[python] Re: Re.: Sketches of four-bar-linkage design
- From: Rasmus Emil Møller <aer@xxxxxxxxxxxxx>
- To: python@xxxxxxxxxxxxx
- Date: Wed, 25 Aug 2004 13:00:54 +0200
Hi,
Thanks to Torben Scheel, I got my first Python-
hand-scratch, plans, measures, 20 m of exp. and
a lot of enthusiasm! I am gonna get there soon!
Before I get too much real-life experience,
now is the time for me to theorize :^)
Disclaimer: I may be stating the obvious,
or utter nonsense - you have been warned!
>> what is the purpose, or the gain, of the
>> four-bar-linkage design?
>
>4bl could be a solution for the main problems
>of all center-steered lowracer constructions by
>generating a virtual pivot with positive trail.
>
>If you want positive trail with a normal pivot,
>the steering angle has to be very flat, resulting in
>gigantic turning diameters and unacceptable
>wheel flop.
I have given a lot of thought to the + trail relation
to stability, and voila some ramblings:
At low speeds, where gyro effects are negligible,
the + trail for a normal bike automatically corrects
small inclinations to the left or to the right.
If by accident you and the bike lean a little to the
left, your weight pushes the front wheel to turn left
because of wheel flop - which corrects the inclination
to the left by steering the bike in under where you
would otherwise be falling. The slight bend at the fork
just before the axle limits the turning angle caused by
this effect to 30-45 degrees. You can notice the effect
when parking a normal bike with a normal left-sided stand.
The front wheel will automatically turn a little to the
left at equilibrium.
Ok ok, I see that you can obtain a positive trail seen
from the virtual pivot, but
- if the seat is fixed to the rear, the turning of the
front wheel makes it touch the inside of you thigh
(upper leg).
- if the seat is fixed to the front, your own weight
makes the bike behave exactly opposite of what you want
(like if - on a normal bike - your fixed a chair to
the steering bar and put yourself in it)
In any case the FBLD severely limits the max steering
giving VERY large turning radius.
>I tried the negative trail with the python, to avoid
>this. The result is surprisingly satisfying,
>but downhills with speed above 50 km/h I miss the
>safe feeling of a bike with positive trail.
I wonder if the unsafe feeling is because it is hard
to brake and steer at the same time? Anyone tried that?
Rasmus
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