Bicycle Mechanics - Front wheel oscillation at speed

I ride a rocky mountain hammer mtb. I've noticed that when I descend down smooth (paved) slopes that my front wheel will begin to spontaneously oscillate, beginning slowly but quickly reaching the point where the oscillation is dangerous. It's self amplifying. I must add that this only happens when I'm relaxing with both hands off the grips and going around 30km/h. It does not happen at slow speeds and there is no noticeable warping in either the front or rear wheels. My question is if there are any other factors that can cause this kind of thing other than the wheels being warped? Mabye frame issues? Any engineers or physicists in the house?

I ride a rocky mountain hammer mtb. I've noticed that when I descend down smooth (paved) slopes that my front wheel will begin to spontaneously oscillate. It begins slowly but quickly reaches the point where the oscillation is dangerous. It's self amplifying. I must add that this only happens when I'm relaxing with both hands off the grips and going around 30km/h :o. It does not happen at slow speeds and there is no noticeable warping in either the front or rear wheels. My question is if there are any other factors that can cause this kind of thing other than the wheels being warped? Mabye frame issues? Any engineers or physicists in the house?

There has already been some very good posts on this topic. Do a search under "wheel wobble" and you will find some good information.

BR

I had a similar problem on my road bike last year. Self-reinforcing vibration on fast descents, to the point where I did not feel completely in control. A bit of research led me to believe the cause was fork flex. So I replaced the fork (mid-range carbon fiber with aluminum steerer) with a much stiffer fork (high-end carbon fiber with carbon steerer). Problem solved, the bike descends like it's on rails now.

Harmonic imbalance, where the resonance of two or more things coincide to generate a big old shimmy.

I ride a rocky mountain hammer mtb. I've noticed that when I descend down smooth (paved) slopes that my front wheel will begin to spontaneously oscillate, beginning slowly but quickly reaching the point where the oscillation is dangerous. It's self amplifying. I must add that this only happens when I'm relaxing with both hands off the grips and going around 30km/h. It does not happen at slow speeds and there is no noticeable warping in either the front or rear wheels. My question is if there are any other factors that can cause this kind of thing other than the wheels being warped? Mabye frame issues? Any engineers or physicists in the house?

Your wheels are probably not warped. Most likely there is nothing wrong with the frame, except the designers put in too little "trail" in order to make the bike feel more agile. Large frames are even worse because the manufacturers don't seem to want to lengthen the wheel base very much when they make larger sizes. This results in a steeper head tube angle, and less trail for the larger frames. Lack of trail reduces stability with speed. You've seen the same thing on shopping carts.

Too little wieght on the front wheel helps it along, too, because there is no damping. That's why it happens when you sit back and take your hands off the bars. The next time you experience the oscillation, gently touch one of your knees to the top tube and it will immediately stop.

In case you are wondering, yes, motorcycles do it, too. On a mororcycle, I can just accelerate out of a speed wobble, but that's not really an option on a bicycle!

Here is a good explanation of what is happening.

http://sheldonbrown.com/brandt/shimmy.html

Shimmy is independant of wheel imbalance or trueness. All bicycles will do this at some characteristic speed, although with some bikes it is too high for you to ever experience it.

I had a similar problem on my road bike last year. Self-reinforcing vibration on fast descents, to the point where I did not feel completely in control. A bit of research led me to believe the cause was fork flex. So I replaced the fork (mid-range carbon fiber with aluminum steerer) with a much stiffer fork (high-end carbon fiber with carbon steerer). Problem solved, the bike descends like it's on rails now.

I just wanted to follow-up on my earlier comment above, in light of some of the other comments. In my case, the replacement fork was exactly the same geometry as the old fork, in fact they were pretty much identical except for the materials (same manufacturer, different models). So whatever solved the problem in my case had something to do with the materials of the new fork. The only functional differences between the two forks are that the new one is a bit lighter and much stiffer laterally. I have to admit though, that perhaps the solution was related to better vibration damping by the new full carbon fork rather than simply flex in the old aluminum steerer fork.

Thanks everyone. It's no longer a mystery!

Here's some physics to add to the mix:

A spinning rigid body has a property that causes undamped oscillation when disturbed by an impulse. This undamped oscillation is called nutation, which is different from precession. (Precession is what makes a spinning top stay upright and makes an angle grinder feel weird when you twist it around when it's running.) Nutation is not often seen. You can see it by taking a bike front wheel, suspending the axle by a piece of string, spin it up as fast as you can and when it's spinning, give it a sharp bump. The spinning wheel should wobble around the spin axis. The frequency of wobbling is a function of the wheel inertia and the angular momentum.

Now when you ride fast enough, nutation becomes a danger as the angular momentum of the wheel increases with speed. While you are holding the handle bars, your arms provide enough damping to overcome nutation and prevent it from growing unchecked. Releasing the bars, there may be enough disturbance in the wheel system to provide a nudge to start the nutation; if there is insufficient damping then the nutation amplitude will grow. The tracking will also tend to damp the nutation out. I remember once as a boy riding a chopper bicycle and letting go with one hand at speed. The front wheel immediately started wobbling and I could not control it until reaching the end of the downhill and the speed bled off. Those choppers had very poor tracking.

I am not sure if all front wheel shimmies can be explained by nutation.

If you happen to have an oscillation problem again....sqeeze the top tube with your knees.
When decending, always keep the feet at the 3 and 9 position so your knees are level.
You squeeze that top tube....oscillation gone !