Hold a bicycle wheel at arm’s length in front of you, hold the hub spindle with your left and right fingers. Now move your hands in the motion of pedaling a miniature bike. The wheel may be spinning or not for the purpose of this demonstration.
This is the motion of the bicycle or motor cycle’s front wheel during a speed wobble of shimmy. It is not simply a fluttering back and forth in the horizontal plane, about the bike’s steering, but it is fluttering, or wobbling in a two dimensional fashion in both the horizontal and vertical planes, violently shaking the whole bike and the rider.
I used to think this was a design flaw, but now I am inclined to think it is a natural occurrence that happens because it can. On a two wheel vehicle the front wheel is able to move in both planes, whereas on a four wheel vehicle the front wheel can only move left and right about its steering axis.
However, when the steering bearings wear out on an older car, the wheel can then move in both planes, and wheel flutter will occur. This will happen at a definite speed. (It might be 60mph.) Drive beyond that speed and the fluttering stops, only to return again as the car slows, and the speed reaches that certain critical level, then will subside again at a lower speed.
Most bicycles seem to shimmy coasting downhill at around 45mph. A loose pannier, or saddle bag will often cause a bike to shimmy at a lower speed. Therein lays a clue. It seems if there is something flapping around loose, it amplifies the shaking. The tighter the rider grips the handlebars, he then becomes this fluid extension of the bike.
This is especially true of motorcycles where riders have been tossed around like a rag doll. Some have even broken arms. Expert motorcycle riders have demonstrated that one can ride “No Hands,” induce a shimmy by tapping the end of the handlebars, then stop it by simply leaning forward.
Tall riders riding bicycles with large frames seem more prone to shimmy. Why? The seat tube slopes backwards usually at an angle of 73 degrees. The taller the frame the more the rider’s weight is directly over the center of the rear wheel. This provides a near vertical pivot between the rider’s weight on the saddle, and the rear wheel contacting the road. The front end of the bike can now shake about this pivot. (See above.)
If the rider is sitting fairly upright, the pressure of wind on his chest is forcing even more weight onto the rear of the bike. If the rider were to lift his weight from the saddle, that weight is now on the pedals. Lower and further forward. Get down into a low tuck position, and the weight is now towards the front of the bike. Pressing a knee or leg against the top tube will often stop a shimmy. This dampens the shaking without being actually attached to the frame thereby increasing the problem.
Go to this YouTube video for a montage of motorcycle shimmy’s where the riders quickly get out of it. In most cases it does not even seem to faze them. It is the same with bicycles, the bikes that are shimmying are the same ones that the pros use in the Grand Tours. The pros do not seem to have a problem descending mountains, reaching speeds as high as 55 -60 mph.
Finally the Fuso frames I built did not shimmy. (As a general rule, there have been rare occasions.) So what did I do different? The frames all had the stiffer Columbus SP chainstays. This gave the frames more lateral stiffness.
If a bike and its frame are anchored at the rear by the rider’s weight, and the front wheel starts to wobble, (As it seems it will at a certain critical speed.) it will only do so if it can. In order for the whole bike to shake, either the frame or the wheels are flexing. Add lateral stiffness to the frame and/or wheels and the front wheel can’t shake.
Move the rider’s weight forward and you are effectively holding the front wheel so it cannot wobble. It is okay that the wheel can move left and right about its steering axis, in order to wobble the wheel has to move in both the vertical and horizontal planes.
I was prompted to write about this subject again because I read this article. Written by a mathematician, it was a little beyond my understanding, so this is an attempt to look at the problem in simple terms. When I built frames I never had this issue, so I never addressed it. It is only in recent years I have started to study this and my views are still evolving.
I’m sure if the motorcycle manufacturers had all the answers they would fix the problem shown in the video link, but it is a complex matter. Feel free to add your views and ideas.