Let’s say I’m 9, riding down the steepest hill in my neighborhood at top speed on my banana seated bike. Suddenly, the front wheel begins oscillating. I strain to control it, but the death shimmey is unstoppable. I face plant on the asphalt, awaiting the road kill patrol. Anyone know why the shimmey starts?
Hmmm, a couple things spring to mind:
1)Bad/non existent bearings in your front hub.
2)Out of true/ bent spokes leading to the dreaded “taco”
3)Loose or bad races or bad bearings on your headset.
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too many GI Joes in the spokes to get the really cool “thwappa thwappa, clunka clunka” sound.
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were you holding on to the handlebars? try it, esp. on steep inclines DAMHIKT!!
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- When I was a wee lad (Stingray frames were still in abundance), I remember that this would often happen with a regular 20-inch (wheel) bike that someone had put a 16-inch wheel on the front of. ~ Many years later I’ve ridden more than a couple mountain bikes down grass/dirt hills at 40+ mph and none of them did it ever but then I didn’t try to use a 16-inch front wheel on any of them, either. - MC
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a strange attractor of non-linear dynamics coupled with aeolian-induced harmonic resonance
…either that, or what MikeG said.
My bikes have 16" and 20" front wheels (respectivel) and don’t have any vibration problem at any speed up to 40 mph and above, so it can’t be just the wheel size. They were designed for those wheel sizes though.
There was recently a discussion on a mailing list about a similar vibration problem on a racing HPV (human-powered vehicle) but they haven’t solved the problem yet, as far as I know. It seems that a certain combination of frame flex, load and speed causes resonance.
A smaller wheel than what the bike was designed for could cause instability. Stability on a bike is a result of the “trail,” the distance between where the front wheel contacts the ground, and the imaginary point where the angled pivot point of the handlebars intersects the ground. (A bike’s stability is not the result of gyroscopic forces, contrary to what many think.)
But a bike with too little trail would be hard to ride at any speed, and wouldn’t just oscillate at higher speeds. This sounds like some kind of flex problem with the fork, maybe? I’ve never heard of a bike having this problem.
Skateboards are another matter. I’ll never forget the time I saw a friend hitch a ride holding the tailgate of a pickup truck. It got to about 25 mph, and the “loop gain” of the skateboard-rider system went into oscillation. It did about three or four quick back-and-forth motions, each larger than the previous, before it threw my friend to the ground.
An article on Trail can be found here: An Introduction to Bicycle Geometry and Handling.
And if you look at the rest of the site you’ll know why these nutcases are concerned about it! 
It is most likely a geometry problem (trail is definitely a possibility.) I’ve got several road bikes, all of which I can purposefully induce oscillations in the steering axis, but only one of which will spontaneously oscillate itself at high speeds.
All but the one bike will dampen out quickly - the difference I can tell is that the ‘bad’ bike is a smaller frame I’ve fitted a very long stem to to make up for my height. This long stem coupled with my riding position seems to invite the ‘dance of death’.
The first time this happened I thought I was going to die. But, I remembered something I had read years ago - squeeze the top tube with your knees. This killed the oscillations immediately, and I changed my underwear.
If you squeeze the top tube with your knees you suddenly change the vibrational properties of the bike configuration so that the resonance frequency is different. Anyway, I survived and now I don’t ride that bike down steep hills anymore.
To my mind ‘shimmy’ is something different, but from the descriptions it sounded like you were talking about this type of steering oscillation.
Poor frame design is one reason as has been pointed out by the mentions of trail.
The first thing you should ask yourself though is 'Is this something that the bicycle has done since being brand new or is it something that has developed ? ’
If it came with the bike then there is a strong chance that it is poor geometry but if it is something that has developed I would check the tyres.
On small bikes such as these tyres are not well made, they usually don’t need to be, and as a result they do not always sit on the rim perfectly concentricly.
Lift the front end off the floor and spin the wheel.Hold someting like a ruler or straight edge as near to the tread as you can.If there the gap varies by more than ,say, 1/4 inch it might be this causing it.Try swapping the front and back tyres around and see what happens.It doesn’t bother the bike much if the back end tyre wobbles.
Alternatively the wheel itself might be badly damaged and not concentric, the spokes will likely be loose on one half of it, from top to bottom rather than left/right.
Kids bikes get a lot of hammer and it might have been stacked head on. Kerbs are a favourite for doing this.
This will push the forks backwards slightly, it may not be very obvious but if you follow the line of the steering head down, the top part of the forks should continue it, if it is slightly steeper then you could try forcing it back with some ‘technical persuasion’(brute force and ignorance)
If the forks are strong then the frame head may have been set slightly more vertically.Look under the lower of the two head bearings where the main tube joins the sterring column tube. If the paint has small cracks across the tube or it bulges then this is what has happened and the frame is(for the vast majority of bicycles) a write off.
Thank you all for the replies. I don’t think that the oscillations were caused by any of the wear and tear issues raised by casdave or MikeG. When I experienced this as a wee lad, it occurred on separate occasions with separate bikes in distinct states of repair (disrepair??). I recall the problem occurring on my first two wheeler (a 16" banana seat job) and my second bike (a 20" Apollo 3 speed with the shift lever located in such a place as to threaten my future fertility). Once I graduated to a 20" BMX type bike and a 27" road bike, I never experienced the problem again.
I like the idea of an intrinsic property of the bike/rider system governing this behavior. The suggestion of trail being involved (thanks Shiva for the link) is intriguing, but I cannot quite get to a mechanism for velocity induced harmonic death oscillation (henceforth termed VIHDO).
I wish I had the bikes so I could provide more info regarding the actual trail, but since they were designed for kids, greater stability (ie greater trail) was probably a consideration. Such a bike (according to the link) has a greater tendency for steering correction and I like the idea of hyperactive steering correction inducing the VIHDO. But the bike was driven into this behavior by riding fast on a smooth surface with no turns. The rider (me and probably many other hapless victims) was small and light. Can anyone synthesize these different components into a complete explanation of VIHDO?