[python] Re: PSI

  • From: Howard Stevens <hstevens94@xxxxxxxxx>
  • To: python@xxxxxxxxxxxxx
  • Date: Sat, 14 Aug 2010 08:05:16 -0700

*More thoughts on PSI( at least this is what I think is the situation)*

PSI happens when there is a pivot in front of the line of the hip joint.  At
the line of the hip joint there could still be some PSI if the force on the
pedals is not applied close to the centreline of the cycle and the
pivot-pedal angle is significant.  Therefore the bottom bracket should be as
short as possible to reduce this lateral distance (known as the Q factor).
If the pivot is behind the hip joint the pivot-pedal angle reduces further
and so the PSI becomes insignificant.  If this is so for Pythons it is
curious that Juergen found more PSI with his PX5 python, with the pivot
behind the hip line.  If the pivot is taken further back, the whole
mechanical situation changes.  As it approaches the rear wheel, the pivoting
movements are more and more of the rear section rather than the front, like
in the RWS trike Trixstar.  The pivot is actually having a castor effect on
the rear wheel and so we get rear wheel steering with positive trail on the
rear wheel.  This is generally considered to be more unstable at speed and
counter intuitive.

What is the best counter measure if you end up with PSI, despite minimising
the Q factor and putting the pivot as close as possible to the hip joint?

   - Modify pedaling style with even cadence, reduce pedal force by use of
   gears, and lateral pedaling Charlie Chaplin style.
   - Handlebars as stabilizers while being held, however it is really
   necessary to permanently counteract any instability.
   - Motorcycle dampers may be the best solution as the dampening effect
   increases as the oscillation increases in range.
   - Rubber cylindrical dampers are used by some Python builders, however
   these would seem to limit the range of rotation?

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