[python] Re: Geometry
- From: "Sam van Herwaarden" <sam@xxxxxxxxxxxxxx>
- To: python@xxxxxxxxxxxxx
- Date: Mon, 7 Mar 2005 22:52:16 +0200
Hmmm...
And I noticed a mistake:
pivot angle should be (90°-pivot angle), if done otherwise the centering
effect would become more with an increased angle.
Maybe I should write an essay with diagrams instead of trying to explain it in
words this way :)
And I have to think of a reason why in the translation from wheel descendance
to seat rise somehow 55° overrules all other angles...
Lots of stuff to think about still!
Sam
> Oops - I forgot to add to that that I don't think seat height
> changes the steering very much (except for leaning) and I wouldn't
> be surprised if the difference in agility between PX.1 and PX.4 is
> because of the increased trail
> (I don't know if you measure effective trail or projected trail, but
> with a 33% increased effective trail I think the descendance of the
> wheel increases by the same 33%). Sam
>
> > OK, I've been thinking about this stuff for quite some time now -
> > and I think I've sort of figured part of it out: Because the pivot
> > is mounted under an angle, when the frame is twisted around the
> > pivot, the entire front side moves downward (when visualized without
> > a ground) - this is easy to imagine when you put an hand at an angle
> > and twist it, the tips of your finger clearly move down. The amount
> > the tip moves down when the hand is rotated 90°, is equal to
> > "tan(pivot angle) x finger length". You can see that it is at the
> > same height as the "pivot" if you do the hand-trick. The finger
> > length in bicycle terms would be the true trail (not the horizontal
> > distance between the pivot and contact point of wheel and road, but
> > as shown in the picture as EN - effective trail, not clearly
> > readable but in the middle of R PN and BH). To know how much the
> > wheel descends for a certain angle, somewhat the same has to be done,
> > but more complicated :) : "tan(pivot angle) x effective trail -
> > tan(pivot angle) x cos(turning angle) x effective trail" - first you
> > calculate how much the contact point is *in front of* the pivot
> > (neglecting the distance to the side), then you calculate the height
> > of that point (relative to the height at a turning angle of 90°),
> > and in the end you determine the difference in height between that
> > point and the contact point. For clarity: what was just calculated
> > is the amount the wheel moves down in a no-road-situation with a
> > certain pivot angle and a certain turning angle. In a "road-
> > situation" this would basically mean the shape and distances and
> > stuff are the same, only instead of the wheel moving down, the bike
> > goes up. I'm sorry if this is unclear, I'm not very good at
> > explaining and this is a hard subject, if anything is unclear please
> > say so. Calculating the rise of the seat is much harder, but with
> > logic you can think up that you need the variables effective trail,
> > head angle and wheelbase. I have been thinking about this for like
> > the last entire year (not full-time, don't worry :)) and would like
> > to hear all opinions. Thanks to all of this, I believe that the
> > Geometry part of the python site is incorrect when mentioning that
> > the pivot should be below the axles, since I don't believe axle
> > height has anything to do with the geometry. Also I think there
> > should be added that not only a 90° head angle doesn't work, but
> > everything with a negative trail. Note: I didn't do this because I
> > want to prove Peter Oliva wrong or anything, but maybe it clears
> > matters up a bit. Next to that I would really want to know if and
> > why anyone thinks I am wrong :) Sam
> > P.S.: Maybe you noticed, but I am one of the over-planners/over-
> > thinkers that thinks way too much before starting to build, but I
> > think I am finally ready to build a Carbon Python (with an adapted
> > frame though) this spring/summer.
>
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