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.
CJ: For starters, speed jobs can be caused by a number of different reasons. Loose trucks can give you too much turning ability, making your set up hyper-responsive for any speed over 10-20mph.
Its very natural for the human body to lean back when going an uncomfortable speed. Place your front foot as far forward as possible without hanging your toes or heel over the railing.
Sam: Before we get in to the nitty-gritty details on how to eliminate the horrible fishtailing known as speed wobbles, you must first understand the primary cause of speed wobble. Speed wobble occurs when the rear of the board begins turning before the front of the board, the front truck will stay relatively straight while the back truck will turn back and forth violently resulting in a hefty amount of road rash.
One of the most common mistakes that beginners encounter when learning how to ride is putting all the weight on the back truck of the board. When riding make sure to keep your knees bent and most of your weight on your front foot.
Sudden movements make speed wobbles worse, so when you shift your weight, do so with caution and confidence. Lower angles will cause the truck to turn even less which will make for a more stable ride.
A lot of us here at Motion Board shop ride with 184 mm Caliber Trucks with 44 degree baseplates. It’s easier said than done when you’re bombing a hill and start to wobble, but relaxing your body will go a long way.
Bending your knees will lower your center of gravity and give you much more stability, but it won’t always get rid of speed wobbles. Get low, relax your legs and focus on keeping your upper body over your board and positioned slightly forward.
Attack the hill and if your board starts to wobble, stay relaxed and confident. It’s much easier to feel relaxed while bombing a hill if you know you could throw a speed check or slide to a stop at any time.
Learning various stopping techniques and being comfortable doing them is one of the most important skills any long boarder can have. 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. 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.
Toucan find out if the bike has a history of accidents that could reveal particular mechanical vulnerabilities. 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.