Road Cycling - 61cm S-Works E5, God Awful Shimmy

Friend of mine bought this frame, built it up with Ultegra, Mavic Open pros. Tried to bomb a descent on it, and it almost shook him off. He weighs 250 lbs, and is not all that road savvy. I rode it over the same bridge, albiet with some 30mph winds kicking around. I have to say, moving weight around a bit helped, as did pushing on the bar, but this thing was down right scary. Couldn't get over 33mph. However 33mph on the flats, it tracks fine.

It seems to have a very short wheelbase, the rear is tucked in tight, and very little rake to the fork. I remember the old fit adage, when you're on the drops, the bars should cover the view of the hub, but no chance on this frame. He does have about an inch of spacers betweent the top of the headset and the stem. The fork has a carbon steerer tube. I will admit, this thing is light, and I know that lightness always has a penalty. These are pretty much team issue for our club down here, and mine is on the way, but after riding this thing today, I am having 2nd thoughts...

Any ideas, or info would be appreciated. Is it a weight transfer issue, other than transfering weight from the plate to his mouth?

I know Mario rides one as well as the rest of the Domina team, granted they don't weigh 250.. Im weighing in at about 215, but dropping quickly. I think (hope) that at around 195 to 200 I should be able descend worry free.


NOTE: This is normally a 45mph plus descent for me, on a 2.8 Cannondale. Yeah, I know its old.

I would try swapping wheels from the specialized to another bike and see if they are the culprit. It seems odd that the bike would only shimmy on a descent. I would for sure take the frame to a shop that sells specialized to have them check it out, or contact specialized. Unless it has something to do with the wheels there is probably not much you can do about it yourself. I have a specialized allez pro from a few years ago that I think has close to the same geometry, frame design and the rear felt a little light in sprints at the beginning, but after I got used to it, it was fine. My neighbor has the same s-works frame and we hit over 50 on a descent that we ride all the time. He has never complained of any problems. I would for sure take it to a shop/framebuilder.

Is it an 02 or 03 model? 02's were supposed to have had some teething problems, though what your talking about seems a tad excessive.

Its an 03, brand spankin new.....

You can check that the headset is not loose, but most shimmy is caused by resonance. When the resonant frequency of the frame/rider matech the resonant frequency of the front wheel, the bike will shimmy. A lighter front wheel may help, but that would mean converting to sew-ups and it might not work anyway. Another solution is to descend with one pedal down (as when you are cornering), and your weight off the saddle. That changes the resonant frequency of the frasme/rider and usually solves the problem. Shimmy is a pretty common problem with light weight bikes and it's not caused by mis-alignment, so no amount of alignment will resolve it. If it's a persistent problem with a particualr type of carbon frame, the manufacturer might help you but I doubt any dealer would replace the frame without the support of the manufacturer.

Smoothie 104,

Before going to the fancy answers, check hubs, true wheels, head set etc. Another words, make sure that the bike is firmly together and tracking straight. Just because it is new dosen't mean it is necessarily tight.

And, enjoy the 2.8. Mine is my favorite bike and saved my butt on a tight, fast turn when the rear end hit some sand. Cannondale should revisit building them again!!

Cyclefrog

Like the others recommended make sure everything is tight. If that does not fix the problem frame alignment could be an issue.

There is an interesting write up on Zinn Cycles website (they specialize in extra large frames) concerning shimmy that can be inherent in larger frames. Seeing that this is a 61cm, you might want to check it out.

It doesn't take much headset looseness to give a frame
the willies. If torquing down on the headset doesn't
change things, have the set checked to be sure it is
well seated and bearing races parallel. Steve

Found the problem, It was in the headset. The top cap for the threadless carbon steerer tube was adjusted wrong, everything appeared tight, but there was some play inside. After re assembling and re tightening I took it over the same bridge. 43mph solid as a rock.

Thanks all!