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Feedback on high-speed shimmy
I enjoy your technical column very much. Your recent answer to a question on fork rake confused me, however. You first stated that a bike is less stable with more rake, but then went on to say that shimmy can sometimes be cured by increasing the rake. Since shimmy is an unstable condition, wouldn’t you want to decrease the rake?
Shimmy is not an unstable condition, rather a vibration causing instability. Decreasing rake causes more shock to be transferred into the frame and increases the vibration.
I could demonstrate that decreasing rake increases stability with a simple demonstration, but not over email, so I did it in the curren tissue of the magazine. You can read about the relationship between fork rake, head angle, fork trail and stability, as well as shimmy in detail in VeloNews #11, July 1, 2002.
I sent you a question a few weeks ago about frame shimmy on a Lemond bike with a Reynolds 853 frame and I appreciate your answering the question. I was wondering if I could follow-up to your response?
In your original reply, you suggested that both the frame and riderare probably on the larger size. The frame size is a 57cm (c-c) witha carbon fork with a 43 rake and I am 6′ tall and weigh about 180lbs. I am using an 11.5cm stem with a 90-degree rise. Except for changing forks to get a longer rake, none of your other suggestions will help me eliminate the shimmy unless I get a new frame. Is there a way to minimize shimmy by changing my riding position? Should I think about a slightly longer stem so that I can get more weight on the front wheel when descending? Should I get weight off the saddle? What can I do to minimize shimmy with the frame and fork that I have? Anyfollow-up answers to your original reply would be greatly appreciated.
I understand that some frames are more susceptible to shimmy than others, given materials, frame size, and rider weight etc., but do things lik echanging position on the bike while descending, either being more backor more forward to get more weight on the fork, how one grips the bars, either with a death grip out of fear of shimmy or a light touch, have aneffect on shimmy? Second, what about fork materials? I currentlyhave a carbon fork (that came standard with the frame) with a 43mm rake. Would a different fork make a difference or will all forks, regardlessof material, cause shimmy at the same speed?
Unfortunately, selling the bike is not an option in that I spent sometime saving to get this one and do not have the means to buy something else. Shimmy did not come up in the test ride since there are no long straight downhills near the shop.
I have tried all of the things you mention with anumber of problem bikes. Changing your weight distribution is worth a try, but it has never worked on bikes I have tried. It also flies in the face of establishing your position on the bike based on pedaling comfort and efficiency, rather than on characteristics of a frame you are trying to adapt to – the biggest sizing complaint people have with bike shops already. And I think the death grip is hard to avoid if you have fear of a shimmy. I think it probably is beneficial, too, since you need that energy absorbed immediately into a more immovable object (i.e., you).
I have found that if the bike has a bad shimmy, you can only marginally reduce it. Also, fork material does come into play, because what you want is more energy absorption in the fork. So, a softer, more deadening fork (maybe aluminum?) should help. Only mild shimmies can be alleviated (or placed outside of the speeds you normally ride) with a change in fork rakeor other reduction in fork stiffness, and sometimes with balancing, truing and dishing the wheels and replacing pitted headsets. Again, see VeloNews#11, July 1, 2002 for more discussion of this.