[python] Re: PSI

  • From: Dirk Bonné <dirk.bonne@xxxxxxx>
  • To: python@xxxxxxxxxxxxx
  • Date: Mon, 16 Aug 2010 12:19:10 +0200

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.


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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