[python] Re: Python, of course.
- From: Jürgen Mages <jmages@xxxxxx>
- To: "Python Mailinglist" <python@xxxxxxxxxxxxx>
- Date: Sat, 20 Aug 2005 09:35:17 +0200
Hi all out there,
this is a PM that I got from Przemysław Abramowski, Szczecin, Poland.
He agreed to discuss it in the public list.
First let me say thanks to Przemek for his detailed questions.
Ok, here it is:
From: "Przemysław Abramowski"
Sent: Friday, August 05, 2005 4:21 PM
1. Stability / neg trail issue. I downloaded those XLS spreadsheets made
by
some friend of yours already in winter, but I could not figure out the
actual bike stability calculations. What I mean is that I could not find
the calculations of balance between destabilising forces (surface friction
force acting on front wheel during turn, which is caused by negative
trail)
and the stabilising forces - the effect of seat rise causing
straightening
moment, which is otherwise quite well calculated, but only this part of
directional stability equation. I would like to get this part well
understood first, because I have my own ideas about frame geometry. I
would
like to know the exact solution for the directional balance. I know Python
is stable enough to ride, but this is not enough for me, as I would like
to
make a fully conscious decision about such things as steering column angle
and steering pivot height. Possibly I can finally calculate it myself
(already started, but then had to drop it for personal reasons :), but
first : perhaps you already know everything about Python's directional
stability by now ? I'd be very grateful if you could give me any feedback
on this thing.
The calculations made by Ray and Dirk are a big progress towards the
evaluation of stabilty factors in the python frame geometry. But the
ultimate solution is still not found (and I guess it never will be ;-)
2. I am not going to build the frame myself - I did electronics, control
systems and stuff, I can't weld. It would take me months and too many
failed attempts to do it properly. Fortunately, we have professional frame
builders in our Szczecin here, and I want to get them interested. Apart
from that, their prices are not that crazy, I could afford it myself.
Perhaps you'd be interested in getting one Python build by a pro frame
builder?
I am a biologist and had no idea of metal works until I started the project
two and a half years ago. I had lots of drawbacks, but the experiences
that I made during the building process were unpayable. Not to mention
all the fun, that I had.
Of course if I intended to trade a small batch of pythons I would let them
build by pro welders. But until now I am not interested in going commercial.
The python project was aimed to be "Open Source" from the beginning,
and it had big benefits from the ideas of other people that joined the
experiment.
I am quite sure that a commercial version will be built sooner or later,
but I am not the right person to do this.
3. Frame flex and rigidity - vertical and lateral. Do you have any
specific
information on these? I would like to have something to show to a
framebuilder, so that I can persuade him easier that the thing will not
collapse or move in a fashion which would be too snaky for a professional
to be proud of his job ;).
Take the project page as a hint for the dimensioning of the tubing.
My P3 frame shows no flex at all, and I think that some lighter tubing
might do it as well. E.g. Olaf and Dirk used 30x10x1.5 mm for the front
part, while I took 40x10x1.5 mm.
A pro frame builder would use elliptic tubing instead of the rectangular.
4. I read on your webpage that Python directional stability is an issue at
high speeds. This is something I would like to avoid. Possibly I could be
able to design a stabler Python. Could you write me how this instability
shows, and how the bike rides at low/medium speeds (is snakelike motion
apparent?).
Snakelike motion is not apparent when the rider is experienced - similar to
a normal bike.
Low speed is no problem. When coasting faster than 50 km/h you have the
feeling that you are riding on the ice of a frozen lake: Everything is ok,
as
long as you manage to stay calm - if you get nervous then say "bye-bye".
However high speed coasting can be trained. I find myself getting slowly
adapted and more relaxed while doing fast downhills.
But in general I would say the python is perfect for flat and undulating
terrain.
It makes no sense in mountainous regions.
5. Handlebar. I don't want to have anything to do with Polish police when
they catch me on a handlebarless bike :), we have a regulation saying that
the rider should have both his hands on a handlebar, so your incredible
first Python would not be street-legal in Poland. Is other Pythons'
handlebar just attached rigidly to front part of the frame?
I would like to have a fully functional handlebar.
In my personal opinion handlebars are not very important. My brake levers
are beneath the seat and if some policeman would ask me for a handlebar
I would show him the levers and he will be satisfied ;-)
Best regards from Jürgen.
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