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a bike descending shimmy-free) can stop operating normally when one parameter in that system changes. “Hopf bifurcation” is a mathematical theory which suggests that a system operating as it should (i.e. There’s another way of looking at speed wobbles too. This is because shimmy happens in a system that’s in a state of equilibrium and pedalling has the effect of throwing off that equilibrium. Interestingly, speed wobbles only seem to happen when a rider is coasting. This stops the oscillations from growing larger and larger until even the most experienced rider is simply thrown from their bike. Thankfully, speed wobbles have what’s called a “stable limit cycle” meaning the oscillations will only ever have a limited amplitude (maximum range of oscillation on either side of centre).
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This is one of the reasons speed wobbles are much more likely on a road bike than a mountain bike - MTBs have suspension that helps damp out the unwanted oscillations before they become a problem (not to mention uneven trails which make it hard for the bike to reach a stable equilibrium, plus slower speeds to begin with). It’s widely understood that speed wobbles are more likely to happen on bikes that aren’t particularly stiff, or that aren’t particularly well damped. Think of the sound a crystal glass makes when you run a wet finger around its rim - this is another example of a system vibrating at its resonant frequency. When shimmy does occur, it’s a case of the bike-and-rider system oscillating at its “resonant frequency” - the system’s natural frequency of oscillation, as determined by the physical parameters of that system. “At most speeds, and often even at the shimmy speed, such a perturbance is quickly damped out and no shimmy occurs.” “At the right road speed (the “shimmy speed”), and if conditions are right, once the system is perturbed, the front part of the bike starts to oscillate: that’s shimmy,” he says. As he explains to CyclingTips, for a speed wobble to occur, a handful of factors need to line up perfectly. This could be something as simple as the rider shivering on a cold descent, the rider sneezing, a gust of wind, a bump in the road, or perhaps even a wheel that’s not quite true.ĭamon Rinard is the Engineering Manager for Road Bikes at Cannondale and a man with a wealth of experience when it comes to frame design. In bikes, speed wobble starts when something causes the front wheel to accelerate to one side. Speed wobble, or shimmy, can be defined as “an oscillation of the bicycle steering assembly at frequencies too high for an effective reaction of the rider.” It’s a phenomenon that’s also seen in motorbikes, skateboards, shopping trolleys, and aeroplane landing gear. So how does it happen? What’s actually happening to the bike in the midst of a speed wobble? And most importantly, what can you do to stop it from happening? It’s one of the most terrifying things you can experience on a bike. Rattled, you clip back in and continue the descent, noticeably slower than before.
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You unclip a foot and, to your great relief, the bike gradually comes back under your control. Terrified of crashing, you pull over to the side of the road, making the quick judgement that if you’re going to fall off, you’d rather do so on the grassy verge than the unforgiving bitumen. The handlebars are shaking violently from side to side and you’re doing all you can just to stay upright. What starts as a small wobble quickly becomes much more.
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But suddenly, as you pick up speed, the bike takes on a life of its own. You’re not riding recklessly you’re not even pedalling. You finish climbing, crest the hill, then start flying down the other side.