[python] Re: Lean disturbances
- From: Ray Schümacher <mtb@xxxxxxx>
- To: python@xxxxxxxxxxxxx
- Date: Sun, 27 Feb 2005 08:36:18 -0800
Hi Eric,
At 04:26 PM 2/26/2005 -0600, you wrote:
> If the Python
>were rolling along while this happened, its front wheel should promptly buckle
>backwards.
But, it would depend on the angle of the pivot: It would definitely be true for
nearly vertical angles, with negative trail. At larger pivot angles with larger
negative trail, the rotational force at the pivot of a small angle of
perturbation is more than counteracted by the self-centering force. It would be
interesting to calculate this, maybe using Dirk's code as a start.
Also, with negative trail, a perturbation of the front wheel due to wind always
turns the wheel in the direction of the wind, providing corrective steer. How
large this effect is (and the sign) would depend of the windage of the front
vs. the entire bike.
>While the riderless situation is a contrived one, this still begs the question
>of how a Python and its rider can stay upright if the lean angle is perturbed.
It would seem only by active correction, but riders of all bikes seem to be
able to correct for lean moments easily (as in very low speed riding), maybe
because they are easiest to detect via the inner-ear sense. The python would be
harder than an upright since the moment about the contact patch is less due to
lower mass height.
>Now, there are four Python stability mechanisms:
>
>1) hanging-pendulum
an interesting question here: what is the natural frequency of the pendulum? Is
it close to an oscillation frequency at speed?
>2) pulling of front wheel forward by drive torque
only during accel or hill climb...
>3) pushing of front wheel forward by rider's legs
I think of it as stretching the frame out, assuming that the seat is attached
to the rear.
>4) gyroscopic effect of spinning wheels
This is pretty low and easily calculated, the code I wrote could be a help. It
would be good to see how heavy the wheels must be to turn stability positive;
the range of self stable speeds is already noticeably different with MTB wheels
vs light road wheels (~150lb-in vs 50lb-in).
>Therefore, I propose that,
>when a Python's lean angle is perturbed, its rider provides the majority of
>the force needed to steer it into the lean.
agreed
Ray
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