[python] Stability on coasting

  • From: "Tom Browne" <tcbrowne@xxxxxxxxxxxx>
  • To: <python@xxxxxxxxxxxxx>
  • Date: Sat, 11 Feb 2006 17:50:27 -0500

Hello Pythonauts:


I came across the Python while browsing through a site showing a range of
home builts: http://mywilson.homestead.com/gallery0.html, about three
quarters of the way down you will find 25Hz's version with the full fender
and rear luggage box.


I'll start by stating that I've never ridden a recumbent, partly because
from an engineering standpoint they all look clumsy with lots of doodads to
run the chain from the BB to the rear wheel, more doodads to join the
handlebars to the head tube, etc. Say what you will about the comfort, an
upright is about as simple and elegant a machine as possible. (Of course I
may be converted from this opinion the day I try a recumbent!)


However the Python is a real elegant design (congratulations, Jurgen)
compared to just about any other recumbent I've seen, and I've been
interested enough to go through the list archive for 2005. I was thinking
about the stability issue when coasting downhill, and it occurs to me that
this is similar to the problem of a tractor + semi-trailer combination which
wants to jackknife in the right (actually wrong!) conditions. Like the
Python, there is a pivot (with 90 degree angle), drive is on the component
forward of the pivot, and the weight is distributed between the forward
wheels (through the 5th wheel pivot) and the trailer wheels. As long as the
tractor is pulling the trailer, there is no problem, but when trying to
brake the system can fold up. My vague recollection of this is that a study
of accidents involving jack-knifed trucks showed that no matter what the
driver did (i.e. brake or not, steer into the skid or just let go, downshift
to brake only the tractor drive wheels, some combination of the above, etc
etc), it didn't help any. I also have a recollection (I haven't been paying
attention to research in this area for many years now) that there is a lot
of work ongoing to try and design a tractor-trailer system that won't


So anyway, maybe there is something to learn here from the trucking
industry. The Society of Automotive Engineers (www.sae.org
<http://www.sae.org/> ) has a large database of technical papers; a quick
search turns up a total of 78 papers on the topic of stability of
tractor-trailer combinations, for example "Dynamics of Tractor-Semitrailer
Vehicles: the Jackknifing Problem", by E. C. Mikulcik of the University of
http://www.sae.org/servlets/productDetail?PROD_TYP=PAPER&PROD_CD=710045. (In
the search box I used the keyword 'jackknife' and selected 'Papers' only
instead of the entire site). Unfortunately the papers are not cheap at $US
12 each, and they don't post an abstract on the web page so you can't really
evaluate if this is the paper you want, but perhaps some more digging on the
web will turn up less expensive info, for instance
http://www.linkrad.com/anatomy.htm. The one major difference is that a
tractor-trailer (like a trike or any 4-wheeler) goes around bends due to
side forces on the tires while a two-wheel vehicle (motorized or not) leans
into it so there are no side forces on the tires, and thus tire slip is less
of an issue (maybe not an issue at all). 


With no immediate access to welding gear I won't be building anything just
yet, but I look forward to following the Python saga, and will look for
decent scrap bikes to serve as donors for an eventual project once I have
the time and cash to invest (I just spent a pile on a new 8.5 kg traditional
road bike, so a welder is off the budget for now). 


Meanwhile I dream through the Canadian winter of the annual summer tour of
Quebec: http://www.velo.qc.ca/voyages/travel.lasso?page=evenement
r> &surtitre=grandtour. A handful of recumbents usually turn up for this;
typically it represents a fairly hard 650 to 800 km in a week, with a large
number of short but challenging climbs. The social side is fantastic with
2000 cyclists, live outdoor music and pub in the evening, obscure art films
that don't play in the Cineplex, etc etc. Great fun and very highly
recommended. Anyone interested?


Sorry about the length, I'll try to keep future posts short.


Tom Browne

Montreal, Canada 

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