[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.
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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