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do a search on "shimmy." complex phenomenon, endlessly discussed.
 

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Big is relative
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Your bike shouldn't shimmy unless you have one of the following: loose headset, lumpy tire, front wheel considerably out of true, or the front wheel isn't clamped correctly in the fork. Before you go digging further, I would check the front quick release and ensure the wheel is centered and that the QR is tight. Try a different front wheel if that doesn't fix it.

The front wheel I used on Saturdays century needs truing. It isn't bad but it needs a little work. I was doing 45+ mph descents with no shimmy.
 

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Jan Heine has a brief article on shimmy in the current issue of Bicycle Quarterly suggesting that, at least in some cases, the lack of precise parallelism between the top and bottom edges of a frame's head tube causes the headset ball bearings on one side to run looser than on the other side, creating just enough looseness in the headset to cause shimmy even when the headset is adjusted correctly. Jan has experimented with needle-bearing headsets (such as the Stronglight A9) attempting to reduce shimmy and claims that on a given frame with a quality ball bearing headset (Chris King, Campagnolo), simply replacing the headset with one that uses needle-bearings can eliminate or substantially reduce shimmy on hands-off high speed descents.
 

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Ichijin said:
Do all sealed cartridge bearings use needle bearings?
No. All sealed cartridge bearings that I'm aware of use ball bearings.

These drawings are from Barnett's Manual:

Here's a headset with sealed cartridge bearings (the bottom stack shows an integrated headset; the upper stack shows a pressed-mount headset) -



Here's a headset with needle-bearings (floating, or non-integral races) -

 
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