Hatching the Python

This page was made as the baby python got reality.

The wheels are 406. There is no suspension. The backpart is out of 30x30x1.5mm, and the fork to the rear wheel is a shortened down fork of a 26" mountain bike.

wheel size406
wheel base1080mm
pivot angle60degrees
seat height270mm
BB height478.5mm
tube size 130x30x1.5
tube size 230x10x1.5

The Pivot

The axle is a simple M12 bolt. The head of the bolt is hex. The adjustment of the bearings is done using two nuts at the bottom.

In principle the bike is splitable at the pivot. But mounting again could be a problem. I do not know if adjusting the bearings while under load is going to be easy. May plan is to tension the nut until I can not feel any play on the front part.

On the picture two nuts are used. In real this is going just to be a nut with a plastic ring.

The bearings look as follows. They are good for both radial and axial loads.

inner °12
outer °32

Front Dropouts

The dropout is near to vertikal related to the line between the hub and the BB. I combined several sources to create this drawing. I went measuring around Town and there doesn't seem to be much agreement between the distance of the derialleur hole and the hub axis. But most of them were near to 30mm. So I have based my drawing on that.

The Blog

This rest of the page describes in a blog manner the progress I made throughout the months I was busy constructing my bicycle. Do note I was going through the larval stage here.

I build it at a place called kvisten. If you live in Copenhagen, and you need a place to work on a bicycle, have a look at it. The membership is low priced, there is a nice atmosphere, and the activities are not limited to only screwing around with cycle parts.

It has been great fun creating something and I have had very happy moments sawing away at kvisten! And I learned something new every day I worked on it ...mostly from my faults ;-).

Okay, here is the blog:

Thought it a good idea to begin at the end. ;-)

I used a fork of a MTB, shortened it, and brazed the drop-outs on it again. And oops, brazed the drop-outs the wrong way.. Oh well, the the v-brakes will have to point downwards then.

Somebody at kvisten said to prevent things deforming to much, I should make the welding by points. Choosing the next welding point as far as possible from the last made point.

Pfew! Finally got to work on the python again --- I was busy with the trailer. Learning from the trailer experience I decided to make a jig. The idea is to have the important points fixed by the jig (the rear axle, the pivot, the front axle and the BB), and then just connect them. The jig made here is not very good, and next time ---yes, I'm already thinking about next time :-) --- I'll put a lot of effort in the jig. As a principle, instead of measuring the cycle itself, I should put all the measuring work into the jig and then work "free hand" using that jig.

The front piece is now finished now that I have welded the U onto it. The U itself is also welded. I first tried to bend a 3mm plate but it was to difficult to get within the tolerances. I hope the 3mm are strong enough.

Olaf visited me, and that got me to work again. The vertical dropouts are finished, and we welded them to the front forks.

The bends in the fork are only welded at the bend that is sharp. The other bend is simply heated up and bend.

Soon I'm going to weld the front fork to the pivot.

Brazed the fork to the pivot. It was some time ago I brazed and there is a lot of material coming together at this point. With a lot of swearing I finally got the brass to melt. The joint must be made strong, but I think it will hold now.

You might be wondering what this middle tube is doing there. Well originally I thought to put the wheel more forward, being more conform to the amount of negative trail "dictated" by the original python. Dropped that. Unfortunatly I had already brazed it. My speciality is to make bad things good ---and good things bad), so the advantage is here that I can use it as a reference line.

After brazing the bearings didn't fit anymore :-(. The pivot body deformed. I sanded it but it is a shame. Dunno how to solve that problem in the future.

Juergen gave me the solution, his words: "when welding the warping is even bigger. To prevent this, I always place old bearings into the shell before welding. They stabilize the whole thing".

Welded the BB to a tube. The simple trick to align the BB in the two direction I got from the website of Juergen (construction page). Just mount the pedals, to aligne, put a rod through it. For extra ease in aligning I welded another one perpendicular onto it.

Well all this work and not a very good result anyway. After welding I saw that the alignment wasn't very good, but this doesn't present a big problem because I have a second chance when welding the tube to the pivot. So I decided to leave it as it is, and correct it later. When welding the square tube to the fork the square tube is slightly rotated, but the BB is nicely aligned to the front wheel axle.

Today I did finished the front part. With the surplus tubing sawed away the front part is delightfull light. I can immediatlya feel this bike is going to be much lighter then my flevo.

I also braced the steering clamp and the cantilever bosses. Looking at the picture now I am a little worried that there may be not enough place for the v-brakes because the pivot is in the way. Hmmm, if so I'll re-braze the bosses under another angle, so the clearance is better.

Note the bosses are at a distance of 98mm. That is actually to far apart. The ideal is 80mms. May be I will change that to by making some kind of "balcony" on the frame.

Still need to weld the derailleur tube onto the bottom bracket. But the derailleur I want to use is for a very large diameter tube diameters. I do not want to weld such a big tube onto it, so I wait for a good idea to happen.

I had only a few hours of time, so I did not do much. Worked a little on the seat mounts. Assembled the bike so I could stare at it. It really needs a seat, it looks very naked like this.
Put the seat mounts on. The seat is a challenge seat (I am going to replace it later with an own construction) and is fixed at two points.

The seat mounts are simply clamped onto the main tube. So the seat is easily movable. The clamp is of a very simple design: I just bended a U over the tube (1.5mm thickness), and brazed on each side of the U a nut. The bottom is made of a thick aluminium plate.

Hmm. Torben Scheel just warned me. The combination of aluminium, metal and salty Copenhagen will lead to corrosion of the aluminium. So paint is definitly very important to put a barrier between the materials.

The seat is not changeable in angle. If I ever will want to adjust, I will just dril another hole in the seat.

20041201 20h00
After patientely welding and brazing the clamps, I went into a frenzy, and inside a an hour I had mounted all the basic stuff needed to ride it and was doing my first little tour on the backyard of kvisten. Tom took pictures of me.

My first impression is that handling feels very much the same as my flevo bike. It amazingly feels even more stable. I rode through Town with it and I didn't experience any trouble (excepts with the stupid slippery wet pedals, my feet kept slipping of).

I attribute it to the smaller negative trail. The self centering effect is not so strong, so unlike a normal python geometry, you do not feel like the bike wants to go straight all the time, even if you do not want to.

Further I noticed that the pivot has play. And I cannot get it removed. So I will have to change the design of it. At the same moment I will change it so, that I can get and easier detachable bicycle.

The frame is flexing when I push the pedal very hard (while holding the brakes). One of the causes is the 3mm thick U-profile for the pivot. It is simply to thin. I could feel it bending while pushing the pedals. Heck, I could even feel it bending while just by sitting on the bicycle. So put some more of the same onto it. Now it is 6mm thick and has some triangles into it. It still needs side plating for rust-dirt prevention (on the picture added with gimp). At the same time this will further strengthen the pivot.

Also notice the rust on my bike. I got this from riding two tours in total about 100kms. K°benhavn is a really really salty place.

I also added a steering, but I am not happy about it yet. The position doesn't feel right. Next week I will have to tinker again with this. I am using STI brake levers, in combination with a converter so I can use V-brakes on the front wheel. The rear wheel brake will be a ordinary cantilever break. Still need to mount these.

As an experiment welded another tube under the main rear tube. In the photo I painted the tube pink. The bike suddenly got a lot more stiff. It seemed to be mostly this tube that was causing the flexibility of the bike.

It is still possible to flex the bike a little, but it is difficult to find out what exactly it is that is flexing now. In other words, it doesn't flex much.

I will probably use a seat stay tube and weld/braze that some 50mm lower then the main tube. So I get a simular effect as the heavy tube I have now on it.