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Re: bicycle stability



Don't worry, I hadn't abandoned this thread.  It's just been a busy week!

(Quoted stuff from Leigh Palmer and myself...)
>
>>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"
>>
>
>The only item of importance here is the lower left hand corner.

If one is a bicycle designer interested in reducing wheel weight, I agree.
If one is interested in the physics of bicycle stability/instability, I
disagree.  Given that Jones included the results in his article, he
apparently disagreed as well.

All the lower left hand corner demonstrates is that the human capacity for
actively stabilizing a bicycle is much larger than any gyroscopic
stabilizing effect.  But I don't think that the interesting thing about
bicycle stability, the thing everyone wants to explain, is that it is
possible to ride a bicycle.

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."
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.

>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.

------------------------------------------------------------------
-- 
--James McLean              Taking "Question Authority" to heart...
jmclean@chem.ucsd.edu
post doc
UCSD
------------------------------------------------------------------
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.

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.