At various times and on various bikes I have resolved/minimized a front end
shimmy by:
1. Reversing the front wheel in the dropouts
2. Shifting the saddle back on the seat post
3. Correctly lubricating and adjusting the headset
4. Relaxing my grip on the handlebars
5. Pressing my knee against the seat tube on long fast descents
6. Rearranging the gear on the front rack of a fully loaded touring bike.
In other words, shimmy is caused or aggravated by the interaction of a bunch
of different factors, and yes, headset adjustment can be one of them. How
you adjust your headset will depend on the headset design. Older threaded
headsets have two relatively large ( usually 32mm) nuts stacked one on top
of the other, with a washer and/or brake cable hanger stacked between them.
Set the bearing pre-load by adjusting the lower nut, then lock the lower nut
in place by tightening the upper nut with one wrench while holding the lower
nut in place with another wrench. Newer threadless headsets are held in
place by the handlebar stem, loosen the Allen bolts that hold the stem onto
the steerer tube and slide the stem up and down to change the pressure on
the headset bearings. The optimum set up for both types of headset is the
point where the handlebars turn freely, with little or no resistance, but
there is no up and down play in the headset. This may not be possible with a
worn headset, an incorrectly installed headset, or a cheap, low quality
headset.
An incorrectly installed headset may cause the bike to pull to the left or
right, which may or may not be aggravating the problem. I spent almost 3
years wondering why I couldn't ride my Peugeot PX-10 in a straight line
hands free, until the nice folks at Missing Link (Berkeley's leading bike
shop at the time) explained that the headset was incorrectly installed.
HTH,
--
mark