[python] Re: RE : RE : Re: Python trike ideas

hello everybody,
I found somebody who is crazy enough to help me build the trike and do the welding. 

So I have to decide on one design.

What are the alternatives
a) A very low steering angle, but positive trail. A bit like greg's machine or the banana trike. 
Very easy to build
disadvantage: big turning radius.
What about wheel flutter at high speed and bump steer?

b)      Something like the Jouta: Steep angle, huge negative trail
unstable at speed, even with dampers

c)      Flevo: medium angle, positive trail
no instability issues that I read of, radius okay
disadvantage: high seat position is baaaad for a trike
would it be possible to get the seat lower, keep the steering geometry and still get the legs around the front wheel even in a tight turn? 

d)      P 4
A longer wheelbase cured stability issues. Wheelbase is now 133cm - correct?
What about bump steer and riding on roads with a camber? Does it pull towards the center? 



Why does one build a Python-type trike?

-looks cool
-relatively easy to build, just one steering joint, not two as in a tadpole, short chain, simple drive 



Why should one not build a Python-type trike?
Unless you copy one of the few functioning measured and documented models, there is no guarantee that it will work. Some Pythons behave, others don't.
I believe the physics are not fully understood, or if they are, they aren't by me. No offence to Jürgen! 



What would I like to do?
Take the front boom of the Hipparion with its movable BB (flevo-type). Use a 20" front wheel. A hub gear would avoid all issues about chain alignment that would occur by using a chaingear on this relatively short drive train. But is there a hub good enough besides the prohibitively expensive Rohloff? I'd love to use a 3x8 DualDrive by Sram if I can get it. But again there is the chain alignment issue.
Use Jürgen's idea to take a BB housing for the center joint. Weld pedals to the rear part.
The rear part should be able to be taken apart in some place, both for easier storage and to allow small adjustments in steering angle geometry. Drawing will follow.
Rear wheels (20" like the front, or should/could they be bigger to give a more comfortable ride?) will be put into forks welded to the "T"-shaped back part of the trike- I see they did that in the Raven Tricumbent. If you use BMX forks or unicycle forks you should be on the safe side strengthwise.
Gives you lots of room to store shopping etc between the forks. better than an axle running hub-to-hub. no special wheel hubs (wheelchair-type) needed.
material will be 45x45mm for the front boom, 40x10 around the front wheel and 40x40 for the "T", 1,5mm wall thickness throughout. 

Seat - I haven't thought about that yet... tube frame with webbing?

brakes: Rim brakes on back wheels

So: any fundamental flaws so far?

Yours

Stefan






Jürgen Mages schrieb:
Nobuo's trike - with the small front wheel and large back wheels, the high seat etc. - does it really make use of the "pendulum" effect? Is it self-stabilizing? The center of gravitiy must be too high, I believe! It works, obviously, because he could control it with the handlebars on that mad downhill ride, but why does it work???
The self centering effect does NOT depend on the seat and CoG height, but on the negative trail and the angle of the steering pivot.
The P4 trike: There was some mention, that it could not go faster than 10-15km/h without beginning to wobble. Was that attitude "cured" by lengthening it? Can it go at normal bike speeds (around 30km/h) now? 


Yes, the longer wheelbase (+15cm) cured everything.

Jürgen.
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