[python] Re: trail vs wheelsize

Am Samstag, den 07.10.2006, 20:28 +0200 schrieb JÃrgen Mages:
> On 07.10.2006 09:55, Dennis Budding wrote:
> 
> > when I make a python with the data of for example P3, same trail, 
> > same pivot angle and same wheelbase but with 18 inch wheels can I 
> > expect a bike with the same riding behaviour or wil it be completely
> > different. (so has the wheelsize also something to do with the 
> > steeringbehaviour)
> 
> The steering behaviour is surely influenced by the wheelsize.
> Bigger wheels make the front part heavier and this counteracts
> the centering effect. Also small wheel python riders report
> downhill coasting speeds of 60 kph and above while bigwheelers
> have their limit around 50 kph.
> 
By announcing my project ( http://ovis.dyndns.org/drupal/node/183 ) i
can second that. My quick an dirty python was welded together in two
evenings, just to try the concept. All i had was a 28" front wheel. not
only had i difficulties in reaching the pedal, it felt very sluggish.
The tubing for the front has 2mm thickness, which makes it together wit
the wheel very heavy. Once you get off your desired direction you have
to really fight to get it back on course. Quite often, this leads to a
continuous left-right-left-right pendulum  untill you crash :-((
I must say that i only had only few hours of time riding it. In the end
i used wooded sticks  (imagine skiing a python :-) to counter the
sluggish behaviour. With sticks you don't have to lean over to touch the
ground for a course correction. I immediately got longer runs. I imagine
a tiny slope that gets you just running could be the best training
ground. You don't pedal and just focus on the steering business.


> But as there are too few experienced riders around who can try
> different pythons there is no scientific basis for a good theory
> yet.
> 
That is my question: what is the best python setup? I was trying to do
some calculations, much like those on the website. I realised i need
vector math to figure out the up and down travel when steering... 
I was thinking to produce an javaapplet, that one can start in a
webbrowser. You enter your measurements, and get i.e. your up and down
travel. I think wheelbase, trail, wheelsize, "the angle" and the
distances between the steering pivot and the axles all have the
influences...

> > when I shorten the wheelbase and keep all other specs the same does 
> > that influence the self centering effect or only the turningcircle.
> 
> If you shorten the wheelbase more weight shifts to the front part
> resulting in less centering effect - I think.
> 
> > I'm planning the 18inch bike based on the bike on the foto, only 
> > adding a second front fork and an optima bike seat. because my 
> > weldingskills are a little bit less than zero I'll have to find 
> > someone to weld it for me. hopefully this month
> 
All i had for welding was an old "electrode?" welding apparatus. It was
basically not possible to weld bike frames with that :-)

Would it be possible to build a flexible python, where one could adjust
all things that possibly have any influence in the driving experience? I
would then go on and modify them one by one to the extremes.

Another thing i was thinking about: If you push one pedal forward, the
python wants to line up the middle of that pedal with the steering
pivot. The ground contact point of the front wheel acts like a second
pivot point. If you turn left, the steering pivot moves to the right.
But since the driver sits behind the steering pivot, he can balance out.
So the python PX.5 on the modification list has the driver in the front
of the steering pivot - hence he can not stabelise a starting
sluggishness. Only the self-centering effect it counteracting.


Regards,
Dirk

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