[python] Re: Python Stability Continued
- From: George Durbridge <gdurbrid@xxxxxxxxxxxxxx>
- To: python@xxxxxxxxxxxxx
- Date: Sat, 30 Jun 2012 23:04:11 +1000
Peter,
Thanks for raising these issues.
Some people find (some) pythons easy to ride. A friend built a python,
which I could not ride at all (I spent most of a day falling off it) and
he could barely ride (straight line only) but two other friends, each
with a lot of experience riding low racers, but none on pythons, got
onto it and rode away. Since most people can ride Cruzbikes pretty
readily, I think a lot of the difficulty of riding a python is just that
it is a low racer. The fact that low racers in general are hard to learn
to ride is one of several lines of evidence suggesting that most bikes
are in fact kept upright by continuous rider input.
I think you are much too sanguine about the effects of wheel-flop on
stability. As you can see with a stationary or very slowly moving bike,
wheel-flop, however moderate, causes instability unless corrected. The
stabilizing effect you describe is not wheel-flop pure and simple, but
an interaction between moderate wheel-flop and a corrective centrifugal
force. My experience with choppers and the like is that higher than
normal wheel-flop makes bikes unrideable, at moderate speeds. At higher
speeds, such a bike may become rideable. Some dynamic factor must be
increasing with speed, at some point overcoming wheel-flop, which is a
static effect and doesn't increase with speed. One candidate is
centrifugal force, which increases with speed. Another is the squirm of
a tyre running at an angle to the direction of travel, which also
increases with speed.
The observation that pythons become harder to control at still higher
speeds indicates that a third force is at work, this one destabilizing,
and seemingly also increasing with speed. This observation seems to
apply to python trikes, as well as bikes, which should narrow the field.
I take it your swing seat would have a vertical pivot at the rear, and
the front of the seat supported on a mechanism which allowed it to pivot
so that the seat and the rider's body lay on a line from that pivot to
the bottom bracket? In that layout, pedalling would obviously tend to
push the front end back to the straight ahead position, but (a) the
rider would have to permit the front end to point in the direction of
steering, and (b) this illustrates that to manage a python at all, the
rider needs to exercise fairly fine control over the effects of
pedalling on steering: why is this control not already sufficient?
George
On Sat, 2012-06-30 at 19:40 +1200, Peter Clouston wrote:
> Thank you all for your replies. I am a bit surprised at comments that
> the python is easy to ride however. Jurgen's account of learning to ride
> hardly suggests that. Nor do the reports by several builders that they
> have been unable to learn to ride their creations. Several of them have
> given up and gone to trike conversions as a result. I think that with
> its other advantages and general coolness the Python would be a real hit
> if it was a bit easier to ride.
>
> Olaf’s comment that riding a Python had similarities with riding a
> unicycle confirms my impressions from following this and other lists.
> Both are ridable, but not by everyman.
>
> A bike that requires active action by the rider to keep straight and
> upright is ridable, but not stable, by my definition at least. Nor by
> the definition of vehicle design engineers. A problem with requiring
> continuous rider input just to keep upright is that as speeds increase,
> things happen far too fast for the rider to react in time. In fact, the
> rider's delayed action may in fact make things worse in the case of
> oscillating instability. This is known as "pilot induced instability" in
> aviation.
>
> I was once saved from very serious injury, or worse, by the true
> stability of a cheap town and country diamond frame bike. I was
> descending from one of our hill suburbs on a very steep, very long
> section of chip-sealed road when I hit something very well-camouflaged,
> on the road surface. The impact wrenched the handlebars out of my hands.
> I didn’t have a computer on the bike but I was definitely going the
> fastest that I have ever been on a bicycle, at least 70 km/h, possibly
> quite a bit more. I was saved by the instant reaction of the bike
> itself. It recovered in a split second, no doubt actually helped by the
> fact that I didn’t have any contact with the handlebars at all. The fact
> that I was sitting as upright as possible, to act as an air brake, meant
> that I was not dependent on the bars to stay on the bike. Not
> surprisingly, I have a strong preference for bikes to be auto-stable at
> speed, especially in a hilly country.
>
> @Patrick,
>
> Forces applied to the bike directly above the rear tyre contact patch
> are resisted by that contact patch only and not at all by the front tyre
> contact patch. Hence there can be no effect on steering. Of course, if
> you add a whole lot of extra weight to any bike it will affect the roll
> inertia, which can affect handling. However I am not discussing an added
> weight situation here.
>
> @Vi,
>
> At only 15Km/h, aerodynamics isn’t all that important, except in a
> strong headwind. I like to average about 30km/h over a couple of hours
> in rolling country, so I would be doing 40 or so a number of times a
> ride - And 5 or less sometimes too :-) I’m not a fast rider, so a bike
> that isn’t comfortable to ride at 40 is not going to be ideal for many,
> in similar country.
>
> Has anyone any thoughts about the utility or not of my idea of a “swing
> seat”?
>
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