Vehicles that can experience this oscillation include motorcycles and bicycles, skateboards, and, in theory, any vehicle with a single steering pivot point and a sufficient amount of freedom of the steered wheel, including that which exists on some light aircraft with tricycle gear where instability can occur at speeds of less than 80 KMH; this does not include most automobiles. The initial instability occurs mostly at high speed and is similar to that experienced by shopping cart wheels and aircraft landing gear.
If shimmy cannot be designed out of the system, a device known as a steering damper may be used, which is essentially a notch filter designed to damp the shimmy at its known natural frequency. Shimmy is usually associated with the deformation of (rubber) tires.
The phenomenon can be explained by introducing multicomponent dry friction forces, apart from the usual forces considered in the literature. Another explanation is that speed wobble is a Hope bifurcation, whereby a system changes from one state (a stable ride) to a second state, (constant amplitude oscillation), when one parameter (forward speed, or air speed) progresses through a critical point.
Wobble or shimmy begins when some otherwise minor irregularity accelerates the wheel to one side. As the wheel rotates, it will exert a cyclic load to the vehicle frame, which is matched with the system's (vehicle and attached accessories) resonant frequency, can produce a speed wobble.
During the wheel rotation, a restoring force is applied in phase with the progress of the irregularity, and the wheel turns to the other side where the process is repeated. If there is insufficient damping in the steering the oscillation will increase until system failure.
The oscillation frequency can be changed by changing the forward speed, making the bike stiffer or lighter, or increasing the stiffness of the steering, of which the rider is a main component. While wobble or shimmy can be easily remedied by adjusting speed, position, or grip on the handlebar, it can be fatal if left uncontrolled.
The top five influences on wobble have been found to be lateral stiffness of the front tire, steering damper, height of bike center of mass, distance of bike center of mass from rear wheel, and cornering stiffness of the front tire. An academic paper that investigated wobble through physical experimentation and computer modeling concludes: “the influence on wobble mode of front tire characteristics, front frame inertia and chassis stiffness were shown.
In particular, it shows that increasing front tire inflation, chassis stiffness, and front frame inertia about steering axis and decreasing sideslip stiffness of front tire, wobble mode damping is improved, promoting vehicle stability.” Proficient Motorcycling: The Ultimate Guide to Riding Well (2nd ed.).
Tank slapper: a speed wobble so severe that the handlebars bang alternately against the sides of the fuel tank ^ a b c Wilson, David Gordon; Jim Papadopoulos (2004). Lime beer, Robin S. Sharp, and Malcolm C. Smith (October 2006).
For machines with a stiff front frame, a steering damper is required to stabilize the wobble mode at high speeds, while older, more flexible machines may require a steering damper at intermediate speeds. CS1 main: multiple names: authors list (link) ^ Zhuravlev, V. Ph.
“Technical FAQ: Bifurcation and high- speed shimmy”. ^ Manfred Local; Johannes Edelman; Bernhard Approach; Christoph Out (7 Jul 2011).
Study of stability of a two wheeled vehicle through experiments on the road and in laboratory (PDF). This oscillation of the handlebars and the front of the bike violently back and forth is due to an occurrence called Hopf-Bifurication.
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. The rigidity of the frame simply increases the critical speed, so you have to be going faster to induce the 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.
It’s a term that is used to describe an oscillation of your bike’s handlebars that occurs at high speeds. Any vehicle that has a single steering pivot has the potential of experiencing speed wobbles.
Other names for speed wobble include: speed shimmy, death wobble, tank slapper, and the bicycle shakes. The first theory is that your bikes tires aren’t perfectly aligned.
Because the alignment is off the tires are fighting against each other which causes the steering wheel to shake at a certain speed. Modern bikes are stiffer than older bicycles, and are known to be less prone to speed wobbles.
There are many cyclists who experienced a scary speed wobble, because of a strong gust of wind. An easy way to help prevent speed wobbles is by holding onto the handlebars, and keeping a proper distribution of weight throughout your bike.
Instead, you’ll want to make sure you’ve got a relaxed grip on the handlebars, and to bend your arms to act as suspension. Just remember to bend your arms, squeeze your knees on the top tube, and use your rear brake to slow down.
Keep your weight distributed throughout your bike, a relaxed grip on your handlebars, and arms bent to act as suspension. Just because your friend has no trouble with speed wobbles on a particular bike doesn’t mean you’ll have the same experience.
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. 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.
“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,” Richard opines. 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.
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. 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.
“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.