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by Matt de Neef
July 6, 2020
Photography by Jered and Ashley Gruber (feature image) and International Journal of Vehicle Mechanics and Mobility
You finish climbing, crest the hill, then start flying down the other side. And then, out of nowhere, it happens.
You’re not riding recklessly; you’re not even pedalling. But suddenly, as you pick up speed, the bike takes on a life of its own. What starts as a small wobble quickly becomes much more. The handlebars are shaking violently from side to side and you’re doing all you can just to stay upright.
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. You unclip a foot and, to your great relief, the bike gradually comes back under your control.
Rattled, you clip back in and continue the descent, noticeably slower than before.
Speed wobble. It’s one of the most terrifying things you can experience on a bike. 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?
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.
In bikes, speed wobble starts when something causes the front wheel to accelerate to one side. 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.
Damon Rinard is the Engineering Manager for Road Bikes at Cannondale and a man with a wealth of experience when it comes to frame design. As he explains to CyclingTips, for a speed wobble to occur, a handful of factors need to line up perfectly.
“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. “At most speeds, and often even at the shimmy speed, such a perturbance is quickly damped out and no shimmy occurs.”
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. 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.
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. 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).
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). This stops the oscillations from growing larger and larger until even the most experienced rider is simply thrown from their bike.
Interestingly, speed wobbles only seem to happen when a rider is coasting. 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.
There’s another way of looking at speed wobbles too. “Hopf bifurcation” is a mathematical theory which suggests that a system operating as it should (i.e. a bike descending shimmy-free) can stop operating normally when one parameter in that system changes. In the case of shimmy it might be that speed is the parameter that changes, and that at a certain speed, the system becomes unstable and starts oscillating.
Thankfully, you’re much less likely to experience speed wobbles these days than you would have been several decades ago.
“Today, most frames are stiff enough that shimmy is less common, and with fewer riders experiencing it, the need to know has also decreased, and in some areas the knowledge might be almost lost in the mists of time,” Rinard opines. “Today’s stiffer bikes are still capable of shimmy, just at higher speeds (stiffer systems resonate at higher frequency.)
“Tour Magazin [ed. A German publication well-regarded for its rigorous lab-based tests] estimates a frame stiffer than 75 Nm per degree is generally shimmy-safe (that’s head tube stiffness or torsional stiffness.) Frames in the ‘70s and ‘80s were often close to that number or even below. Many racing frames these days are around 90 to 100 Nm per degree. The stiffest I know of is around 140, but at that level other aspects of ride quality suffer.”
So what’s actually happening to a bike once a speed wobble starts?
Most of the research in this space has been based on mathematical models and supported by laboratory tests; there’s been little in the way of genuine on-road measurements of shimmy to help validate the numerical models.
But some studies have included real-world measurements, such as the latest paper in this space from Italian researchers at the Politecnico di Milano (Polytechnic University of Milan). With on-road tests, they were able to capture data from an in-progress speed wobble, then analyse that data to explain the actual motion of the bike and its parts during shimmy.
Led by Nicolò Tomiati, the researchers rigged up a 52 cm road bike with six inertial measurement units containing accelerometers and gyroscopes (IMUs 1-6 in the image below). All of the units were connected to a Raspberry Pi single-board computer on the down tube and a power bank installed on the seat post.
One day in 2017 (the wheels of academic research turn slowly) a test subject rode the instrumented bike down a hill near Lecco, Lombardy to see if they could detect shimmy.
The rider was in the drops, holding on firmly, and aimed to reach a speed between 50-65 km/h (31-40 mph) by the bottom of the descent — theoretically enough to initiate a speed wobble. Two different sets of tyres were used during the trial — one set being 22 mm wide, the other 25 mm wide — and the descent was ridden three times for each set of tyres.
While it’s possible to initiate a speed wobble by bashing on the handlebars mid-descent (as in the terrifying video below from Damon Rinard) the test subject in this case didn’t apply any deliberate stimulus to promote the onset of shimmy.
The researchers detected shimmy in two of their six tests, both while using 22 mm tyres. They then took the larger of the two wobbles as a case study and dove into the data.
The researchers found that the shimmy began at a frequency of about 6.9 Hertz — that is, with the front wheel turning left and right at nearly seven times a second. That’s certainly faster than any rider can react to and bring under control just by counter-steering. In the course of the “wobble event”, the frequency jumped to 7.2 Hz, then 7.4 Hz, before fading out.
The researchers suggest that their data provides further evidence of what other studies previously revealed — that the frequency and amplitude of the shimmy appears to be independent of speed. That is, the front wheel doesn’t oscillate faster or more violently the faster (or slower) you go.
Previous modelling also suggested that shimmy should disappear once the rider’s speed drops below 51 km/h (31.8 mph). The researchers found this in their data, albeit with a bit of a delay. Their hypothesis is that the system takes some time to reach a stable equilibrium once the necessary speed has been reached.
A delay was found when speeding up too: “forward acceleration has a stabilising effect on oscillations and delays their onset at a forward speed,” the researchers write in their latest paper. Such delays were found in previous studies as well.
When the front end of a bike oscillates slower than about 6 Hz, that’s what’s known as “weave” (see animation below left). At these frequencies the front and rear of the bike both appear to act rigidly — that is, there doesn’t seem to be any twisting or bending of the frame. Once shimmy frequencies are reached though, things get considerably more complicated.
Which is where the data from the latest study comes in — the six sensors mounted onto the experimental bike show us just how the bike moves around during shimmy.
From the Italian research we can see that the amplitude of the oscillations is largest near the front of the frame (particularly at the head tube) and decreases as you head towards the back wheel. This makes sense: the front assembly of the bike (the stem, fork and wheel) is the part with the most freedom to move.
Data from this latest study shows something less obvious too: oscillations at the seat stays are out of phase with those on the top tube. In other words, when the front of the bike is accelerating in one direction during shimmy, the back of the bike is accelerating in the opposite direction, effectively twisting the bike beneath you.
The fork, meanwhile, twists slightly upwards in the direction the wheel is facing during shimmy. The handlebar twists downwards in the same direction, creating torsion in the front steering assembly. The left half of the following diagram shows this more clearly.
So that’s what we know about the how and why of speed wobbles. But what about something more practical: how do you stop shimmy once it starts?
First of all, try not to panic. While the experience can be terrifying, it’s relatively easy to bring the bike back under control. To do that, you’ll need to change something about the system.
If the shimmy happened when you had your hands off the bars, gently put them back on again (see the Rinard video above). This should stop the wobble. If it started when your hands were on the bars, resist the temptation to squeeze the bars tighter — this is likely to make things worse. Instead, maintain a solid but not overly firm grip on the bars.
Anecdotally, the easiest way to bring shimmy under control is to clamp your knees to the top tube.
“This works by adding damping (your body contacting the frame) and increasing the stiffness of the system,” Rinard explains. “By making the system stiffer, you’ve increased the system’s resonant frequency. For bikes that means you’d need a higher speed to shimmy. In practice that’s OK, because you’re not riding at that higher speed right now, you’re riding at the shimmy speed. So the shimmy stops.”
You can also try lifting your backside off the saddle slightly.
“This works by decreasing the stiffness of the system, by removing an inertial anchor point (your body on the saddle),” Rinard says. “By making the system less stiff, you’ve decreased the system’s resonant frequency. For bikes that means you’d need a lower speed to shimmy. Again, in practice that’s OK because you’re not riding at that lower speed right now, you’re riding at the shimmy speed. So the shimmy stops.”
Ultimately, you’ll probably want to slow down as well. Try to avoid the front brake if you can; instead use the rear brake gently. You can also adopt a more upright position to use your body as an airbrake — the increased aerodynamic drag will help to slow you down.
The Italian researchers have some recommendations when it comes to stopping shimmy too.
“Resist as far as possible the temptation to jump off the bicycle, which may appear to be the lesser evil in that situation,” they write. “Try not to fall, and not to hit other vehicles, or go off-road, for the time necessary to decrease the speed below the threshold of the disappearance of shimmy [ed. somewhere below 50 km/h or 31 mph].
“Unfortunately, this is a rather difficult task if the road remains very [steep] and curvy because brakes cannot be used effectively when cornering, and they may, at first, even cause an increase in the amplitude of the steering oscillation.”
Again, the key seems to be to remain as calm as possible, to alter something about the system to destabilise the oscillations, and to reduce your speed safely. If you can do that, you should be able to bring the bike back under control with little more than a huge fright to show for it.
But what if shimmy keeps happening? Well, just like when you’re in the middle of a speed wobble, you need to change something about the system to discourage the scary equilibrium of shimmy from happening again. That’s likely to be a process of trial and error.
“To stop a bike from shimmying again, change the mass or stiffness of the system,” Rinard says. “Less mass and more stiffness both increase the resonant frequency of the system, making shimmy less likely.”
The nuclear option is to get a new frameset.
“The frame and fork are the biggest springs in the system, so changing to a stiffer frameset is a good option, though expensive,” Rinard says.
If you’re looking for a far more affordable alternative, start swapping out components.
“Because many components participate in the spring [ed. in the bike’s spring-and-mass system], if you were thinking about changing something, go ahead and change it – it might reduce the chance of shimmy,” Rinard says. “Considering a new set of wheels, tires, handlebar or stem?”
More-supple tyres seem to be a good (and relatively inexpensive) place to start, in order to provide greater damping at the road: “Softer or less structured tyres, may altogether prevent the onset of shimmy,” write Tomiati and co in a 2017 paper.
If changing components doesn’t work, it might be worth checking that your wheels are round and true. It could be that slight imperfections in the front wheel are giving your bike the encouragement it needs to generate lateral movement once at speed. Other interventions you might have heard of are less likely to make much of a difference.
“If you ask around you will no doubt hear about various mechanical fixes: adjust bearings, check alignment, tighten spokes, etc,” Rinard says. “None of that makes a significant difference unless it changes the mass or stiffness of the system.”
You might also want to look at your position on the bike: shimmy seems to occur more often when a rider has a higher position on the bike, and when there’s a lot of weight out front or way back on the bike. Try switching up your position and see if that helps.
Again, finding something that works to stop the shimmy will likely be a process of trial and error, often with an associated cost. But as anyone that’s experienced speed wobble will tell you, once it’s happened the first time, you’ll do just about anything to stop it from happening again.