Re: bicycle stability - A book report

[palmer@sfu.ca (Leigh Palmer)]

Well, I'll contribute two other book reports, one from an older book
and the other from a newer book which analyzes Jones' results. Both
maintain the lack of importance of gyroscopic moment to riding a
bicycle normally, as did Jones.

> 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 only item of importance here is the lower left hand corner. 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 as is clearly demonstrated by the experiment in which the bicycle
is ridden normally with zero gyroscopic moment. That it affects no-hands
riding is no surprise to me.

> From the Sharp text, section 168. Gyroscopic Action in Chapter XVII
"Stability of Bicycles", pp. 207-208:

  ...

 "Thus, in estimating the stability of a wheel rolling along a circular
  arc, both centrifugal and gyroscopic actions must be considered.

  [theory and numerical example]

  "From the above example [including gyroscopic moments] it will be
  seen that gyroscopic action in bicycles of the usual types is
  negligible, except at the highest speeds attainable on the racing
  path, and on tracks of small radius."

I should point out that the numerical calculation is for 28" wheels
weighing eight pounds! Remember, this text is 100 years old.

This is all old stuff; it has been known for more than a century. Just
because "the latest idea" epitomized by a textbook passage asserts the
bicycle is stable and offers the mechanism of gyroscopic moment to
explain this phenomenon does not mean that it applies in the real world.
It doesn't.

> 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.

I do, perhaps because I'm a cyclist - and an experimentalist.

> 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.

A Freudian slip? Clearly you interpret the results differently from the
author. To 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.

I find it unsurprising that this particular bit of nonsense has survived
for so long in our physics textbooks. It is scarcely unique, and I fear
that there is some small justification for the attitude of students that
somehow physics doesn't apply to the real world.

The bottom line I learned at Berkeley well before the sixties:

                          Question Authority

Leigh