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bicycle stability - A book report



I thought I might tell everyone what I just read in the famous
bicycle-with-a-gyroscope article (Physics Today, April 1970, by David
Jones).  Take home message for those who don't want to read this whole
message:  the article does *not* establish that wheel angular momentum is
of negligible importance for bicycle stability.

Consider the results of these experiments.  The bicycle had a third bicycle
wheel attached to the front forks, so that it could be spun in either
direction (I don't think it was coupled to the rotation of the front wheel
- it was just spun by hand).


extra wheel   | counter-spinning     stationary         co-spinning
------------------------------------------------------------------------
bike pushed   | "collapsed ineptly"    no data          "showed a dramatic
with no rider |                                           slow-speed
              |                                           stability" 
              |
              |
bike riden    | "almost impossible"  "not easy to ride"     no data
"no hands"    | "invited continual   "lacks balance and
              |   disaster"            responsiveness"
              |
bike riden    | "easily riden"       "easily riden"     "easily riden"
normally      | "felt a little       "felt a little     "felt a little  
              |   strange"             strange"           strange"     

Somehow, dispite the "no hands" experiment, the author may suggest that
"The light, riderless bicycle is stabilized by gyroscopic action, whereass
the heavier ridden model is not."  On the other hand, he may not; the
article is presented historically, and it is difficult to tell whether he
finally accepts this idea.  In the article's conclusion, he says "In
addition to the rider's skill and the gyroscopic forces, there are, acting
on the front wheel...", so clearly he is not completely dismissing the
gyroscopic forces.

The distinction between with and without rider arises because "The
lightness of the front wheel distresses some theorists, who feel that the
precession forces are inadequate to stabilize a heavily laden bicycle."  I
personally don't understand this, since the torque leading to precession
increases proportionally with the load weight.

In any case, it seems clear to me from the above experiments that
gyroscopic action *is* significant in bicycle stability.  Clearly there are
also other factors which are at least equally important.  The remainder of
the article nicely describes a principle one: stability due to a steering
geometry which tends to turn the wheel during a lean in order to lower the
bicycle's center of mass.  But that involves some complicated geometry -
you'd best read the article if you want to know about that.

-- 
--James McLean
jmclean@chem.ucsd.edu
post doc
UCSD