Bike Shimmy!!!

I encountered a very violent "shimmy" on Saturday while descending a mountain. My speed was 44 Mph when the shimmy started. I was shaking so violently that my water bottles were ejcted from the frame. I managed to hold it together until I found a grassy area (just in case I needed to bail out). As soon as I hit the grass I was able to regain control of my bike and resume (shaken not stired). I believe I may have come out of the saddle once I hit the grassy area in preperation to bail. The shimmy was so great that I was unable to apply my knees to the top tube.

I have had a similar experience about a year earlier on the same bike at 42 mph. Managed to use the knees and hold it together. My concern is the cause of the problem. I ride a Cannondale six13, 58cm. I had this bike fit at the LBS and they replaced the stem with a 100mm. The rest of the bike is stock. I am 6' tall 178lbs with long legs and short torso. Have been told that I look stretched out, arms not bent when in the drops. Any advice on this problem would be greatly appreciated.

 

I am assuming that your wheels are true. If so, I would look at the front hub components as something is coming out of alignment at your high speed. I would check the bearings and bearing races and definitely the axle for a hairline crack. It also might be worthwhile to check the tires to make sure that they are running true, i.e. that they are round instead of eggshaped on the wheels. You also should probably check the headset to be certain that it is not loose.

 

Buy a bike with a shorter top tube, 56cm, and use a 120-125mm stem! (I have one forsale )

Assuming wheel balance, tyre seating, wheel bearing play, headset adjustment all check out ok, then you may not have enough weight over the front wheel with the present set up.

I build flat bar road bikes and this can be a problem, I use a set of bathroom scales under the front wheel during bike set-up to check.

 

yeah what he said.

i ride a rode bike to class and wear a 30lb bookbag. i was only going about 15mph and i sat up straight, hands off the bars, to coast for a bit and the front started to shimmy shimmy. theres almost no weight on the front wheel when i sit like that so yeah.

maybe the air resistance at such high speeds is tending to rotate you/bike in the direction of doing a backflip which would take more weight off the front of the bike than normal.

 

Shimmy is said to relate to the lateral stiffness of the bike and the frame length, and also to the size and weight of the rider. I guess that fork stiffness is probably also important. I recall that the longer the bike and the less laterally stiff the bike, the higher the risk of shimmy. If it keeps happening, you may need a different bike.

 

I got a 2003 Jamis Eclipse that has done the identical thing at about the same speed. I think mine is caused by the carbon stays on the bike not being insync with the steel frame, which cause vibrations that start the wobble. I have changed my position on the bike and tried every trick in the book to remedy it, all to no avail. I have done extensive research and nobody has a clear answer. Here are a couple sites that will help you research your problem:

http://davesbikeblog.blogspot.com/2005/11/high-speed-shimmy.html

http://www.calfeedesign.com/pdf/Fork_Symmetry.pdf

http://davesbikeblog.blogspot.com/2006/08/shimmy-re-visited.html

There is a ton of info on the net about the shimmy/wobble. My solution; it was an excuse for me to get a new frame (Curtlo steel), as the Jamis is useless over 35+ mph.

FYI: I have never had this problem on any other bike, even at 50+ mph.

 

I've never experienced the speed wobbles, but recall discussing the issue with a local framebuilder when I ordered a custom frame (al/cf rear) a few years ago. When talking tubeset options for my 57.5 TT frame, he advised that the Zonal weight, with "megatube" TT and DT shape was the way to go for a "big frame". We have a lot of high-speed descents around here, and he said he wanted me to have a wobble-free bike at 55 mph. Don't know if I could have "gotten away" with something lighter, but the frame (with Ouzo Pro fork) has been rock-solid on descents.

 

I have taken the bike to the LBS. The tech told me that it was just "one of those things" that has no real explanation. They are in the process of going through the bike to check for any damged, loose or warn parts, as well as the symetrical balance of the fork. They feel that a smaller frame would cause me to lean over to far with the relationship between my saddle height and stem position.

Do you think that a high tire pressure could enhance the possibility of the "wobbles", I currently use a tire pressure of 120 psi. I thought that If I would lower the tire pressure it would assist the bike in absorbing some of the road irregularities. I am also looking at my riding position. Stay off the saddle, Peddels weighted evenly with a good balanced position on the front of the bike.

Thanks for the replies and look forward to other opinions.

 

Tire pressure will have some impact as changing the tire pressure changes the tire's damping properties and spring rate. This will the then change the response of the frame.

How heavy are you? At around 175lbs, I run 110f and 105r.

High pressure is not your friend.

 

That's interesting. I thought that it's more common to run higher pressure at the back, exactly reverse of what you have.

 

That's because I wrote it backwards. That's what happens when you don't sleep for a few days.

 

Alienator is fallible...

 

Of course I am. I have great powers of, uhm, fallibility. I can make the most idiotic math errors on test, for instance. However, while I am fallible, chicks still dig me.