[python] frame geometry (Was: Re: How tall are you?)

Hillel,

> We had a discussion a little while back about the relationship
> between trail and stability.  Jurgen pointed out that the python design
> depends on negative trail (with the 80-degree pivot angle) for
> stability.  Other center-steered recumbents with higher seats require
> positive trail (and a pivot near 45 degrees).  I am not sure at what
> seat height the requirement changes.  Perhaps it is whether the seat
> bottom is above or below the front wheel axis, or maybe whether the
> pivot is above or below the wheel axis?

The negative trail itself destabilizes the bike. The python geometry is
special in one thing: The pendulum effect is stronger than the NT force
and the wheel does not flap when braking. If the seat is not lifted through
steering there is no pendulum effect. So the seat height does not really
matter. Maybe Peter can explain this better because he is more
familiar with english.

Jürgen.

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