[python] Re: Stability Theory - bug :-(
- From: mtb@xxxxxxx
- To: python@xxxxxxxxxxxxx
- Date: Tue, 22 Feb 2005 23:16:24 -0800
Hello Jürgen,
At 06:02 PM 2/21/2005 +0100, you wrote:
Thanks a lot for the metric version.
I'm afraid that I must apologize for a serious bug I just found in the
calculator.
I omitted the conversion to radians for one of the head angle lines. This
had little effect for small head angles, like the "standard" bike, but
erroneously suggested stability regions for the typical python.
Recall that an unstable bike by these criteria can still be ridden, but
with a bag of sand in the seat it will not ride by itself.
I added a verbose output to a command widow to simulate the output by Hand
in his thesis paper:
http://ruina.tam.cornell.edu/research/topics/bicycle_mechanics/papers/comparisons_stability_analysis.pdf
if a formula goes negative, it prints a letter.
The interpretation of the Routh-Hurwitz criteria it prints is probably best
done near the end of
http://ruina.tam.cornell.edu/research/topics/bicycle_mechanics/papers/benchmark_results.pdf
My previous comment about a de-centering spring is still correct; it
appears that a negative spring K can compensate for some of the negative
trail, and can be tested with the new slider (dimensionless units). I'm not
sure yet how best to increase the second criteria, "B", which seems to
mainly limit the python's stability, in theory. I'll be looking into that
and post later in the week....
The head angle is now measured from vertical, as in most papers.
The default values are for a Schwinn Varsity 10 speed, as per Hand's paper.
I also added both metric and imperial into one version, which is where I
discovered the error when testing :-(
http://rjs.org/Python/FrameGeometry.zip
However, I did find another junk bike today for parts
Ray
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