A Python For Marc

In the September month of 2005 Marc & Leen came to visit us here in Copenhagen. While Leen enjoyed the very fine weather, Marc and I where at kvisten to built a recumbent bike together. The plan was to built it inside the period of a week so Marc could take it back home with him.

Not wanted to create the same design again, we decided to build the frontpart out of thin tubing. Mostly out of recuperated seat stays of junk bikes. The bike is nearly wholy built out of recycled material, an aspect I am very much fond of. Still, some of the tubing is bought --10x1mm.

As a result the bike is *really* stiff! On my own python I can see the BB go down, when pulling the front brake and pushing the pedal hard. On this bike no visible flexing happens! So even though I do not think it saved weight, it did make a better bike.

What follows is the development of the project during the week. They are in cronological order. You can click on a thumbnail to see a larger picture, and then if you want to see even more detail, click on the larger picture.

We used qcad to draw the recumbent. The dxf file is here.

The front is made according to the birdcage principle. The hope is that the bicycle will be much stiffer then my own python. And it is! The bike bends under pressure like a stone.

Although I think the design could save weight, this won't happen in this verson: we both agreed not to experiment too much. Even without very complex analysis, it can be easily seen that the two red coloured tubes receive all the hits of the road. The green tubes are only loaded by pedalling. So for the green tubes we could try with smaller diameters, like round 10x1mm.

The pivot is a recuperated headset from a bike (1inch headset), and has an angle of about 60degrees. The trail is ... mm (thats a little more then the baby python). Although I would prefer a smaller trail, I am forced because of construction reasons (pivot is higher).

The bike has to survive the belgian cobblestones so we put suspension on it. Instead of some expensive solution we use a rubber cylindric block. If it proves too bouncy, we just will have make it harder by putting a ring over the cylinder.

The seat is movable by sliding it backwards or forwards.

The backbone of the bicycle. We used the bottom tube of a mountain bike. The angle between the bottom tube and the head tube happened to be around 60 degrees, so that was fine.

The suspension pivot is recuperated from a rather cheap looking fully suspended kid bike. It was the pivot around which the rear triangle rotates. The pivot uses bronze bushings of at least 20mm diameter

It is a 1 inch headset. - I would have prefered an oversized one, but couldn't find one.

The way we did it, was to take a steering tube, cut the fork off, close the ends with plating with nuts embedded in them on both ends.. The birdcage has two pads, and are simply screw onto the steering tube.

The jig to make the (complicated) front part. We uses just a 40x40 square tube. This tube is positioned perpendiculum to the pivot. Just need to drill the holes at the right places.

For the BB we drilled a 8mm hole and then welded an U profile around it. Between this U profile a BB of 110mm fits.

A (bad) picture of the jig. The important parts are to be fixed onto this jig. It is not perfect (holes not perfectly aligned), but hopefully will do.

Originally we thought to use threaded rods, but that was a bad idea. Cutting a thread inside a 2mm thick wall is enough to offset the hole.

Building up the front piece inside the jig.

The jig seen from the front.

Hmmm. I see I need to get a hair cut soon. ...And get my clothes washed.

A first peek at what we have been welding. Marc was very satisfied!

Detail of the front part. The two tubes along the pivot aren't very useful: they are to limit the amount of turning of the pivot. Prevents the cranks hitting the ground when not sitting on it.

The two top tubes are 10x1. This is very thin and could be risky, but my reserve of seat stay tubes was depleted. Anyway, when I make one for myself, I want to use 10x1 or 12x1 tubes, so Marc is going to be the guinea-pig.

After welding the front part, we put the wheels between two heavy profiles to align them, and the we welded the rear fork onto it.

.... I was too lazy to find profiles to also clamp in the top of the wheels, and now they seem not to be aligned in the vertical sense. I will need to do them again. :-(

In the mean time I did them again. I cut through the backbone just after the suspension and welded things together again, after taking my time to make sure things are okay.


A fresh t-shirt.

We use a rubber block as a suspension. It is a block used to mount motors in cars or trucks (I think). On both sides of the block are metal plates with in the middle a hole with M8 threading in (about 1cm deep).

We drilled through the rubber block destroying the M8 threading on the left side, but leaving the threading okay on the right side of that block.

The trick is that we translate the pulling force on the screw, to compression on upon the block. And the advantage of this trick is that we can pre-load the block. You just need to thighten the screw.

To prevent the screw from loosening there is a counter-nut.

I beefed the picture a little up so its easier to see what is what. The picture was taken while Marc was sitting on it. You can see the rear hanging onto the M8 screw (yellowish), creating a distance to the backbone of about a half a centimeter, making the block bulge outward.


The seat is on it! We used 10x1mm tubes to mount the seat. That part of the rear has the same look/feel of the front part.

The seat is slidable over long holes. There is a problem though, when turning the screws the screw moves inside of the hole. May be solvable with big rondel.

The seat mounting. The seat is movable for over 5cms.

Time to try it out. First Marc, ...

And then Dirk.

Marc tries to show me where the handle bars should be.

At the Nørrebro festival, explaining everybody about kvisten and the bike. The tandem is made by me.

In April 006 Marc painted his bike. The is a photo gallery here.