[python] Svar: Re: PSI

  • From: "Thomas Paul Jahn" <tpj@xxxxxxxxxx>
  • To: <python@xxxxxxxxxxxxx>
  • Date: Tue, 17 Aug 2010 10:52:10 +0200

and who won the race? ...just for curiosity ;-)

>>> Jürgen Mages 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.


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.
> people I know who ride pythons have no PSI to talk about. Of course
> 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
> there is some occilation but it is probably a few mm's (diffcult to
> a number), in turning degrees it must really be dwindling minimal. PSI

> on a handlebar recumbent on the other hand is large. Ride together
> some of the mainstream handlebar steered recumbents, and then you'll
> 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
> 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
> 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
> Though I do not think that PSI and trail are related, I do think that
> 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
> 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
> 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
> With the flevo, it regulary happened that I fumbled when starting at
> 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
> 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
> trail that did it in the case of the 70degree pythoon).
> note2: I also tried a friends 60degree flevo and felt it rode very
> - 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
> 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
> (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
> bike, instead of lessening it.
> Moving the pivot to the back of the seat might or might not help -
> 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
>> 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
>> (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
>> like in the RWS trike Trixstar.  The pivot is actually having a
>> 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
>>       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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