[python] Re: Geometry
- From: "Sam van Herwaarden" <sam@xxxxxxxxxxxxxx>
- To: python@xxxxxxxxxxxxx
- Date: Mon, 7 Mar 2005 20:08:43 +0200
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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