We received a detailed email over the weekend from reader Drew Clark of Erie, Colorado. Drew’s own experiences with “speed wobble” led him to research it in depth, and he shared his finding with us, in easy-to-understand terms. Here’s what Drew, a retired math and physics teacher, wrote:
I have experienced speed wobble many times and in some very scary, dangerous situations, which prompted me to delve deeper into the cause and cure.
A speed wobble is essentially a harmonic vibration.
As such, it must have an anchor point. Imagine, if you will, a meter stick placed on a countertop so that most of the meter stick projects out from the countertop, and you firmly hold the few inches over the countertop with the heel of your hand. Then you twang the meter stick and it vibrates. But if you remove the anchor point (your hand), or even slightly relax the force with which you press on the end of the meter stick, the vibration ceases immediately.
The Key is the Anchor Point: Your Butt!
A speed wobble on a bike is very similar. And the key thing to know is: Where is the anchor point?
Many people believe the vibration is centered around the headset, but it is not. The anchor point is where the bike meets a constraining mass, your butt.
And the cure for a speed wobble is simple and easy: unweight the seat.
You do not even have to be completely off the seat; a slight unweighting is all that is needed. The wobble will cease instantly.
Some other considerations.
A speed wobble needs two things: a trigger and a sustaining input or feedback.
You are correct that a sudden crosswind can be the trigger, as can a patch of rough pavement. But, contrary to expectations,a speed wobble is very unlikely to sustain on rough pavement, because the roughness contributes DISRUPTIVE feedback to the speed wobble (i.e. the roughness is very unlikely to occur in harmony with the frequency of the wobble). So, when you get a speed wobble, rough pavement is your friend, and smooth pavement is more dangerous.
Some bikes are more prone to speed wobble, but it can happen on ANY bike. Speed wobble IS NOT an issue of loose components, nor of tight components. A very common feedback that magnifies a speed wobble is the human shiver, which CAN closely match the frequency of the speed wobble. (This is also why speed wobbles are more likely to occur when you are chilled on a fast descent.)
It is virtually impossible for a speed wobble to occur on a tandem. Go back to the meter stick example and imagine a meter stick placed across a gap between two countertops and with both ends anchored. With both ends anchored, inducing a vibration is virtually impossible. The two rider’s butts on a tandem act in similar fashion to the two anchors on the meter stick.
Other actions that can help:
Grip the top tube between your knees.
Keep pedaling, even without force applied to the pedals. You are not going to be able to pedal at the frequency of the wobble, and the pedaling motion works to disrupt the harmonics of the speed wobble. But this is difficult to do with the seat unweighted, and unweighting the seat is by far the most effective method.
Be sure you put on warm, dry clothing before a cold descent.
Reducing speed is NOT a sure cure. Reducing speed MAY make a wobble worse.
On more than one occasion, when I reduced speed to try to stop a speed wobble, the result was that that frequency of the wobble reduced, but conservation of energy simply transmitted the extra energy into INCREASING the AMPLITUDE of the wobble.