I am reminded of some relevant experiments I did in LA in the forties
and fifties. My first bike was a pre-owned "balloon-tired bike",
heavy with longhorn handlebars. It was also quite rugged. I lived
very near to Burnside Avenue Elementary School* which had a large,
open asphalt surfaced play area. I learned that if I got going fast
enough on my bike and jumped off, the bike would continue to move on
a curving path. On rare occasions it would complete an almost
circular orbit and I could catch it before it crashed. Of course I
never could get it to go straight after I jumped off, though I did
try hard to do so. Of course I now know why I failed. I think that
this experiment could be repeated by running alongside and launching
a small child's bicycle which one is willing to scratch up a bit. I
would like to hear any observations made by members of this group who
try it. Beware of the dreaded "pedal hop" on tight fast turns, a
disease my first bike had.
Despite my demonstrated lack of respect for that bike my parents
bought me a Bayard "racing bicycle", one of the very few thin-tired"
bikes owned in my peer group. It had only one gear, but it was fast,
and I started organized racing on it. I never abused it as I had the
other bike. I did experiment with a new challenge, however, and it is
one I still carry out to this day. Find a stretch of four-inch wide
white line somewhere on a flat, unbusy road or playground. See how
far you can ride on the line without leaving the white. You will find
this to be very difficult for the reason I mentioned; you will be
constantly turning to avoid falling**. If you try this experiment you
will understand my explanation kinesthetically as well as
intellectually, a good thing to do, in my view.
The old gyroscopic argument may have made some small contribution
with my old bike; at least I once thought so. The line riding
experiment, however, should bust that myth permanently. Riding fast
(with increased gyroscopic moments for the wheels) doesn't make line
riding any easier. The best feeling bikes I've ridden also seem to be
those with the lightest wheels. Just put those old gyroscopic
arguments away with the Benoulli "explanation" of how airplanes fly.
We owe our students sound arguments or else they will think that
physics is just a collection of pat answers to be memorized and
regurgitated. Do mention that this simple system is not sufficiently
well understood that it can be completely modeled. That won't make
the students think any less of you, and they will get the idea that
perhaps all the easy stuff hasn't already been discovered.
Leigh
*After much thrashing with Google and Google Earth I determined that
Burnside Avenue Elementary School has a new name. MLK Elementary? No.
JFK Elementary? No. It is now called "Saturn Street Elementary
School". I wonder how much General Motors paid them to rebrand the
school.
** As you get better, you will make smaller adjustments to keep on an
even keel. I find that it is impossible to do well at this task while
looking directly at the front wheel on the line. Look into the
distance and keep the line and wheel in your peripheral vision. I
used to find that I got more proficient with practice during the
commuting season, and I lost all my ability over the winter when I
was not riding regularly. I can't do it at all well now.