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« Back on the Bike: The First Month | Main | Clueless »
Friday
Aug182006

Shimmy Re-Visited


Speed wobbles or shimmy occur on bicycles and motorcycles because the front wheel is free to turn about its steering axis and at the same time the whole bike can move from side to side along a horizontal axis with the pivot point being the front and rear wheels being in contact with the road. Think of the motion of climbing a hill out of the saddle and you are swaying the bike from side to side.

So if the bike is swaying from side to side and the front wheel is turning from left to right at the same time the front wheel is in Nutation. Rotation is an object spinning around a fixed axis; Nutation means the axis (or axel in the case of a wheel) is also moving as the object is spinning. Think of an orbiting planet; the Earth spins but its axis also moves as it orbits the Sun

To demonstrate to motion of a front wheel in a shimmy hold a bicycle wheel by the axel in your outstretched hands (not spinning) and move your hands in the motion of pedaling a miniature bike. If the wheel was spinning while you were doing this the wheel would be in nutation. If you spin the wheel you will notice that the axel is difficult to move because the gyroscopic action of the spinning wheel is preventing nutation.

Now get someone to tap the side of the wheel as it is spinning; it will nutate (wobble) briefly but quickly return to spinning straight as the gyroscopic action dampens the nutation. So nutation is a constant and natural occurrence as a bicycle is being ridden caused by the movement of the rider pedaling the bike, side winds, bumps in the road, etc.

We do not normally notice this because the nutation is constantly dampened out, by gyroscopic action; the bike’s trail which provides a caster action keeping the front wheel straight, and the damping effect of the rider’s hands on the handlebars. However at a critical bike speed, the front wheel nutation frequency matches the bike + rider natural frequency amplifying or sustaining the nutation. And you have shimmy.

During a high speed shimmy the front wheel is not just fluttering back and forth about its steering axis but is also moving side to side in the horizontal plane shaking the head tube violently from side to side; the rider’s weight on the saddle provides an anchor point, the rear wheel on the road provides another making a pivot point for the front end of the bike to move from. Adding a pannier or saddle bag behind this pivot point will increase the likelihood of shimmy and cause the bike to shimmy at lower speeds because it gives an added sling-shot effect, especially if the load is loose and free to move.

It is a well know fact that tall riders on large frames are more likely to experience shimmy. I believe this is because the seat tube slopes backward and as the frame gets taller and the rider’s weight is more directly over the center of the rear wheel. This provides a near vertical pivot line between the riders mass on the saddle and the rear wheel on the road for the bike to shake and weave. With a smaller frame the rider’s weight is more forward with a less than vertical pivot line, making it less prone to shimmy.

Pressing your knee against the top tube will often stop a shimmy; in doing so you have dampened the shaking top tube through the muscles and tissues in you leg without actually connecting the leg to the top tube. This is also a clue that the rider needs to be holding the handlebars lightly so that you are damping the nutation rather than being connected to it by grasping the handlebars tightly.

In extreme cases if the rider is gripping the handlebars tightly the body starts to shake along with the head tube and handlebars. It becomes difficult to loosen your grip on the bars with your body shaking violently and because the shaking mass now includes your body it is much larger, higher and more fluid making the situation much worse and a crash may ensue.

This is more likely to happen with motorcycles and it has often been observed that a rider will be thrown from the bike, the bike will then stop shaking (because there is no longer a rider in harmony with the machine’s vibrations) and the bike with continue on for a while on its own before it hits something or looses momentum and falls.

An article on Wikipedia stated that frame flex has nothing to do with shimmy, but I am not so sure. Frame flex may not be the cause of shimmy but I believe it can sustain it. If the seat is not moving because the rider’s mass provides an anchor. And the rear wheel is not moving sideways at its point of contact with the road; but at the same time the head tube is shaking side to side, something has to be flexing and twisting, either the frame or the rear wheel.

Frames I built do not shimmy as a rule; so what did I do different? My bikes had a little more trail so possibly the damping effect of the extra trail helped. But I believe another factor is that all my California built frames had Columbus SP (heavier gauge) chainstays, making the rear triangle much stiffer and less likely to flex.

There could possibly be flex in the rear wheel if you consider that the upper spokes are under tension and the lower spokes are not; making the wheel likely to flex at the bottom. Also a dished wheel has unequal tension on the drive and non-drive sides. So if you have a bike that is prone to shimmy maybe think about switching to a stronger more tightly built wheels.

Most high speed shimmies occur while coasting down hill so here are a few things you can do to avoid this phenomenon. Lifting your weight from the saddle without actually standing up will transfer your weight to the pedals which are a much lower and more forward point of contact. Keep one pedal down and most of your weight on that pedal; you can switch pedals as you corner keeping the lower pedal on the outside. This will make the point of contact between you and the bike very low, but also off center of the frame.

Keep your knee lightly against the top tube as I have already mentioned, and hold the bars lightly. Many riders report that a shimmy it gets worse when applying the brakes. Well of course when you squeeze the brakes you automatically grip the handlebars tighter. So practice applying the brakes while loosely holding the bars.

Try switching your hands to the brake hoods and applying the brakes over the top with your fingers. It may take some nerve to do this if your bike is already shaking, but letting go of the bars for a split second may bring you out of the shimmy. Remember it is the connection between you and the bike that is causing the shimmy and the more connected you are the worse it will get.

Reader Comments (10)

I tend to agree that frame flex can make shimmy worse. After I got my Fuso, I turned my previous road bike (a upper mid-range Univega from around 1984) into a commuter/rain bike. After many years, the Univega became more and more flexy, much like an old spring that has gone soft. I finally had to retire the thing for good because it became dangerous to ride. You couldn't ride it no handed and pedal at the same time because if you didn't press your knee to the top tube, it would try to pitch you off.
August 18, 2006 | Unregistered Commenter warthog
Hi Dave

I wrote most of the wiki article on shimmy that you linked to; nice blog. Just one clarification, nutation is a _property_ of rigid spinning bodies, so it is not a question of gyroscopic effect dampening nutation, but that nutation is a type of gyroscopic effect itself.
May 10, 2007 | Unregistered Commenter Anonymous
Dampening=getting wet.
Damping=reducing vibration.
August 7, 2007 | Unregistered Commenter Anonymous

I remember reading an article when I was a kid, in a skateboard magazine, on how to reduce speed wobbles: shift your weight to the front wheel. It worked. Years later, I tried this whenever I got speed wobbles on my racing bikes, and it worked as well. I've always known and used this one simple method for eliminating speed wobbles, just lean over the handlebars, sometiems even resting my chest on them for a most aerodynamic position.

July 28, 2008 | Unregistered Commentersupertimm

From a mechanical standpoint the bike and rider are a system and the system has a natural frequency influenced by the various masses (rider), stiffnesses (frame, wheels), and anchor points (wheelbase, wheel diameter, riding position) within the system. When some external force (gyroscopic nutation) matches the frequency of this system, you'll get shimmy no matter the bike stiffness, rider weight, rider height, speed, or even the damping provided. As the various factors in the system change, you're more or less likely to get shimmy. And changing one ore more of those factors (knee on top tube, shifting weight, changing speed, etc) will fix the problem if it starts. But coming from a guy who has had to wrestle his bucking 1985 Ciocc back into submission at 50 mph, it's really freakin' scary no matter how it starts.

June 24, 2009 | Unregistered CommenterMichael Brooks

I had my first shimmy during a descent off Monitor in CA. I did every thing wrong except stay up right and on the road until the shimmy stopped under 20..... I was starting to look for a soft rock to land on. Learned what I did wrong and relaced the front wheel to heavy straight guage spokes. Haven't been down that hill since.

April 7, 2010 | Unregistered CommenterRalph

Very nice article i learn a lot from this blog thanks for the post

November 17, 2010 | Unregistered CommenterMartin Carter

I've watched "The World's Fastest Indian" years ago and he had the shimmy problem with his race motorcycle. All he did was to get himself as low as he could with his machine to get rid somehow of the wobbling. Since then I've been able to get rid of shimmy by lowering myself down on the handlebars.

November 22, 2010 | Unregistered CommenterPedals Cycling

Shimmy on a motorcycle is colloquially known as a 'tankslapper' :-)

March 1, 2013 | Unregistered CommenterYoav

I suffered this "Shimmy" recently and find this word a little underestimating, Death wobble is more in line with the feeling it creates!! I too picked my spot to stop, problem being it was on the Snake Pass doing 42mph with traffic front, behind and opposite direction, barrier on my left looked more like a razor blade and the only gap was on a bend with a huge drop. I also did everything wrong as I had never heard of this before, however since the experience a few, but not many have experienced "shimmy" as it is so politely called!!! Anyway, the fact that I write this passage tells you I survived.
This should be something all speedy cyclists are aware of as it seems it is not mentioned enough especially given the potential consequence?
If it was not for this site I would not know how to combat it in the future so thank you.

September 17, 2013 | Unregistered CommenterKen Dawson
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