Date: Fri, 13 Feb 2004 12:07:59 -0800

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 poor straight ahead preference and would riding no-hands
difficult.  Many bicyclists never master riding no-hands because the
gyroscopic forces are too small for them to detect.  Hands on the
handlebars completely obscure these forces.

For those who ride no-hands, countersteer 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, although the opposing lean is unnecessary,
countersteer is still needed and can be done without counter-leaning.

That there are gyroscopic forces is evident from the riderless bicycle
test in which a bicycle is shoved at a brisk speed (from another
bicycle) and allowed to coast on its own.  If the initial course is
straight, the bicycle will continue this path until it slows to a
speed where gyroscopic forces are too small to correct steering.  Then
the bicycle takes a steep turn as it falls.

Gyroscopic forces are also used to walk a bicycle, holding it by the
saddle and steering it to either side by quickly tilting the bicycle.
The effect can be observed by resting a road bicycle (with a
horizontal top tube) on the shoulder tilted forward just enough to
make the front wheel aim straight ahead.  Spinning the front wheel by
hand forward will make it steer as one expects, left for a left tilt,
right for a right tilt, all moves performed in less than a second.
With the wheel spinning backward, all responses are reversed.

A good example of a bicycle with no gyroscopic forces is the ski-bob,
a "bicycle" with short ski runners in place of wheels.  This bicycle,
having no rotating parts, is ridden downslope easily by anyone who can
ride a bicycle.

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