*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?

**Follow-Ups**:**[python] Re: PSI***From:*Mike Channon

**[python] Re: PSI***From:*Dirk Bonné

- » [python] Re: PSI - Howard Stevens
- » [python] Re: PSI- Mike Channon
- » [python] Re: PSI- Dirk Bonné
- » [python] Re: PSI- Jürgen Mages
- » [python] Re: PSI- Matthias P. Nowak
- » [python] Re: PSI- Jürgen Mages
- » [python] Re: PSI- Gerald
- » [python] Re: PSI- Rhisiart Gwilym