Friday, August 18, 2006

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 dampening 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 dampening 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 dampening 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.


warthog said...

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.

Anonymous said...

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.

Anonymous said...

Dampening=getting wet.
Damping=reducing vibration.