[python] Re: some thoughts
- From: "25hz" <25hz@xxxxxxxxxx>
- To: <python@xxxxxxxxxxxxx>
- Date: Thu, 13 Oct 2005 22:29:31 -0400
> But the gyroscopic torque is 90degrees to the pivot axis. In fact the
> gyroscopic torque was there, and it was working in on your bench. If the
> bike would have been free standing, then the bike would have fallen over
> when rotating the front end.
Ok, if at first you don't succeed . . .
So, tonight, I took the little wooden stirrup I use for a kickstand, and put
it under the main frame section with another 2 x 4 on it so the python would
be suspended, at pretty much the center of balance, with the wheels off the
ground. I steadied the python with one hand and initially put the rear
wheel on the ground to temporarily aid stability. I put the drill to the
wheel again and spun it up. Then, holding the front fender, I gently
rotated the python even, so it was balanced again, and gently pivoted it
from side to side. Again, the python felt no different than when I did the
same thing, but the wheel was not turning. As the front wheel both moves to
the side AND rolls slightly to the left and right AND drops down, to the
rear as it turns, I can't see any axis that the wheel doesn't change as it
turns. At any rate, if you want to call it gyroscopic action for steering,
that's cool. I can feel the force you mention on a normal mountain bike or
road bike (but only very slightly), but none on the python.
> AFAIK, The gyroscopic torque felt will be lineair with the speed. And is
> quadratic in the wheel size.
Granted, but the point I was trying to make was that I didn't hardly need to
rotate the unmounted wheels at all, and the gyroscopic force could easily be
felt immediately, even when the wheels were in a normal upright bike. I
couldn't feel anything close to those same sensations in the python at all.
> My question is why would a bike be different at high speed than at low
> speed? I can really not believe that the answer is "pivot angle". If you
> are willing to drop the notion of gyroscopic torque, what is there left
> to actually give you physically the feedback of that something is
> qualitatively different?
I think it could be different for a number of reasons.
- first, it handles differently when you are applying drive power to the
front wheel and when you are coasting. Applying power produces a rotation
of the BB up and to the rear, while coasting produces a rotation of the BB
down and to the front. Applying power puts more down force on the pivot,
which is good, while coasting reduces the down force on the pivot, which is
bad for "feel". Applying power, even if you are at a "neutral cruise speed"
is still at least NOT allowing forward drag that will allow the down force
to decrease. If you were able to get most of your weight below the front
axle height, then even on a downhill while coasting, with your weight trying
to rotate down and under the axle, that should add down force to the pivot -
which would add to stability.
- next, high speed on level ground produces an entirely different feel than
high speed on a hill, at least on the three pythons I've ridden on hills.
On the flat, I have lots of time to make fairly coarse steering corrections,
but at higher speeds on the flat, I have to be faster, anticipate and have
much more finesse. So, technically, while it feels different to me, it's
not the python, but my riding ability. On a downhill though, the steeper
the hill, the more it feels like the PC2 when I raised the pivot angle well
over 70 degrees. I can't tell which way I need to correct the stering
because I've lost the down force "feel" that tells me what the python and I
are doing. So, in that case, the python rides differently because gravity
is no longer acting on the pivot at the same optimum angle that I need it to
be for comfortable control. Conversely, on the uphill, if I'm able to keep
my pedal rpms up, I have no problem with the very steep angle (relative to
the direction the gravity is working) but if I shift too late and start
grinding a little, the "relative" pivot angle can be lower the 45 degrees
and I have a devil of a time keeping a straight line.
- lastly, many bikes, especially two wheel recumbents (more "conventional
designs), have steering geometry that is optimized for a certain riding
style/speed. It's not hard to find comments all over the message boards
about bents that are vicious at low speeds, but absolutely beautiful at high
speeds, or vice versa. This behaviour, on all these bents, is absolutely
related to their rake and trail. Additionally, location of CoG also has an
effect by loading up either front or rear wheel and producing more
instability at certain speeds. I rode a Vision SWB with full suspension
that cornered great above 30kph, but at anything lower than 20kph it had
crazy oversteer that you had to be n top of or you'd wipe out very easily.
> It would be interesting to know how the mechanical trail has an
> influence. Hmm, making the pivot angle more horizontal will make any
> gyroscopic torque more vertical, creating less influence on the side
pull...
>
> Dirk
Mentally changing the traill and picturing the effects, it would seem that
increased traill would tend to have the same kind of effect as increasing
the wheelbase. The longer the traill, the more you would have to deflect
the front end to get the same amount of direction change - the steering
qould feel "slow". Another side effect is it would likely make the pivot
"weight" feel heavier (sort of like using a very low pivot angle).
Conversely, if you shortened the trail right up, it would make the steering
feel very quick and would likely reduce the feel of the self centering
weight. Like anything else, there is likely an optimum traill where the
quickness and steering effort (weight) would be most acceptable to the
majority of riders.
Trying to tweak, or more accurately maximize, the handling of the python
seems to be like squeezing a wet bar of soap. It's a very cool design, but
the 4 or 5 major design components seem to be able to overlap and produce
the same effects as other parts with certain adjustments. It's sort of like
the Olympic rings. With the python, I don't know whether you want all the
component "rings" overlapping enough to produce a common area they all
overlap, or if you want them to just get close enough to have a common
point. Maybe all 2 wheelers are like this and I'm ignorant to these common
problems with any 2 wheel design. I know that building a trike is MUCH
easier :)
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