BlueDevil

Ok, so here is the situation. Today, going down a quite steep hill in the middle of my ride, I hit a max of 44mph. When I was at the max, a large bird flew out in front of me, and missed my by about 3 feet (it was a large Hawk or something.. stopped in front of me, switched directions when it saw me, and just missed me!! ). Anyway, I really tensed up when I almost hit it, and immediately my frame started shaking like paint mixer (which is not exactly the best feeling at 44mph!!). So, I tried as best I could to loosen up, and kind of pinched my top bar with my knees, and just coasted until the vibrations stopped. This seemed to work, but it took a while (was probably only a couple of seconds, but it seemed like an eternity) to settle down. I was afraid to tap the brakes, as I thought I might have been thrown off. was that the correct way to stop it?? Or is there a better way? What should I do differently if it gets the shakes like that again?? ... phew...

J-McKech

wow i have no answer but that does sound scary as hell!

ShinyBaldy

that is shimmy - caused by vibrations working like a spring. Every frame has a vibration frequency where there is shimmy - whether we get in conditions that cause them in another thing. Wheels/fork etc contribute to this as well as rider.

My guess? When you tensed up - you became the spring board... the uneven osciliation started between the front and the back of the frame.

next time? Don't tense up on the bars and rest your knees on the toptube.

roadfix

Your body weight may be a major contributing factor. Are you on the light side? My 110 lb skinny friend on his 54cm Moser Ti bike used to shake like hell every time going down this hill. Absolutely nothing wrong with his bike. I rode his bike down the same hill.....no shaking. I'm 165.

dexmax

I had an experience.. 10 yrs ago. It was a near encounter w/ a dog. What I did was I tapped my rear brake lightly until the shaking stopped.

BlueDevil

Thanks everyone.. The explanation of shimmy makes a lot of sense (seeing as my research is in structural response to earthquakes, etc.. I shoulda thought of that myself!! lol) It was a bit hard not to tense up, but that seemed to be the factor that slowed the vibrations down.. I guess when you are loose, your body weight acts as a damper which would greatly reduce, or eliminate any free vibration in the frame.

The Fixer- I wish being too light was my problem.. I am weighing in at around 185lbs on a 6' frame.. would like to get down to 170-175ish, so no, I am not a featherweight..

telenick

This has happened to me when I'm cold and shivering on a decsent. In your case, I'll bet dollars to donuts that it was adrenaline causing you to shake from your near miss with the hawk.

jfmckenna

Scary is'nt it. Happened to me at 50mph once. I had a rack on my tourer and when I took it off it did not shimmy anymore, of course then on out I made sure I clenched the tt before decending fast. But that is the method for stopping it you did right. You may want to stand a bit too on the pedals too which shifts weight. There's lots of reasons this can happen. Is it a large steel frame? Thats what my tourer was. The AL/Carbon frame I ride now is solid as a rock at 50mph

tbick

My understanding is this is common on downhill rides in which there is more weight on the front wheel than usual. The extreme situation is when the front wheel starts to really go crazy... I have been told "put more weight on the back wheel"...but have not quite figured out how to do that on a downhill decent!

mymilkexpired

Slide back on the saddle.

Lone Ranger

To me, never having been through it, you need to slow down and to center your weight over the bike and as close to the bike's center of gravity as possible - down and back when riding downhill. I would try lightly putting your rear brakes on. I wouldn't stand up as spreading the weight out can't have a good effect (harder to get back in balance). I would crouch more into the bike and slide back somewhat on your seat to center the weight and to remove some from the fromt wheel, which is the "loose link"

Swimjim

This phenomenom is alot more scarey on a motorcycle traveling close to triple digits. On a bicycle get your chest up and use it as an air brake to slow you down. You will find that a high speed wobble will disappear pretty fast. In scuba diving, the first rule you should be taught is "what ever happens don't panic. Stop, think, react." A good rule to remember here.

Jim

Dchiefransom

[QUOTE=BlueDevil]Ok, so here is the situation. Today, going down a quite steep hill in the middle of my ride, I hit a max of 44mph. When I was at the max, a large bird flew out in front of me, and missed my by about 3 feet (it was a large Hawk or something.. stopped in front of me, switched directions when it saw me, and just missed me!! ). Anyway, I really tensed up when I almost hit it, and immediately my frame started shaking like paint mixer (which is not exactly the best feeling at 44mph!!). So, I tried as best I could to loosen up, and kind of pinched my top bar with my knees, and just coasted until the vibrations stopped. This seemed to work, but it took a while (was probably only a couple of seconds, but it seemed like an eternity) to settle down. I was afraid to tap the brakes, as I thought I might have been thrown off. was that the correct way to stop it??



Knees against the top tube is one method of helping. Loosening up is another. Light rear brake only will help also. You ended up with a harmonic vibration. We had a guy that lost it last year like this on a ride. Two weeks in a coma. He's back riding now, but does downhills much slower. We did some internet research and everything you did was recommended. One of our race team members said he's had to stop completely to make it go away. He's a tall guy with a large frame.

froze

This problem believe it or not was virtually unheard of in the days of lugged steel frames. I knew plenty of racers back in the late 70's and early 80's and no one ever experience this. A bicycle tire can have a balance problem but due to it's far lower weight then a car tire the problem is not anywhere near as severe as you mentioned, and it only happens in a 5 mph window. But now with these tig welded frames we are seeing more vibration problems especially with mid to low end bikes and ultralight highend bikes. It appears the higher end tig welded bikes are more thought out in design. Also I have not heard that carbon frames were experiencing this either.

But the best way to reduce the effects of the vibration is to clamp the top tube with your knees and slow down gradually by coasting.

jfmckenna

I don't know as I mensioned before it happened to me on my '83 lugged tange 2 frame. It is a large 62 cm frame though and a touring geometry which I probably had no business breaking 50mph on but that was all I had at the time and I had a record to beat

ShinyBaldy

Uhh... where did you get that idea? I have a schwinn lugged cromoly 4130 that can shimmy. My Trek 5200 can also shimmy given the right conditions. Trying to prevent a bicycle from shimmy is like trying to elude physics, you can't! All bicycles have shimmy potential - in fact, the more properly aligned frame is more likely to shimmy than one that is off. Just because the nature of occilicisations requiring a balanced object - get a balanced pendulum and it'll swing back and forth quite a bit with no extra force. Get a pendulum with a uneven ball/balance and it'll swing once or twice and stop going back and forth.

froze

Right back at you, where did you get those ideas? A bike in perfect alignment will shimmy more than one that is not? That poky crock! (whats poky crock?) Anyway, a pendulum is not a example of frame alignment. Take a car for example, a car with a bent frame or out of align tires will shimmy, whereas one that is not out of align will not. Cheap made lugged steel frames that were not align accurately could have a problem of shimming; but in my experience when I raced back in the days of lugged steel, the mid to high end bikes never had that problem.