This is a high-end physics term used to describe the point at which a system’s stability changes to that of a periodic solution. According to Eric M. Bolt, W. Jon Harrington Professor of Mathematics Clarkson University, this is not the case as there is no periodic forcing nor is there a building of amplitude in a speed wobble on a bike, both required for a resonance phenomenon.
Secondly, when a speed wobble occurs, it doesn’t begin as a slight shimmy and build to a larger and larger amplitude but rather begins at a point known as critical speed, or the point at which the bike crosses the Hope bifurcation value, and shakes violently back and forth at a more set amplitude. The second cause of a speed wobble is insufficient rigidity of a bicycle frame itself, particularly the top tube.
This often occurred in older bikes, but newer models are now stiffer and more rigid thus reducing the occurrence of a speed wobble. In addition to keeping your weight more evenly distributed, a quick, simple way to control the bike and stop the speed wobble is to squeeze your knees together onto the top tube with the pedals at 3 and 9 o’clock (where they most likely already are as this is how you should descend unless going through a corner).
To help avoid it from happening, keep your weight more evenly distributed between the front and rear wheels and always make sure you have a grip on the bars but not a death-grip. I ride a 30-year-old, vintage Motorcade 10- speed that is in excellent condition.
Six months ago, while making a high speed turn in a 90-degree corner (about 22 mph) the front end and wheel began an uncontrollable wobble and shimmy, causing a crash, resulting in a bent rim and handle bar. Did notice some front wheel wobble when riding no hands above 20 mph, but not bad.
The frame flexes to allow this to occur until the action is overpowered by the self-correcting forces created by the bike’s steering geometry. The energy stored in the frame then causes the steering to over-correct in the opposite direction, which starts another round, and thus the oscillation begins.
These questions will provide a good starting point for diagnosing the cause of the wobble. When you experience a speed wobble or tank slapper while riding your 3 wheel motorcycle or 2 wheeler, it can be downright terrifying.
Whether you’ve dealt with a speed wobble before or not, it’s vital to understand why it happens and how to prevent speed wobbles on a motorcycle to stay safe when you hit rubber to the road. A speed wobble typically starts when the front wheel elevates from the ground when you accelerate, do a wheelie, or run into a pothole.
However, sometimes the effort of aligning the forks is so great that your bike will overextend the straight line and land on the opposite side. Rapid acceleration, hitting potholes, or doing a wheelie all have the potential to displace the weight of your bike.
After the wobble starts, the external forces increase and can worsen to the point where the ride loses control. However, any motorcycle, especially when you corner, puts increased strain on the parts and could lead to wobbling.
Touring motorcycles can be particularly vulnerable to rear-wheel wobbles if the weight they carry isn’t properly distributed across the bike. It is crucial to understand how your bike operates both when it is empty and when it carries a heavy load.
Once you are familiar with how your bike should feel both empty and loaded, toucan take care of any wobbles right away before the situation spirals. Now, if your bike is heading towards a wall or another dangerous obstacle, and it becomes apparent that a crash is imminent, it your best choice is to bail off.
Typically, a motorcycle that is speed wobbling will slow down at a rate of one-eighth g. A sliding biker will slow at a rate of approximately 1 g. The point is, even if the worst case scenario happens, and you have to bail, you’ll come to a stop more quickly and in a shorter distance than your bike. Finally, understand that speed wobbling is greatly dependent on the rate of speed you operate your bike at.
Frequently check components of your bike such as its suspension, aerodynamics, steering damping, and wheels, especially if you purchase used motorcycles. Have your bike regularly checked by a professional, to confirm that the main components are in optimal working order and reduce the likelihood that you’ll experience dangerous wobbles.
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.
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 airplane landing gear. In bikes, speed wobble starts when something causes the front wheel to accelerate to one side.
Damon Richard is the Engineering Manager for Road Bikes at Cannon dale 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.
But some studies have included real-world measurements, such as the latest paper in this space from Italian researchers at the Polytechnic di Milano (Polytechnic University of Milan). Led by Nicole Tomato, 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 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 Echo, Lombardy to see if they could detect shimmy.
While it’s possible to initiate a speed wobble by bashing on the handlebars mid-descent (as in the terrifying video below from Damon Richard) the test subject in this case didn’t apply any deliberate stimulus to promote the onset of shimmy. “I shot it because I had just gotten my first helmet cam (they were new back then, before GoPro) and I was just setting it up,” Richard says of the video.
Previous modelling also suggested that shimmy should disappear once the rider’s speed drops below 51 km/h (31.8 mph). When the front end of a bike oscillates slower than about 6 Hz, that’s what’s known as (see animation below left).
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.
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.
If the shimmy happened when you had your hands off the bars, gently put them back on again (see the Richard video above). 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,” Richard explains. “This works by decreasing the stiffness of the system, by removing an inertial anchor point (your body on the saddle),” Richard says.
Again, the key seems to be to remain as calm as possible, to alter something about the system to destabilize the oscillations, and to reduce your speed safely. If toucan do that, you should be able to bring the bike back under control with little more than a huge fright to show for it.
“The frame and fork are the biggest springs in the system, so changing to a stiffer frame set is a good option, though expensive,” Richard says. More-supple tires seem to be a good (and relatively inexpensive) place to start, in order to provide greater damping at the road: “Softer or less structured tires, may altogether prevent the onset of shimmy,” write Tomato 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.
“If you ask around you will no doubt hear about various mechanical fixes: adjust bearings, check alignment, tighten spokes, etc,” Richard says. Again, finding something that works to stop the shimmy will likely be a process of trial and error, often with an associated cost.