[python] Re: geometry
- From: Dirk Bonne <dirk_bonne@xxxxxxxxxx>
- To: python@xxxxxxxxxxxxx
- Date: Tue, 27 Jul 2004 10:11:20 +0200
Bernard Boden wrote:
hello
on the python webpage you point that the center pivot has to be lower
then the wheel axis
I am not convinced by your arguments,
Do you ever tried a higher position (on a scale model?)
Bonjour Bernard,
Me neither. I do not not think that is important at all. Its the trail,
size of the wheels and angle of the pivot that determines if/how much
the seat position goes up when turning. If you move the position of the
pivot along the pivot axis nothing will happen: the bike will have the
same properties (which is good for the 20inch versions ;-) ).
Did anybody actually try to calculate seat position in fct of pivot
angle? I have tried (with octave ---a kind of matlab), but I am still
debugging the formulas. The debugging goes awfully slow because I am
rusty in linear algebra. May be there is a somebody here on the list
with a fresher mind and/or lot's of time!
I am going to lean out of the window here, and make a fool of myself.
Here is my theory: it doesn't matter.
1) at slow speed. Look at a flevo: a positive trail will cause the seat
to go down when turning. And yes it is also ridable at slow speeds. The
center seeking property of the negative trail may help, but seems not to
be necesary ---you can learn to cylce nearly anything. The real
problems/dangers come at high speeds:
2) when speeding up, negative trail and front wheel drive can only be
good. It is like pulling a trailer.
3) when riding fast, it is the momentum of the wheel that stabelizes the
bike. And these forces are higher then any effect of rising the seat a
few millimeters (you aren't turning the pivot a lot at high speeds).
IMO, positive/negative trail doesn't matter here, simple because we
aren't actually turning.
4) when braking, there is this possibilty about doubling up the bike
(folding). If you brake in a straigth line trail doesn't matter (forces
are swallowed up). If you brake when the bike is not aligned, it will
clearly depend on the steering angle (amount of turning). At high speeds
again, you never turn much, and your legs are attached to the pedals
which counteracts this folding force.
So in my view the weak point of negative trail is when braking at high
speed (not caring about low speeds). But how much does it matter? It
needs to be related to the other forces acting on the bike. This is
over my head to put together a theoretical model. But seeing that Jürgen
and (many?) others are still alive and well, it seems to be no big deal.
Does anybody think this makes sense?
Dirk
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