[python] Re: pivot angle

I finally got moved in and set up enough to get back into building in my new
garage.  The first order of business was to make a new seat for the python
out of 3/4" EMT.  It seems solid enough to support my weight and is pretty
light.  I'll add pictures when I get some time.  I also raised the seat by
adding about a 5" riser to get the seat height up closer to the axle height.
I'll take some more precise measurements tomorrow as well.  I hooked up a
rear brake and the "rear" derailleur so I can shift and stop.  It's been
raining pretty steady here for a few days so haven't been able to test ride
it.  I need to paint it though as rust is a continuing problem lately.  I'll
take a final measurement of the pivot angle as well because I have a
slightly fatter front tire on it so the angle might be closer to 68 or 69
degreees now instead of the original 70.

I am already planning the 2nd python, and went out and bought some 12.7mm
female rod ends to use as the pivots - similar to what Mike Channon did, but
with a little twist.  I made a quick 3D version of it and U/L'd it here:
http://bikes.jkcc.com/python%20pivot%20mk2.jpg

The rear frame material is essentially identical to the first python, but
the chainstay steel is 1 1/2" x 1/2" instead of 2" x 1/2" like the first
one.  Instead of a "C" shaped section to connect the rear of the bent to the
front, I am only going to use a 1/4" plate and will drill 2 holes to put the
1/2" rod ends through.  The pivot tube that is welded to the chain stays is
nothing more than a piece of 3/4" steel tube, with 1/2" ID.  I'll use a 1/2"
bolt for the pivot, and when the 1/2" bolt is tightened, it will tighten the
two rod end balls against the end of the 3/4" pivot tube, and the pivot is
complete.  I can fine tune the actual pivot angle by simply adjusting the
length of one or both of the rod ends.  This way, I can find out what angle
is best for the trail and CG height of the bike.  A little oil or grease on
the rod ends should be sufficient as they don't actually have to rotate
much.


> > Is there any consensus on the best pivot angle?
>
> That is THE question!
>
> If my name was "Deep Thought" I would have said:
> 42 degrees ;-)
>
> Serious: I believe in degrees between 60 and 70, whereas
> 65 is my favorite.
>
> The optimum pivot angle would be the one where the self
> centering effect is at max and the wheel flop effect does not
> occur.
> The wheel flop effect depends on pivot angle, trail and the ratio
> between weight of the rear mass and weight of the turning front mass
> (including legs!).
>
> If the angle is very steep (70+) you are on the safe side concerning
> wheel flop, BUT self centering force is weak and the bike might get
> too nervous and instable at high speeds.
>
> If I had to build another python, I would choose 65 to 67 degrees.
> (Anyway - as you have seen it´s easy to change the angle later on)
>
> Regards,
> Jürgen.
>
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