Bike shimmy on Descents
 

I have a Cervelo S2 i have the Zipp 404 rims . seems when descending over 60 km. I feel a slight shimmy all the time on the bike. often i have to hit the brakes to get some control.

Is this due to the Aero rims? As my rims are quite true?

I used Tubed rims

anyone else have similar situation? thanks

My Kestrel is a bit twitchy on fast descents. I have to use my brakes to calm it down.

A couple of things come to mind assuming your wheels are perfectly true. First is a defective tire, sometimes you just get a tire that is not perfectly round. Turn your bike over or place on a stand to spin your wheels as fast as you can and look at one spot right on top of each tire looking for any kind of hop. If you can see it with your eyes, it will translate into a shimmy and get worse with speed. There's a possibility that if you see a hop in one of the tires that the bead of the tire is not fully seated in one spot on the rim causing the tire to be out of round?

Also consider that the long valve stem needed for a deep section rim will by nature throw the wheel out of balance all by itself, especially if you have valve extenders on there. You can find spoke weights to attach opposite the valve stem to balance the wheels if needed. With your bike upside down or on a stand, roll your valve stems to about the 3 o'clock position and let go of the wheel. Does the rim instantly spin to have the stem at the 6 o'clock position? If so, it needs balancing. I've heard of some guys simply taking a spoke magnet used from an old cycling computer they've had laying around to balance their wheels out, just a thought?

Do you ride in the drops on descents? (if not try it - adds a little more weight to the front end).
Properly adjusted headset?
I have had a similar issue when cycling 60km/hr - but only on inclines :)

Unless you mistakenly replaced your fork with one having the wrong rake, your frame was somehow badly damaged, or the wheels are drastically out of true as said above, usually shimmy and its drastic cousin, the speed or death wobble, are an inherent characteristic of the entire bike package including the whole bike, the rider, riding position, speed, etc. More often than not it has nothing to do with anything singular and specific being wrong, misadjusted, broken, etc. Therefore the problem usually can't be fixed. My advice is that if your bike shimmys for you at speeds you like to ride, then sell it and get a new one from a different manufacturer. The chance of having a repeat is very small.

Just a word of caution, hitting the brakes without doing something else doesn't always stop the "speed wobble". Usually, change the dynamics somehow such as unweighting your saddle. Counter-intuitively, loosen up.

It's a kind of harmonic, I think, which depends on a lot of factors coming together. Balance, rider input, properties of the frame, wheels, perhaps tires. I would even experiment with tire pressure to see if that altered it.

A collection of the best articles I've found on the topic:

Dave Moulton's Blog - Dave Moulton's Bike Blog - High Speed Shimmy.
Dave Moulton's Blog - Dave Moulton's Bike Blog - Shimmy Re-Visited
Dave Moulton's Bike Blog: Why are large frames more prone to shimmy?
Dave Moulton's Blog - Dave Moulton's Bike Blog - More about shimmy
Technical FAQ: Bifurcation and high-speed shimmy | VeloNews.com

I became keenly interested in the topic after a new bike build threw me in the ditch on fast downhills on the first two rides.

The other technique for quelling the shimmy is to lock your knees on the top tube. This works for speed wobble and should also be helpful for lesser shimmy. But it isn't as easy on a sloping top tube bike as it used to be with a horizontal top tube.

I had a poorly behaved Ti frame that exhibited severe speed wobble once and overall squirrellyness often. Well I fixed that sucker...sold it's butt. New Ti frame is perfectly stable.

BTW, for those concerned about the new owner, I have had no complaints. As I said above, the misbehavior is often a characteristic of the whole system including the rider, rider weight, riding style, etc. So the chance of two dissimilar owners both observing the shimmy is fairly small. Nevertheless I would have gladly taken the frame back and refunded the price if the buyer had been dissatisfied at all.

I had a secondhand 2002 Litespeed (custom-built for a racer).
Once I figured out the issue, (too-steep head tube), I fixed it by ordering a custom fork with only 37-degrees rake.

While waiting for the fork to arrive, I tried out a descent technique recommended by Dave Moulton:
- Crank vertical, with all weight on lower pedal.
- Weight off the saddle.
- Light grip on the bars, and no weight on them.

I descended a 45mph hill that way.
Previously, I'd lose control at ~35mph.

Once the new fork was installed, I never had a shimmy issue again.

If the shimmy gets bad at speed then as a last chance before crash, try clamping your legs against the top tube.

Try unweighting your handlebars and seat. Always works for me.

-knees to the top tube (not constantly, necessarily, but think of a tuning fork vibrating -- touch it to stop it from reaching resonance.)
-don't hold the handlebars (the drops) in a deathgrip. Hold them enough to not have them jump out of your hands if you hit an unexpected hop in the road, but dont hold them tightly like a baseball bar or something.
-butt off the saddle. You should be on the pedals with your legs properly extended/bent to handle the tilt/curve of the road coming up to meet you. If it's just going straight down, pick a leg to be in front (like a surfer, you'll figure out your natural 'front' leg') and get your pedals horizontal to the ground. If you're turning right, your left foot should go down (both for balance and to avoid a pedal strike.)
-You want to push weight down through the bike to the road. Your inside handlebar and outside pedal are your surfaces on which to balance when in a turn. Shift between the two most extreme left/right iterations of this, as necessary for the characteristics of the curve you're dealing with.
-Brake before the turn. Use both front and rear brakes. Brake during the turn if you need to, but gently/gradually, not drastically. If you need to take the inside foot out and put it down on the ground to make a tripod of balance out of your wheels and this foot, do it -- the alternative being flying off the road and dying.

This is very effective. I battled a real nasty wobble on a Ridley frame (hit me at over 40mph the first time). I eventually started knees on the top tube as a routine down hill position. Something about how I interact with that bike. Never under power, rock solid in a sprint. No mechanical issues. My son rides it without issue.

Even on another bike I have yet to totally regain confidence descending.

The knees pressed against the top tube works well before the shimmy really gets worse. Once you sense the shimmy, then do the press. My understanding is that the knee pressure against the top tube does not need to be like a clamp. At the same time, the pedal/crank position should be horizontal and foot pressure on pedals is equally important, if not more important.

What you are doing is creating a triangle (Isosceles). But this knee lock thing is compensatory. Its just trying to address the shimmy effect and not the root cause. Some responses have suggested some causes.

As others have suggested, put your knees on the top tube and make sure everything is adjusted/balanced properly. Knees should not be tight, just enough to absorb vibrations. I prefer to orient them assymetrically. Also, be sure to relax. If you hold the bars too tight, you can make the shimmy much worse.

One thing I find interesting about your description is you mention the problem being for speeds over 60kmh, not some specific range. Harmonic vibrations typically kick in at some speed but if you go faster, they'll disappear again.

60kmh is not that fast so something is definitely wrong.

It could be that the random rider motion during the frantic pedaling in a sprint keeps the bike from reaching a harmonic resonance. Much like soldiers marching out of phase to keep from bringing down a bridge. Speed wobbles usually occur when coasting when the leg motions aren't breaking up the harmonic vibrations.

It is not resonance.
In the link I posted earlier, read the section titled "A math prof explains high-speed shimmy":

Technical FAQ: Bifurcation and high-speed shimmy | VeloNews.com

I had that happen once trying to get a KOM on my 2014 Specialized Roubaix. Scared the crap out of me. Luckily, the terrain changed and the bike slowed down on its own. This was some where around 38-39 mph.

I've had my steel frame bikes in the 40's many times with a peak of 46 mph and never a wobble.

I have to check. on my front rim i do notice that the where the stem is is where the wheel ends up when i let it suspended
would adding some weight on other side work? what is a good way to add weight to a rim?

I don't recall hearing about shimmy with the pro races. Fast descents, even with Froome off the saddle and on the top tube.

It's a crabon thing, especially with italian brands.

Well, I'm glad we got that cleared up. Seriously I am not in a position to argue with this guy. As a lowly chemist, I am no match for him in such areas. I will, however, say that the author's explanation is not consistent with my observations and those which I have read of others. My observation is that the phenomenon usually requires a trigger like washboard road, crosswind, etc. This is specifically denied by the math prof who suggests the onset at a certain speed is inevitable. I know that I could ride fine almost always at high speed on my Ti frame, but one time not so much. His explanation sounds quite reasonable, but not the phenomenon I and many others have observed. Actually it doesn't matter what the real cause is. When you have the problem, the solution is to ditch the bike.

could be, my Italian does

No need to ditch the bike.
See my earlier posts.
I solved it by reading Dave Moulton's writings, taking some measurements, doing a little math, and ordering a custom fork to put the trail into a reasonable range. Bottom line is it is all about the trail.