[python] Re: Questions about Python-Trike geometry (project similar to Howard Stevens foldable Python trike)
- From: "gdurbrid@xxxxxxxxxxxxxx" <gdurbrid@xxxxxxxxxxxxxx>
- To: python@xxxxxxxxxxxxx
- Date: Thu, 28 Jul 2011 21:19:25 +1000
Frank,
I assume you propose to build a delta trike (two rear wheels), as a Python
tadpole (two front wheels) is a challenging thought. There are several trikes
in Jurgen's hall of fame, but the best documented front-wheel drive
centre-steer delta trike I know of is the Hipparion (Google for it, but avoid
the horses). It isn't strictly a Python, because Esko Meriluoto built it before
Jurgen built the first Python. But it's very much the same sort of design, and
is now in its fourth iteration.
Most successful trikes have a wheelbase of about 1 metre, and a track of 60 -
80 cm. Tadpoles generally have about 1/3 of the weight on each wheel.
Rear-wheel drive deltas such as the Anura and Kettwiesel often have more than
1/3 on each rear wheel. These are not golden rules, just experiences. Your
front wheel must provide traction, steering and braking, so don't unload it too
much.
On any trike, the wider the track, the less the trike will tend to tip over
sideways when cornering. On a Python trike, your weight must be close behind
the front wheel, so the wheelbase should be short. Draw a triangle on the
ground, connecting the points where your wheels touch the ground. Mark a point
under your centre of gravity: the longer the line from one side of the triangle
to the other through this point, the stabler the trike will be. How wide a
trike can be made usually depends on space in the house, door widths and so on.
If the seat is low enough, a track of 66 cm is satisfactory on tadpole trikes.
I would not use less on a delta trike, unless you mean to ride very slowly and
carefully, because delta trikes are very tippy: see the Hipparion site.
One golden rule is that the lower the centre of gravity, the better the trike
will corner and brake. This really means your own centre of gravity: riders
weigh a lot more than trikes. Trikes are completely different from bikes as
regards cornering (though not braking). You should have the seat as close to
the ground as you can arrange: 15 cm is high, and 10 cm is better, unless you
will be riding off road, or over gutters and rocks. The low seat does make it
harder to get off, but you can put a loop of webbing around part of the front
frame and use it to lift yourself up.
Python trikes seems generally to work best with lower pivot angles than Python
bikes. I don't know why, but it seems that 57 - 60 degrees has generally been
found preferable. Since bikes lean when they corner, but (most) trikes do not,
it does make sense that the inverse pendulum should work differently for bikes
and for trikes. Perhaps the lean of the bike increases the effect, so that a
steeper pivot angle is satisfactory, but a trike depends entirely on the
inverse pendulum, so needs more of that effect i.e. a shallower angle. Dirk may
have a view on this.
Best of luck,
George
---- Frank Schmaus <frank.schmaus@xxxxxx> wrote:
> As I got quick reply and really good help last time, I try it again:
>
> I'm planning to build some foldable python trike like this one:
> http://en.openbike.org/wiki/Python_Trikes#Howard_Stevens_Mk2.2C_Australia
> But with larger laguage capacity. On the python page
> (http://www.python-lowracer.de/geometry.html) there's a lot of good input on
> geomtry questions concerning pythons. But how is all that with a python
> trike? I
> gues the requirements and the conditions are quite different. Is there any
> written summary about trail and angle for a trike? I would like to follow
> well
> approved standards instead of having to build several trikes, just because
> the
> first ones are ugly to stear.
>
> Any other parameters e.g. wheelbase, minimum ground clearance, track width...
> are also still quite a myth to me, but those are more bound to other
> requirements of the vehicle, whereas angle and trail are free of choice to
> assure the best performance possible.
>
> Hope to get a lot of good suggestions,
> Cheers from Germany,
> Frank
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