Speed wobble is the term used to describe a quick oscillation of the
handlebars while riding at high speeds. Any vehicle with a single
steering pivot is capable of experiencing speed wobble.
Your immediate instincts are to think that something wrong with the
bike (loose headset, flat tire, etc). Panic can often occur and the first
thing many people do is straighten their arms, lean back and put more
weight on the saddle. Unfortunately these are all the wrong things to be
doing. Speed wobble can be attributed to many different factors.
Incorrect weight distribution is a very common cause of speed wobble.
Quite often, speed wobble has just as much to do with the rider as it does
with the bike.
If speed wobble starts occurring, many people will
intuitively put their weigh towards the back of the bike instead of
putting their weight towards the front to actually stop it and dampen it
out. The best thing to do to get out of a wobble is to put the weight of one of your
legs down at the 6/12 o’clock position, put some weight on the front end
by bending your elbows (use soft hands – don’t grip handlebars firmly!),
and take some weight off your seat (to take the pivot point away) which
puts more weight back on the handlebars, which puts more weight on the
front wheel. This will usually bring stability back to the bike and correct
the oscillation. The reason it’s suggested that the pedals be in the 6/12
o’clock position (rather than the 3/9 o’clock position) is because this will
bring your body weight into a better balanced position which will calm
the bike down. Clamping your knees against the top tube is also a known
solution to calm the frame.
Different riders may experience speed wobble on different bikes. A bike
can descend like a dream for one rider,while another will get the fright of
his life on the very same bike. Things like stem length on a particular
geometry and weight distribution of the rider can have a lot to do with it.
While speed wobble can scare the hell out of any rider, knowing what to
do in such case will instill confidence and safety.
Of course make sure your bike is in good repair and tuned. Proper wheel
alignment, truing and headset adjustment are critical.
This is an excerpt reprinted from http://cyclingtips.com.au/
SAFE RIDING EVERYONE