[python] Re: [python]

  • From: George Durbridge <gdurbrid@xxxxxxxxxxxxxx>
  • To: python@xxxxxxxxxxxxx
  • Date: Wed, 25 Aug 2010 09:17:42 +1000

Dirk, although the graphs only deal directly with seat-rise, not with
forces, if you graph seat-rise against steering angle for one particular
bike, the slope of the graph at any point will indicate the turning
effort at that angle.  You would need to compare similar turning-effort
graphs for different bikes to find which geometry was best.

You are obviously right that restoring force is different from PSI, but
it is relevant to the discussion, if it tends to overcome PSI, in the
sense of restoring the steering to the straight-ahead position after it
has been displaced by PSI.

I am not sure about the basic proposition that PSI is minimised by
keeping the steering pivot in line with the hip joint.  Seen from above,
the hips are always displaced to right and left of the steering pivot,
so there will always be some steering torque from pedalling effort.

On Tue, 2010-08-24 at 17:51 +0200, Dirk Bonné wrote:
> I wouldn't count to much on these graphs I calculated. The just do
> seat-rise related to turning angle. These graphs misleaded even me when
> building the pythoon. The main problem with these graphs are that they
> do NOT deal with forces. What would be useful is turning effort related
> to turning angle for different geometries and weights. That way the
> ideal geometry could be found.
> (But all this has nothing to do with PSI).
> Dirk
> On 21.08.2010 15:23, Howard Stevens wrote:
> > Hullo Sprocket scientists!
> > Yes Dirk, it is very easy for me to change the pivot angle because the
> > pivot is attached to a rotating bar which is part of the folding
> > system.  Reducing the angle however lowers the seat level and makes it
> > look rather weird!  That is why I wanted to get this right before I do
> > the final build.  You asked for photos or drawings and now I have to
> > show my ignorance....I don't know how to post a link!?? I'll try to
> > send to your email.   I haven't noticed any tendency for wheel flop at
> > all when turning, so it looks like I have the balance between flop and
> > centreing correct.   I thought your explanations of why the PX5 type
> > bikes can have PSI was interesting, but I think this would be much
> > less with a trike, because of the stability of the rear end.
> > Thanks George for your comments.  I knew of Dirks graphs but I didn't
> > know that Esko's trike had a pivot angle of 53.1 degrees too...how the
> > hell does he even measure that!. It seems that a trikes "sweet spot"
> > might be around that figure...it certainly was very conclusively so
> > for my trike!
> > Cheers all, Howard
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