[python] Re: Python Stability Continued
- From: Vi Vuong <vi_vuong@xxxxxxxxx>
- To: "python@xxxxxxxxxxxxx" <python@xxxxxxxxxxxxx>
- Date: Sat, 30 Jun 2012 12:01:30 -0700 (PDT)
Hi George,
Very good insight on low riding position being difficult in general. I think
PSI is the third force you are referring, and I suspect a downhill incline
making it worse. Perhaps regarding changing dynamics, we can split the
stability discussion into flat surface vs. downhill.
Vi
>________________________________
> From: George Durbridge <gdurbrid@xxxxxxxxxxxxxx>
>To: python@xxxxxxxxxxxxx
>Sent: Saturday, June 30, 2012 6:04 AM
>Subject: [python] Re: Python Stability Continued
>
>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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