[python] Re: Svar: Re: PSI

  • From: Jürgen Mages <jmages@xxxxxx>
  • To: python@xxxxxxxxxxxxx
  • Date: Tue, 17 Aug 2010 11:20:35 +0200

The ZOX-rider was slightly faster, but he was tailfaired ;-)


On 17.08.2010 10:52, Thomas Paul Jahn wrote:
and who won the race? ...just for curiosity ;-)

 >>> Jürgen Mages<jmages@xxxxxx> 17-08-10 10:32 >>>
I agree - the PSI on pythons, airbikes and flevos is not really an
issue. In the contrary: I once had a little race with a ZOX20 rider
(about 40 km/h) and a friend riding behind us told me later that my ride
was much more straight than my competitor's. So much for that.

Cheers,
Jürgen.

On 16.08.2010 12:19, Dirk Bonné wrote:
 > Hello Howard,
 >
 > Somehow the pivot angle is missing totally from your picture of PSI.
 >
 > But first: it is a little strange to read all this stuff about PSI. The
 > people I know who ride pythons have no PSI to talk about. Of course the
 > BB is movable, so you will end up with some movements - but no large
 > pedalling induced movements. On my bike, I can hardly see PSI, I am sure
 > there is some occilation but it is probably a few mm's (diffcult to put
 > a number), in turning degrees it must really be dwindling minimal. PSI
 > on a handlebar recumbent on the other hand is large. Ride together with
 > some of the mainstream handlebar steered recumbents, and then you'll see
 > the wheel wiggling left right all the time. I am sure they have much
 > more PSI than I have on my python. (I'll see if I can make a video of my
 > BB/wheel movements while riding).
 >
 > That said, I must add that the python is more lively than the flevo. I
 > do remember that in the beginning of riding the python I did have
 > considerable PSI, which went away after getting used to bike.The
 > *tendency* for PSI on the python is greater than on a flevo. With the
 > consequence that it takes longer to learn to compensate (an unconscience
 > part of riding, I couldn't tell you about how it is done).
 >
 > I know for sure, that the tendency for PSI is in direct relation with
 > the pivot angle (and not much with trail). I have now ridden:
 > * 45degrees pos trail (flevo race, 20" wheels)
 > * 60degrees neg trail (baby python/ last version of the pythoon/and
 > tried a 26" python)
 > * 70degrees neg trail (first version of the pythoon)
 > * 80degrees neg trail (second version of the pythoon)
 >
 > About wheel size: my experience is with small wheel sized centered
 > steered bike, which automatically means that the trail is rather small.
 > Though I do not think that PSI and trail are related, I do think that a
 > 20" python will beat a 26" in agility (just like a 16" python beats a
 > 20" python). About experience: Flevo and python I have much experience.
 > The 70/80degrees version less so, but still, I rode each of them for
 > minimum 200kms, so I have had some time to get used to these angles)
 >
 > The tendency for PSI rises with pivot angle. Already at 70degrees I
 > found it annoying, and it was a challenge to completely compensate it
 > away. At 80degrees it could even worse. On the other hand, at 45degrees
 > the flevo rode like a train. The 45degrees pivot is PSI-wise really
 > better. One could ask why not a python with 45degrees? Well I think
 > you'll pay with less agility and more difficult handling at slow speeds.
 > With the flevo, it regulary happened that I fumbled when starting at an
 > intersection in a stress situation. This is something that is really a
 > thing of the past since I began riding the python, and it is the main
 > reason why I like the python so much.
 >
 > note1: interesting/strangely enough even thoug the PSI is larger on
 > 70/80degrees, the agility of them were bad. At 80degrees I felt it was
 > simply the ergonomics of turning that was getting difficult (difficult
 > to fold at the hips sideways to turn). At 70degrees the bike didn't want
 > to turn - there was so much self centering, that I had to some hefty
 > weight lifting to turn, I could even feel my seat rising! (I am not
 > saying that all 70degree pythons are bad turners, I do not know that,
 > and it could have been a combination of the long wheel base, the larger
 > trail that did it in the case of the 70degree pythoon).
 >
 > note2: I also tried a friends 60degree flevo and felt it rode very well
 > - I would describe the experience like: "a python". So I have the
 > feeling that pso/neg trail has little to say about PSI.
 >
 > About dampers. On the flevo there is a rubber damper. I have ridden the
 > flevo with and without (because it tended to rip over after a year). I
 > didn't notice any adverse effect on PSI by the damper. IMO, the damper
 > is not there for riding, the damper is there for parking the bike.
 > Without the damper the flevo folds and falls over when not carefully
 > placed (more so than the python). The damper did have effect on handling
 > (i.e. turning ability) of the flevo - it worsened.
 >
 > So I am not a believer in mechanical solutions (i.e. dampers) for PSI.
 > Except for the handlebar which actually adds to the human input into the
 > bike, instead of lessening it.
 >
 > Moving the pivot to the back of the seat might or might not help - this
 > could be tried. But you have to think about the ergonomics of turning
 > too. It might well get physically difficult to do some sharp cornering
 > with the bike!
 >
 > May be pivots with angles smaller than 60 but still larger than
 > 45degrees might be a real improvement. Might be that for example a
 > 55degrees pivot might keep the easy slow speed handling of the python
 > but make the tendency for PSI less.
 >
 > groeten,
 > Dirk
 >
 > On 14.08.2010 17:05, Howard Stevens wrote:
 >> *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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