The following tells of my exploration into the wonderful world of bicycle building. Some time back I stumbled on to a web page with an extremely interesting center steered recumbent called the "Python" designed by Jürgen Mages. I can't recall exactly how I got there but it was while searching the history of recumbents. The last few stops were basically the Challenge company which makes the infamous Hurricane recumbent. They use Ligfietsshop Tempelman for some development which in turn had its roots in the Flevobike - an early Dutch center steered bike which was one of the pioneers of all modern recumbents. This in turn led me to the pioneering German center steered recumbent called the Airbike. Finally this then led me to Jürgen's page on the "Python", a lowracer evolution of the Airbike.
The design hit me immediately and on many levels! I was extremely impressed! One look at the bike reveals it as a clean distillation of the essence of a recumbent. I loved the idea of what it might be like to sit nestled low and fast between two big wheels (even though I already do own a Streetmachine GT). I was intrigued by the audacity to design a marginally stable bike with no handle bars. I could see the front drive eliminated the usual problem of the long circuitous chain-line on a lowracer recumbent and the itinerant steering issues. I downloaded the 8Mb QuickTime movie "python.mov" and I was sold. I couldn't help but smile watching Jürgen put his feet on the pedals and use his hands on the ground to "walk" forward to get a start then ride off no-handed. Yes this was a bicycle I had to try!
As you can see from the picture above this bike is different than most. The typical front fork is absent and instead a wheel frame is used which pivots under the seat more or less in the center of the bike (hence "center-steered"). On a typical bike a line extending from the fork pivot intersects the ground ahead of the front wheel contact-spot making the bike stable. On this bike the extension is behind the wheel. By itself this would make the bike unstable i.e. this tends to make it like pushing a shopping cart wheel backwards. The front wheel wants to flip around backwards *but* because the pivot is angled backward putting weight on the seat (you) tends to cause the seat to want to attain the lowest vertical position and in doing so it turns the front wheel in a straight forward direction. This is called the "hanging pendulum" effect and it adds stability to the bike. It should be noted that increasing the back angle too much will lead to "wheel flop" - not good. On the other hand as this back angle is reduced sensitivity increases until eventually instability will become a problem. It appears 67° is near optimum. Additionally being a front wheel drive pedaling tends to "pull" the bike along adding yet more stability. From rider commentary it appears these forces tend to reach a balance point at high speed and much discussion on the Python Group List is towards design ideas directed at raising this force balance-point to increase top speed.
Because the bike geometry is so different it takes different skills to ride it and some time must be spent learning to acquire this. The payoff, however, is a ride that is described as almost metaphysical in that the bike and the rider form a symbiosis.
Apart from the dynamic aspects one look at the bike and you can see that Jürgen has come up with design that lends itself to home-building more than any other bike I have seen. It uses low carbon steel tubing and recycled bicycle parts and yet in the end it has a very refined look to it. As I later went over the design I couldn't help but marvel at the efficiency with which Jürgen had achieved all the bike's functionality.
Before beginning I studied Jürgen's info and read the Python Group List (later joining) in order to better understand the bike and to see if I could come up with a design that would work for me. The part that was difficult, for me, is that you either need long legs (inseam measurement) to stay with 26" wheels, go to a smaller wheel (I didn't want to do this as I loved the bigger wheels). The other alternative was to compact the front end beyond what was recommended to the point where deraileur chain-line issues could be a problem and a multi-speed hub would have to be used. Compressing the front end would also require my getting thin 10mm wide metric tubing (to get into the BB past the front chain rings). Not one to give up on something so basically cool I started using Pro/E CAD to develop some plans to work this out. Most of my design time was spent on tackling the many fronts of this single issue. These were fundamental issues and until I solved them there was no point in starting to build. On the other hand I could see once it was solved it looked like it should go quite quickly and it appeared I had a lot of the equipment to do this. If you are unfamiliar with Pro/E most of your time is spent setting up the basic part and drawing relationships. Then you can essentially change the model to your hearts content and the drawings update automatically. This is just what I needed while I experimented changing this and that. The only parts I deviated from Jürgen's design are first, compressing the front end to the absolute minimum (with trade-offs he mentions) to retain the large wheels and secondly, the pivot where I used an angular-contact bearing approach as opposed to the bottom bracket approach. Below you can see my results so far with the plans in PDF form and a screen snapshot of the 3D model.
The link to the construction photos will be activated when I complete the building of the bike (i.e. the link is not active yet).
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