[python] Re: A Bicycle Can Be Self-Stable Without Gyroscopic or Caster Effects
- From: "dirk@xxxxxxxxxx" <dirk@xxxxxxxxxx>
- To: python@xxxxxxxxxxxxx
- Date: Sat, 16 Apr 2011 22:19:38 +0200 (CEST)
Hi Pascal,
pybuen@xxxxxxxxx hat am 16. April 2011 um 20:21 geschrieben:
> I was proposing first a much simpler experiment: putting various weights on
> the seat and launching the python at various speeds riderless (i.e.
> control-less) (do I need to put a disclosure notice? You guessed it). This
> experiment works well with upright riderless bicycles, even without weight.
> From what we can see in the video that Vi posted, the python self-steers to
> the left with a left lean at small weights, which would, it seems, counteract
> the fall, but the opposite self-steering effect is seen at heavier weights (it
> steers to the right with a left lean). Is this latter behaviour the source for
> having to relearn how to ride with a Python?
>
what do you mean by weights? a gentle leaning and a harsh leaning into a curve?
still, this experiment would be a good starting point.
> Redistributing the weight more on the front wheel with a pole could be
> interesting too... However, this would also be quite different from putting
> weight directly on the front wheel (i.e. playing with new sitting positions as
> DirkS suggested!). It also depends very much if this weight is ahead of the
> wheel-road contact point or not.
>
> Anybody went for a bicycle trip with loaded front wheel? Are you saying DirkS
> that this would be unridable? Why do you think the CoM should be as close as
> possible to "the front"?
>
i was trying to derive from the experiment. if the clue is to have more mass
towards the front which is still attached to the rear part in order to improve a
bike with negative trail,then:
- the extreme imagination/exaggeration would be a rider, that sits above or in
front of the front wheel. This bike would probably fall over, at least when
braking
- or there would be an additional mass almost as heavy as the rider, which is
undesirable.
So in practice, i figured, all we can do to approach towards this benefit is to
move the CoM of the rear part towards the front.
A bike with a loaded front wheel only changes the relation of the mass between
front and rear part. i does not alter the CoM of the rear part alone.
> In fact, a substantial redistribution of the weights is effected when riding
> uphill (rear weight) or downhill (front weight). Is one way more stable to
> ride (not speaking of fear of speed)?
>
true, i recall that many on this list reported better or worse rideablility when
going up or down. But you would alter all values at once. CoM distances of both
front and rear in relation to the contact point of the wheel, the pivot angle in
relation to the flat ground...
> One could also experiment unloading: attaching Helium balloons to various
> parts of python... :) A lot of fun and spectacle for the kids... pricey
> perhaps? Mh. Still less than the moon... (some may remember E.T...) A cheaper
> alternative would be to ride underwater :)
>
i think its a good start adding weight here and there and let the bike go to see
what happens. It should be better than the pushing test we discussed the other
day, since there aren't any external, varying forces involved :-)
> In any case, riderless behaviour is not strictly equal to rideability - we can
> ride upright bicycles at very low speed too, where we know they would fall
> without our skillful presence.
correct. i think the essence of this experiment is, that when the bike is
falling towards one side, its CoM is pulled further down by gravity, so the bike
wants to fall over. If this happens at speed, there is a resulting influence on
the steering behavior, depending on mass distribution of front and rear section
and pivot angle and position. If that leads to a correction in the driving
direction, towards that side, the resulting centrifugal forces will pull the
bike up again and over to the other side. The faster the bike is, the more and
the earlier this kicks and the less wobbly it is.
Now how is the resulting influence on the steering behaviour? What geometry
makes the bike wanting to steer to the right side?
greetings,
DirkS
>
> On Sat, 16 Apr 2011, dirk@xxxxxxxxxx wrote:
>
> > Hi,
> >
> >
> > now that explains this forward pointing rod with an extra mass attached to
> > it
> > :-)
> >
> ?? how would we do this in real life?
> > Sit in front of the front wheel, while still being attached to the rear
> > wheel?
> > Or have an extra mass greater than the drivers weight pointing forward?
> > scary :-)
> >
> ?? The only practical clue i can derive is that among other things one should
> have
> > more weight on the rear part than on the front and aim for a centre of mass
> > as
> > close to the front as possible.
> >
> >
> ?? Greetings,
> > DirkS
> >
> >
> ??
> ??
> > "Jürgen Mages" <jmages@xxxxxx> hat am 15. April 2011 um 13:32 geschrieben:
> >
> >> Thanks Christian. The clue is: if the rear part's center of gravity is
> >> shifted far enough over the front part, then the negative trail bike is
> >> inherently stable.
> >>
> >> Cheers,
> >> Jürgen.
> >>
> >> On 15.04.2011 12:11, Christian Andersen wrote:
> >>> Hi folks
> >>>
> >>> In Science there is an article about bikes, that could be interesting
> >>> for those of you guys, who have sufficient knowledge (unlike me).
> >>> Maybe it is worth to have a look at to improve the pythonconcept
> >>> regarding stability.
> >>>
> >>> http://www.sciencemag.org/content/332/6027/339.abstract
> >>>
> >>> Science 15 April 2011: Vol. 332 no. 6027 pp. 339-342 DOI:
> >>> 10.1126/science.1201959
> >>>
> >>> * Report
> >>>
> >>> A Bicycle Can Be Self-Stable Without Gyroscopic or Caster Effects
> >>>
> >>> 1. J. D. G. Kooijman1, 2. J. P. Meijaard2, 3. Jim M. Papadopoulos3,
> >>> 4. Andy Ruina4,*, and 5. A. L. Schwab1
> >>>
> >>> Abstract
> >>>
> >>> A riderless bicycle can automatically steer itself so as to recover
> >>> from falls. The common view is that this self-steering is caused by
> >>> gyroscopic precession of the front wheel, or by the wheel contact
> >>> trailing like a caster behind the steer axis. We show that neither
> >>> effect is necessary for self-stability. Using linearized stability
> >>> calculations as a guide, we built a bicycle with extra
> >>> counter-rotating wheels (canceling the wheel spin angular momentum)
> >>> and with its front-wheel ground-contact forward of the steer axis
> >>> (making the trailing distance negative). When laterally disturbed
> >>> from rolling straight, this bicycle automatically recovers to upright
> >>> travel. Our results show that various design variables, like the
> >>> front mass location and the steer axis tilt, contribute to stability
> >>> in complex interacting ways.
> >>>
> >>>
> >>> greez,
> >>>
> >>> christian a wannabe pythonrider
> >>
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> >>
> >> Listmaster: Jürgen Mages jmages@xxxxxx
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> >>
> ??
> > http://dirk.steuwer.de ;
http://dirk.steuwer.de
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