Re: bicycle stability

[Leigh Palmer <palmer@sfu.ca>]

> Rather, to quote Jones, "You experience a powerful sense, when riding a
> bicycle fast, that it ... could not fall over even if you wanted it to."

That is silly and hardly scientific. Do you ever feel you could close
your eyes and coast forever? Trust me; you should resist that urge. It is
an illusion.

> The interesting part is the difference in stability between stationary and
> moving conditions.  The best way to probe that is to remove or limit the
> active stabilization by the rider - hence the 'no hands' experiments.

"active stabilization by the rider"? Is a wirewalker employing "active
stabilization"? How about someone on stilts? I call this "maintenance of
equilibrium". What you should say is that it is easier to *balance* on a
bicycle while moving than it is while stationary, or that the task of
"active stabilization" is less difficult on a moving bicycle than it is
on a stationary bicycle.

I guess that I have to point out the obvious again. When riding a bike fast,
no hands, an important force affecting balance is *aerodynamic*. If you
will try the experiment I suggested you will find that a bike with a very
light front wheel is easily ridden no hands at speed, downhill at 60-70
km/hr. A heavier front wheel with slight dynamic or static imbalance will
render this a dangerous exercise, by the way.

> > I've never
> > said the gyroscopic moment was negligible; I said it certainly matters at
> > speed, for example. My conclusion is that the bicycle is *not* stable when
> > riding in a straight line at moderate speed. "Gyroscopic stabilization" is
> > a myth ....
> Here may be the kernel of our discussion.  If gyroscopic moment is not
> negligible, how is "gyroscopic stabilization" a myth?  I suspect that you
> mean something different by 'the myth of gyroscopic stabilization' than I
> think you do?  Certainly the bicycle is not stable, but that doesn't mean
> it hasn't been 'stabilized,' i.e., made less unstable.

I take the term "stabilized" to mean "made stable". Would you purchase a
contraceptive which promised to make a woman less pregnant?

I'll suggest a Gedankenexperiment (Please don't try it!). Ride a bike no
hands at moderate speed on a parking lot with your eyes shut. I find that
I am absolutely dependent on visual feedback to maintain my upright
condition.

Gyroscopic moment is never a stabilizing influence anyway; that is a myth
of some sophistication. True stabilization must have a restorative nature;
gyroscopic moment has none. When one refers to a gyroscopically stabilized
system there will always be a feedback controlled element in the system
which senses displacement and applies appropriate restorative forces. If
one exerts an unbalanced torque, however small, on a gyroscope it responds
by altering its angular momentum. It shows absolutely no tendency to
return to its former direction. If one applies an impulsive torque the
gyroscope will be seen to nutate and then settle down again, but it will
be pointing in an entirely new direction. (I have often thought that this
nutation is what people mistakenly interpret as "gyroscopic stability".)
Thus the restorative element which is essential to stability is absent in
the bicycle because it has no active system for restoring the staus quo
ante. The gyroscopic moment does increase the inertia associated with
turning the front wheel, but it does not tend to recenter it. It merely
makes the hands-off ridden bicycle respond more slowly than it would with
no gyroscopic moment.

Perhaps a relevant analogy will explain the difference between being
stable and balancing. This is similar to the wirewalker's use of a pole
to increase his moment of inertia about the wire. He maintains *balance*
more easily because the system has rotational inertia; it responds more
slowly. There is no stabilization effect, however, in a kosher balancing
pole. A heavily end weighted pole which droops below the wire could indeed
be stabilizing. A commonly seen toy with a clown on a bicycle riding on a
string works just this way. The center of gravity of the toy lies beneath
the string because of the drooping end weights. The upright position is
truly *stabilized* by the pole.

> PS. With regards to your quote from Whitt and Wilson:
> >  Jones set out to build an unridable bicycle (URB). In his URB I, he
> >  cancelled out the gyroscopic action of the front wheel by mounting
> >  near it another similar wheel which he could rotate backwards. He
> >  found that this made little difference to normal handling, and
> >  concluded that gyroscopic action has little influence on bicycle
> >  stability.
> This conclusion is contrary to the actual article, which says just after
> describing the URB I experiments:
>
>  I was thus led to suspect the existence of another force at work in the
>  moving bicycle.
>
> and later, in the conclusion:
>
>  In addition to the rider's skill and the gyroscopic forces, there are,
>  acting on the front wheel, the center-of-gravity lowering torque and the
>  castoring forces.

Whitt, Wilson and I evidently read it differently from the way you do.
I can't interpret any more deeply as you do in the following:

> The first quote, taken alone, could be taken to mean either "another force
> instead of gyroscopic stabilization" or "another force as well as
> gyroscopic stabilization".  Whitt and Wilson apparently took the first
> meaning.  However, taken in context (immediately after the 'hands-off'
> experiments) and given the second quote, I think it is clear that Jones
> meant "another as well as", and that he did *not* conclude "that gyroscopic
> action has little influence".
>
> ------------------------------------------------------------------
> PPS. Do you have a more complete reference for that Sharp text - I'm
> wondering if I can find it, because I would like to see what he is
> calculating.  In particular, I am led by my reading to believe that what
> stabilizes a moving bicycle is the tendency for the front wheel to turn
> when the bicycle leans.  I don't see what centrifugal force, mentioned in
> the part you quoted, contributes to that effect.

I'm at home now. The Sharp text is (re)published by MIT Press and it is
inexpensive. I couldn't find it on the web. Ask me again tomorrow if you
need an ISBN.

Leigh