[python] Re: Steering, springs and damping (LONG)
- From: ebooth@xxxxxxxx
- To: python@xxxxxxxxxxxxx
- Date: Sat, 5 Mar 2005 20:47:05 -0600
I appreciate your taking the time to respond to my post. You've raised a number
of excellent ideas, which I'd like to discuss.
Quoting Ray Schümacher <mtb@xxxxxxx>:
> >... Such a scheme would require a downward-pointing
> >triangular cam, fastened to the rear subframe with a spring, to press down
> upon
> >an upward-pointing triangular cam on the front subframe.
>
> It could also be done with a spring loaded roller on the front frame and a
> cam shape on the BB shell, the opposite of a detent spring.
>
> The authors which suggested the negative coefficient had the idea of a cammed
> pivot, like on some bathroom stall doors. Their actual finding was that the
> desired effect was proportional to weight, so the cam is ideal. Note that
> downhilling increases weight on the python pivot.
>
Good point; I hadn't thought of that cam-and-roller idea. That would certainly
get rid of the cusp, and thus prevent (at least, forestall or minimize) the
onset of chaos. There's certainly also something to be said for resisting force
with force. I don't quite see where the weight influence comes in for
frame-fixed hardware, though. Maybe if you hinged the front assembly through
the steerer, perpendicular to both the steerer and wheel, that would work. It
would certainly provide a jumping-off point for suspending the frame.
I still have reservations about this system, though. This would be much harder
to adjust--for increasing speed--than a damper would be. You would need to
overcome the force of the spring itself in order to recalibrate its preload; I
doubt that this could be done with the shifter cable and derailleur return
spring. Definitely, I would avoid adding a servomotor for preload adjustment, as
a matter of principle if nothing else. Besides, this cam-and-roller setup would
add more complexity than would a damper, and subject the frame to additional
stress besides.
> My reading of the PDFs lately has me thinking that bike stability has enough
> degrees of freedom, and interaction, to be a chaos problem.
Also, I do worry that this "doubly-forced pendulum" could lead to unpredictable
results, much like a pendulum swinging between two magnets. Unfortunately, my
kinematics experience is not deep enough for me to begin addressing this issue.
> >As the negative damping constant
> >is increased, the stable (or near-stable) region moves to higher speeds.
>
> Remember, in my program, the pivot k value is a spring, not a damper. I have
> not figured out yet where to insert a damping variable.
>
Aaarrrgggh! You got me! :) Seriously, though, I'd definitely be interested in
seeing how the the selective introduction of steering resistance would affect
the bike's stability properties.
Thanks again,
Eric
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