Re: counter-steering, with numbers

[palmer@sfu.ca (Leigh Palmer)]

First let me qualify myself as a bicycle rider, experienced commuter,
and a physicist who has read on and analyzed this problem. I have only
piloted a motorcycle myself on a couple of occasions, and whenever I
rode as a passenger I felt distinctly uncomfortable, so I profess no
knowledge of that, demonstrably very different, physical system.

As often happens, I must disagree with a vital premise, and for that
reason I have included none of the material which follows this quote:

> 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.

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.

The second thing that is done after this fall has started is to steer
the front wheel in the desired direction. There is then a force acting
on the bicycle tending to arrest the rider's fall by seting it back
toward upright. The bicycle exerts a centripetal force on its rider
which causes the rider to move in a circle.

When it is desired to end the turning, the rider turns the front wheel
a little more sharply *in the same direction* until the former upright
condition obtains, at which time he straightens the front wheel. There
being no further lateral force, the rider and bicycle continue moving
in a straight line.

I feel I should point out that if the front wheel is now held rigidly
in this direction the cyclist will fall over in short order, though
with equal probability in both directions. Bicycles being ridden in
straight lines are *not* stable, contrary to the beliefs of textbook
authors who invoke gyroscopes. If you don't believe me try riding a
bicycle in a straight line with its headset clamped. And if you do, be
sure you are wearing a helmet!

Leigh