Re: counter-steering, with numbers

[jmclean@chem.ucsd.edu (James Mclean)]

Leigh Palmer says:
> > There is agreement that when starting a turn on a bicycle or motorcycle at
> > low speeds, the technique is to shift the body weight into the turn so that
> > the bike leans into the turn.
>
> That is not the case with a bicycle. "Shift one's weight" is perhaps a
> kinesthetically satisfactory process, but it is unphysical. In order to
> produce any change in the motion of a body there must be a net external
> force exerted on it. This force can be the reaction to some force
> initiated by the body, but let us speak in terms of such forces. Mere
> "shifting of the weight" is mystical; physics is not mystical.

You bring up two issues:
1) Is "shift one's weight" unphysical?
   Stand on the ground with feet somewhat appart.  Now move your muscles to
lengthen your left leg, so that your body shifts to the right.  You have
just "shifted your weight".  Seeing as it just happened, it is not
unphysical.  It can be analyzed in greater detail if you wish, and I admit
that I did not, as it was not directly relevant to the remainder of the
message.
   I will think a bit about how this can be used to tip a bicycle, or for a
more simple problem, how to go from balancing on a see-saw to tipping one
end down.  I agree with you that it is not as simple as might appear.
However, I am confidant that I can do it (tip a see-saw, that is), and it
is just a matter of figuring out what is going on.

2) Can a bicycle turn be initiated by "shifting one's weight"?
> There is agreement among those who have analyzed the problem correctly
> that when starting to turn a bicycle from a straight course at any
> speed whatsoever, the first thing that is done is to turn the front
> wheel in the direction opposite the desired turn. This will result in
> the bicycle being forced to that side, and the rider will continue in
> the same direction as before. Thus he will start to fall over in the
> direction of the desired turn.

While I agree completely that this is a possible method (my post was, after
all, devoted to analyzing this action in more detail), I should like to see
any references you have which contend that it is the only method.  In
particular, I contend that if it *were* the only method, then it would be
impossible to ride a bike without holding the handle bars (since it would
be impossible to do "the first thing that is done").

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