[python] Re: frame geometry and pivot angle
- From: "jan" <jlaube@xxxxxxxxxx>
- To: <python@xxxxxxxxxxxxx>
- Date: Wed, 28 Mar 2012 21:37:38 -0400
lovely looking bike..but don’t really understand your paper example
From: Patrick van Gompel
Sent: Wednesday, March 28, 2012 7:22 AM
To: python@xxxxxxxxxxxxx
Subject: [python] Re: frame geometry and pivot angle
Hey Mike,
I think Pythons are always made this way:
front / back (the '/' is the pivot angle) So in words that would mean you
measure the angle from the back and the hinge is leaning backwards.
I think a Python with a pivot angle the other way would be very hard to drive,
unless you have some very different setup (like a tadpole trike).
You could make a very easy simulation if you want: get a piece of paper and
make a 65 angled fold in the middle. Unfold and put the paper with the edge on
a table (the fold should be at 65 angle to the table). Now, keep one part
vertical (fixed) and move the other left or right. If the corner of the paper
goes down into the able, you will have the python setup and the Centre of
Gravity will go up. If the corner of the paper goes up from the table, you are
doing it wrong ;-p (the CoG will go down and the bike will be unstable)
I made a few animations to get a better idea about how the CoG is affected.
See: http://cycle.free-creativity.com (bottom left: CoG animations) I consider
a setup with a CoG that goes up when you steer, a stable one and a CoG that
goes down, unstable.
For the above paper example you could consider the rear of the Python as the
fixed vertical element. In my eyes this is because when you are just sitting
(not driving) on a Python the rear is kept upright with either your feet or
your hands. When you cycle, it is a mix of both (both the rear and front will
move when steering) but since most of the body weight is attached to the rear,
the front will move easier.
Hope make rattling makes sense ;-)
Patrick
> Date: Wed, 28 Mar 2012 03:22:18 -0700
> From: mpower.lists@xxxxxxxxxxxx
> To: python@xxxxxxxxxxxxx
> Subject: [python] frame geometry and pivot angle
>
> I assume based on what I have read that the pivot angle is the angle
> from the ground to the axis of the hinge.
>
> Does it matter if the pivot angle is measured from the front of the bike
> versus the back? To put it another way does it matter if the hinge
> leans forward or backward?
>
> I see most pythons with the hinge leaning backwards and I had designs
> with the hinge leaning forward. I am concerned that would affect the
> physics of the bike.
>
> Mike Power
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