This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <email@example.com>
Date: Tue, 16 Sep 1997 11:10:32 PDT
What keeps the bicycle upright?
The question is often asked and, as often as not, is an introduction
to expound on the gyroscopic forces of the rotating wheels that make
bicycling possible. This claim is as accurate as the one that
authoritatively explains that spokes support the bicycle wheel by
hanging the hub from the upper spokes. They don't and it doesn't.
Some who propose the gyroscope theory, also explain that the advanced
skill of making fast turns on a bicycle involves a technique they call
countersteer. In fact, a bicycle cannot be ridden without
countersteer, commonly called balance, and it is this balance that is
used to keep the bicycle upright, just as one does while walking,
running, ice skating or roller skating. To say that the gyroscopic
forces of rotating wheels keep the bicycle upright, ignores that
roller skates are operated the same way and have so little gyroscopic
moment that one cannot detect it. On ice skates the argument fails
entirely. Besides, a bicycle can be ridden at less than three miles
per hour, at which speeds there is no effective gyroscopic reaction.
Those who ride no-hands sense and make use of the small gyroscopic
effect of the front wheel to steer. This, together with trail of the
steering geometry stabilize steering. Without trail, the bicycle
would have no straight ahead preference and would rapidly fall if one
were to attempt riding no-hands. Many bicyclists never master riding
no-hands because the gyroscopic forces are too small to be detected.
Hands on the handlebars completely obscure these forces.
For those who ride no-hands, the countersteer effect should be visible
and obvious because the bicycle must be leaned away from the preferred
lean angle and direction of a curve so that the turn can be initiated.
With hands on the bars, this opposing lean is unnecessary, because the
front wheel can be steered without leaning.
A good example of a bicycle without gyroscopic action is the ski-bob,
a "bicycle" has short ski runners in place of wheels. This bicycle,
that has no rotating parts, is ridden downslope easily by anyone who
can ride a bicycle.