[python] Re: Thoughts on Python Stability
- From: UH-13P <uh13p@xxxxxxxxx>
- To: "python@xxxxxxxxxxxxx" <python@xxxxxxxxxxxxx>
- Date: Fri, 29 Jun 2012 09:09:00 -0700 (PDT)
Like you I have not ridden a Python yet. However, I have ridden a swing bike
for about 5 years. The center pivot below the seat and at the seatpost angle
finctions much as a Python pivot providing seat rise. If just riding straight
and not having to pedal up hill the bike can be ridden by nearly anyone and is
much easier to ride than a recumbent. The seat rise center joint is very
stable with hands on the handlebars. That said I have yet to ride hands off.
Seat rise in this situation simply does not work. Now if you think of taking
your little finger and sliding an empty coffee cup on a smooth table and then
reduce that force to 1/2 of that level to probably 1 or 2 grams. If I lay one
finger on the handle bar I can ride for miles with left and right pressure and
very seldom forward or back on the bar end. With the thought that this very
little force on the handlebars acted at stiffening the headstock I tried to
lock the front
headstock bearings and ride with just the seat rise center pivot and huge
negative trail. I found this impossible. Rider input even at an almost
unmeasurable level is very important.
________________________________
From: Peter Clouston <peter@xxxxxxxxxxxxx>
To: python@xxxxxxxxxxxxx
Sent: Friday, June 29, 2012 1:57 AM
Subject: [python] Thoughts on Python Stability
Hi Everyone!
This is my first post here. I have followed the Python story with interest for
a while. It is fascinating that a bike that breaks so many of the "rules" for
bicycle stability can nevertheless be ridden (By some at least). Even though I
am not a Python rider, I hope that this post may be of some use to those that
are, or are about to be Python riders..
One argument, strongly advocated on the Python website, is that the reason that
the Python can be ridden is the steering centering effect of the rider's
weight. I am not arguing against this but I note that the part of the rider's
weight that rests on the pedals has the opposite effect. Jurgen has also
alluded to this but it is not mentioned very prominently on the Python websites.
The centering effect is technically "negative wheelflop" and is an automatic
consequence of negative trail, since wheel flop is defined as Trail x sin(A) x
cos(A), where "A" is the steering head angle. However, discussions on wheelflop
in standard diamond frame (DF) bikes usually claim that positive wheelflop has
a small but positive benefit for stability. Even if the Python's negative
wheelflop was beneficial for stability, because it's effect is a constant for a
given bike and rider and all other dynamic influences on steering increase with
increasing speed, it will be fully significant at lower speeds only.
It may be that the Python is in fact quite unstable and can only really be
ridden by riders of exceptional balance and trained reflexes. There is a case
of a rear-wheel-steer bike of mathematically proven extreme instability that
"nobody" could ride. It was therefore classified as truly unridable, until a
circus acrobat got on and rode it away, no problems! Maybe the Python is a
less extreme case of that phenomenon. (This is NOT a criticism of the Python,
if you have one, can ride it and enjoy it, why not?)
Several people on this discussion board have posted saying that they were about
to post a mathematical analysis of Python stability, either using JBike6 or
something like Matlab. Maybe I've missed something but I've not seem any
results here, so far. Anyhow, there are potential problems in doing this for
the Python, or for any Moving Bottom Bracket (MBB) design. Pappadopolous has
noted that JBike6 is probably a better model for standard recumbents than for
DF bikes, because the rider does not move his weight relative to the frame, as
happens with DF bikes (Body English). However, in MBB bikes, the effect of the
part of the rider's weight that is on the pedals acts quite differently to the
rest of the rider's weight. For the Python, I have already mentioned the
opposite effect on wheel flop for instance. It would be hard to model this
effect in JBike6 because the riders feet and lower legs are connected to the
rest of him by the hips, which are
neither rigid connections nor free-swinging ball-joints but something in
between. Indeed the stiffness of the joints will vary, depending on how much
effort the rider is putting in and also how relaxed he is. JBike6 would have
to assume a rigid joint.
The JBike6 developers from the Cornell/Delft team have shown that a marginally
unstable DF bike can be stabilised merely by putting a weight in the handlebar
basket. The lower legs and feet of the Python rider have the same tendency.
The effect will be less if the rider is tense, because this will cause him/her
to act as a rigid body instead of a freely jointed one.
I should define what I (and JBike6) mean by a stable bike. I mean a bike that,
without needing any input from the rider, will steer in such a way that it does
not fall over (roll stability) and tends to track reasonably straight on rough
ground (Yaw stability).
With positive trail, if the bike and rider start to fall over to either side,
the imbalanced weight to one side will cause the bike to turn into the
direction of fall. This turning results in centrifugal force that picks the
bike and rider up again (Roll stability).
The bike must also recover from any disturbance of the front wheel (Yaw
stability). If the front wheel is twitched to the left, the bike will of
course tend to turn left. This is resisted by the inertia of the bike and
rider, resulting in a an inertial torque to the right. In a bike with positive
trail, the effect of a rightwards torque is to turn the steering right, thus
canceling the original offset to the left. This is what caster action is. (I
have seen caster "explained" as being due to the tyre contact patch being
dragged behind the steering axis, but of course, unless you have the brakes on,
tyres do not drag and even in braking the drag is directed through the steering
axis and has no leverage to affect the steering.)
Since the Python has negative trail, as normally defined, it is unstable in
both roll and yaw, unless the effect of the component of the weight on the
bottom bracket can overcome the negative trail effects. On this list someone
has already noted that the bike is easier to ride if the shoulders are firmly
braced against the seat back. Some of this effect may be due to the reaction
force of the feet on the bottom bracket tending to stretch the bike out,
therefore straightening it. However, consider taking this to extremes, where
the rider arches his back so all his weight is carried on the shoulders and
feet. The weight on the shoulders is close to the rear wheel and thus has
little effect on steering. All the rest of the weight is on the bottom
bracket, forward of the steering axis. He will have changed the dynamics of
the Python from those of a bike with negative trail to one where the effect is
of quite a lot of positive trail!
Of course, it would be pretty darned hard to ride the bike with your body
arched like that, Riding by planking seems like a lot of work! Also, the
wheel flop would be huge, so this technique could only be useful in stabilising
things in emergency at high speed.
Note that it is mostly the forces sideways to the plane of the bike frame that
affect steering, especially at speed. Forces in line with the frame affect
steering only though wheel flop, where the Python is OK. If the Python seat
was designed so that it pivoted about a point above the rear hub and the front
of the seat was supported but able to swing from side to side, the stability
benefits of the "planking" technique I have described would be gained, without
requiring effort from the rider and without upsetting the balanced wheelflop of
the normal python. Perhaps Jurgen's Hammock type seat may allow some sideways
movement at the hips? If so it may make his bike easier to ride than Pythons
with conventional recumbent seats.
I have no idea whether having a pivoting seat would have a negative effect on
steering and pedaling. Is anyone game to experiment? It won't be me alas, my
bike building skills are nil, my legs are way too short for a Python and if I
don't finish the new house soon my wife will have words to say1
Peter Clouston
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