[python] Re: Python Stability
- From: Patrick van Gompel <patrick_van_gompel@xxxxxxxxxxx>
- To: <python@xxxxxxxxxxxxx>
- Date: Mon, 2 Jul 2012 01:21:52 +0200
Hey Peter,
It's nice to see that you are really digging into it. But in my eyes there are
too things: thoughts and experience. I am not sure why you seem to be 'afraid'
that a Python is unstable, but in general: when you think something is unsafe
and will cause you an accident, it's better not to make use of it. That might
be a 'mind-thing' but I noticed that learning to drive a Python is somewhat the
same. For me it was like switching a button in my brains and suddenly had great
improvement on the Python. But anyway, back on the subject...
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.Yes true. Sorry, I was
a bit confused by the word 'steering'. But if you steer a Python, something is
going to 'roll', either your front or your rear. In my eyes: the heavier the
rear, the less it will roll (and thus the more roll of the front). But you said
you weren't adding weight... Not sure why you mentioned it then. Whether you
stretch or not, does not change anything to the weight on the tires, unless
your CoG of your body goes forward or rearward. And for a Python it's not just
the front that 'steers'.
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.I won't say it's easy. I neither think that riding a
normal bicycle is easy. It has been quite some years since I learnt to cycle as
a child, but it sure took me months (or years?) to fully master cycling. For
example: I had to cycle to school every day, but it took me years -since I
started cycling- till I felt comfortable enough to cycle handsfree.
If you think you can cycle a Python because you already know how to cycle a
normal bike, you have it wrong. You have to start all over again (in my
experience), although you have already learnt more about balance of course in
the past.
You say that some are never able to learn to drive a Python, but how much does
that have to do with the stability of a Python? Most people learn to cycle as a
child and have difficulty to adopt/learn new things when they become older,
some just don't have perseverance, or they might just have a unstable build.
Again, there are many bikes. One Python is not the other. Would you like to go
100km/h on a home build citybike with average brakes and some indecent or
cracked welds? Well I won't. But I think that is exactely the world of this
Python list. All builders build to their own ability. Some might have some
degree and professional tools, others don't. You are not reading cycle
experiences here of factory build bicycles with proven (and well tested)
geometry. I don't want to sound harsh or offending, but that really is a huge
difference to the safety and stability of a bike.
I am not sure about your pivotting seat. Of course, unless somebody tries, we
will never know for sure. There are a few things I would not like (of course
these are only thoughts):
-Pedal induced steering is likely to increase, because you can't really
counteract this with your hips and your steering body pivot (hips) is not
centered at the Python's pivot anymore.
-Steering will be a totally different thing. I might have your idea totally
wrong, but how do you actually steer? If you are steering and stretch your
body, will the bike steer less?
-The more stable the bike will be, the harder it will be to actually steer.
Stability might be positive, but I don't think you want a bike that will be
harder to steer when you push your pedals. In my eyes this is already the case
with the Python. I certainly wouldn't like to increase that property much more.
-I prefer good contact with any bike. Having a pivotting seat and anything that
is not firmly connected to the mainframe, wouldn't be my preferred option. I
once put a less soft and more slippery seat cushion on my recumbent and really
lost the nice feeling with the bike. But I might be totally wrong on this as a
pivotting seat is a totally different thing from what I have ever driven.
Given that
fact, I cannot imagine an average person being able to control a
Python at 95km/h.
My question: how many can on a normal bike? I for sure wouldn't like to put my
mum on any bike with that speed.
My Python delta trike has the same steering as a Python. So if you are only
discussing the steering (pivotting) of a Python, it should work the same.
Though, in my experience it is quite a bit more stable when cycling. That means
that there is more than just the steering. Going downhill with my Python at
high speeds have me a maximum speed at which I felt safe (about 30km/h I
think). With my (leaning) trike I still have to reach a speed that gives me
worries (I guess I went to about 60km/h). Very high speeds are hard for me to
reach since we dont have steep hills here and my gears aren't enough for it.
So the steering of a Python does definately not compare to a unicycle in my
eyes. And trust me, I made my hours trying to cycle both... ;-)
(still can't cycle on a unicycle though)
So, although a very interesting discussion, may I ask why you are kinda worried
about the stability of a Python? If you are a homebuilder, it's not only the
stability of 'the' Python, but also about the stability of 'your' Python and
your ability to learn new things.
Patrick
Date: Sun, 1 Jul 2012 22:25:44 +1200
From: peter@xxxxxxxxxxxxx
To: python@xxxxxxxxxxxxx
Subject: [python] Re: Python Stability
Vi, I’m not claiming 40km/h on the flat! 40 is my speed in the
downhills in the rolling foothills part of my regular ride. The
70 km/h was down a long and very steep slope that I could just
barely ride up (1 in 3.75 to 4 gradient maybe). I was
exploring, I don't think I'll try that ride again! But I like
to go as fast as I can downhill to make the uphill bits easier.
There are no speed limits on the very few bike paths where I
live in provincial New Zealand but I do almost all my road bike
riding on rural roads anyway.
The problem with maximum sideways fixity between rider’s body
and the python frame is that the sideways forces are then
applied in the middle of the pivoted frame, in exactly the wrong
direction for stability. And these forces get much worse at
higher speeds. You don’t need any other explanation for the
high speed instability of the Python than that. Given that
fact, I cannot imagine an average person being able to control a
Python at 95km/h. The guy who did that must have the reaction
speed of Ronaldo and balls of steel as well. You play Premier
grade football Dirk?
As
for the importance of hands-free stability being overrated, it
quite possibly saved my life in the incident that I have
mentioned. So I would take a lot of convincing on that point.
I am pretty sure that, at speed and without pedaling, a
pivoting seat would be more stable. I am not sure what the
effect on pedaling power and PSI would be, it depends on exactly
how the pedal thrust is angled, for which I cannot find any
data. A pivot may be detrimental when pedaling, or it may not.
George, if you read my post carefully you will see that I am
not claiming that wheelflop is the reason for a DF bike’s
stability, just that the small amount of positive wheel flop of
a standard DF is said to have a small beneficial effect on
stability. In fact such a bike would still be stable with zero
wheelflop, provided that the trail remained about the same.
(Zero wheelflop requires a vertical
steering axis, which on a DF makes
providing adequate trail, turning clearance between the front
wheel and the toes and having the handlebar in easy reach all at
once very difficult.) There is no argument that excessive
wheelflop e.g. with chopper geometry isn’t desirable.
There has been a lot of good research on bicycle dynamics. I am
no expert but I have followed as much as I can of the
Cornell/Delft groups work, it’s really fascinating. Whilst the
Python is very different to a DF, it is still a two-wheel,
single track vehicle, so it’s subject to all the same forces as
any other bike. So their findings are still of relevance, with
adjustment.
Peter
Other related posts:
