a different rotor - one that is thicker and stiffer would be a good place to start. May not look as cool or be as light but it’s going to behave differently in terms of oscillation.

If it’s the wind, then I’d be looking at things that the wind might have an effect on and could start vibrating such as handlebars. I suppose certain aero shaped bars could be a problem. You should be able to debug this by changing hand position on the bars. I’d look at putting my hands on the half of the bar that’s nearest the stem and try moving them in and out and see if that stops it. If it doesn’t, then it’s not the bars.

The classic advice given is to clamp the top tube between your knees. This works if it’s the top tube (less but still effective if down tube) that’s oscillating. What this does from a frequency perspective is make your top tube shorter so that you push the resonant frequency higher. Once it’s higher, then it will stop oscillating from the current input frequency because it’s no longer the resonant frequency. Of course, this is going to matter what the frequency is, where you clamp your knees on the tube etc.... So that’s why it doesn’t work *all* the time.

Scary problem. Been there.

Perfect example. Adding mass in the wrong place and changing the spring constant in the tire are two places that could lead to underdamping. That tire is probably stiffer with less damping of the road vibration, and that handlebar bag depending on how it’s attached and where it’s attached is helping to set up an oscillation in the front end but probably in the bars. Stiffer bars might help that.

I agree with the idea of changing tires but make sure you reduce pressure so you have a cushier ride. Generally faster on the road that way anyhow.

J.