You’re going down a fast descent and all of a sudden your bike starts to speed wobble and shimmy beneath you. What do you do? What’s happening? How do you avoid crashing?
What is a Speed Wobble?
A speed wobble, or speed shimmy, or death wobble, is when the front of the bike starts to shake violently back in forth originating from the front fork and wheel. 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. This means that the bike maintains stability through a deflection in one direction followed by a deflection in the other to maintain equilibrium. This is much like that of a top that is spinning, slowing, and beginning to wobble back and forth yet still staying upright.
What a Speed Wobble Isn’t
A speed wobble is often mis-described as a resonance phenomenon where the bicycle hits a resonance frequency and begins to shimmy. 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. In laymen’s terms, this describes that a force such as a bumpy road, which places differing forces upon the wheels and frame, can induce the effects of a speed wobble where in fact it can just as easily occur on a smooth surface. 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 Hopf bifurcation value, and shakes violently back and forth at a more set amplitude.
What Causes a Speed Wobble?
On a bike, a speed wobble can be caused by a number of things. The first is rider placement upon the bike. Distribution of weight centered more toward the rear of the bike causes the front end of the bike to loose stability at speed. It would be like if someone were to shake a swimming pool noodle back and forth holding onto one end while you held onto the other end in a fixed position; the noodle would form s-bends but if you were grab the other end of the noodle by the person’s hands as well, it would stabilize it thus removing the s-bends. On the bike, the person shaking their hands back and forth is the occurrence of Hopf-Bifurication and controlled instability as the critical speed of the bike is reached.
Fixing and Avoiding a Speed Wobble With Weight Distribution
Holding the handle bars and placing more weight on the front end of the bike will stop the speed wobble and if you keep enough weight toward the front of the bike, it will help avoid a speed wobble in the first place.
Insufficient Frame Rigidity As A Cause
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.
What To Do When You Get 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). This will immediately stop the speed wobble and allow you to brake to a more manageable speed for the bike you’re on.
The Take Away
Getting a speed wobble will scare the crap out of you no matter how many times you’ve experienced it. Not knowing what to do can be even scarier. Remember just squeeze your knees onto the top tube whenever it happens, then brake to slowdown. 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.
