Trumpet
Final geometries:
Pivot angle: 64°
Trail: 23cm
Seat Height: 37cm
BB-Seat Diff.: 36cm
Ground Clearance: 5.8cm
Wheelbase: 124cm
BB-FWA: 46cm
BB-EOS: 64.5cm
Wheel Size: 26" 26"
Background
Materials
The Trumpet is based on the skeleton of an old bike with rod brakes found by the dumpster. It was completely dismounted: the previous owners wanted to save everything possible. I don't have a photo of the original frame, but it looked just like this:
At first I was only interested in the fork, which was completely stuck to the handlebars because of rust, and it wouldn't come off.
But when I managed free the fork, I found out that it was pretty damaged. I couldn't use it as a structural part.
Concept
I was ready to throw away the whole frame, when I realised that the fork tube could fit perfectly inside the down tube. Since the head tube has an angle of approximately 120° with the down tube, the frame could be then flipped around, putting the down tube on top and the top curved tube below. The resulting pivot angle, with the down tube kept horizontal, would be 60°.
So with a single screw one can have the rear part and the steering basically for free, no welding or special materials needed. Also this would make for a pretty strong rear part, as the whole fork tube could help sustain the forces applied on the outer frame tube. The only variable remained the front part, that would determine the pivot angle and the seat height. I haven't seen anyone trying this approach, so I thought I could share it. The main weakness is that you cannot have the seat too close to the ground, as the head tube is quite long.
Name
When I started working with the frame assembly, I was roaming around the workshop with this funny looking black and curved steel piece. At one point a friend of mine stopped me and asked: "What are you doing with that trumpet?". I think that's a fitting name.
Frame
Since this is an experiment, mainly to learn to ride the python, I decided to cut the down tube as close to the bottom bracket as possible. This makes for a long wheelbase but i figured I can always cut it again later to shorten it; also since it is quite horizontal, the pivot angle would not change much.
And yes, I could cut the curved part right after the joint between the two tubes, but I thought it looked funnier like that.
The rear fork was fixed to the frame with the small screw of a seat post clamp.
In March I changed removed the screw, because if screw is not extremely tight the fork can rotate slightly with the forces at play, which translate to relatively large rear wheel tilts (not a nice feeling).
After I had to cut the frame (see #march update) I opted for an external seat post clamp.
In May I realized the rear part was under too much torque, as the clamp slipped three times, even if tightened as much as possible. Surely with a non-cut frame it would've worked better, but in my conditions I opted to weld the fork to the frame.
After a while I realized the fork was not going to be strong enough by itself, so I reinforced it with some tubing, a flat steel piece and a second fork.
Front part
For the front part I used the chain stays from the original bike, welded to a fork (which acts as the pivot) cut to shape for this purpose. The fork I used already had bent blades. It was welded to the little bracket designed to clamp the kickstand and to a small extent to the chain stays, for better support.
The angle at which the fork meets the chain stays decides the pivot angle and the seat height, so it is an important parameter.
At first the pivot angle ended up being around 66°, then it changed because of various frame bends.
I welded in place the rear triangle from a donor bike (orange part in the picture #december update)
Seat
For the seat I wanted the half-hammock foldable mechanism described by Jürgen. I didn't really have an easy placement for the seat: the down tube is round, so perpendicular welds on top of it are not too easy. I decided I wanted to use as much of the original bike as possible, so I cut the seat stays and bent them to a weird J shape. Since I had no tube bender I did this by hand on a vice, basically destroying any structural strength the tube had. So I welded a couple of supports to avoid it crumbling under my weight.
I screwed a wooden plate on top and bolted the whole seat to the bottom part of the frame (the curved top tube).
I'm basically using the bracket that connects the seat stays as the second support to keep the seat in place: it leans against the head tube.
The M8 bolt I used for attaching the seat will be used to hold the kickstand as well.
For the rear part of the seat I used the original damaged fork showed earlier and bolted it to an M8 threaded rod passing through the lower tube, just behind the joint with the one at the top.
Update March 2023
The bolt for the back side of the seat failed, so welded a hub. The seat itself also failed, I still have to figure out the new design.
Update May 2023
New seat design! It is elevated by a round tube welded perpendicular to the frame and a "K"-shaped structure that holds the seat plate. The added height serves to fit the stem for the frame triangulation; I tried to keep the seat as low as possible so that I can still touch the ground with my fingers.
For building the K seat frame I used 12mm square tubing with 1.5mm tick walls, which has proven to be quite useful: I also used it in the front triangle and as a horizontal piece in the seat fork.
In the picture you can also see there's a handlebar now: it is connected to the white seat stays that triangulate the front part.
Log
December 2022
I added a rear triangle to the front part so that I can pedal already. I could have more distance between the seat and the bottom bracket, but I decided to keep it smaller, so that people with shorter legs can give it a try as well. The seat is really comfortable now that I zip-tied a couple of wooden planks at shoulder height. Something soft to rest the head would be perfect now.
I'm learning to ride it, but it's really really hard! At the moment I can only go in straight lines for two meters without touching the ground. I hope to improve soon.
With my weight on top I have very little ground clearance, as the bike flexes a lot under the pivot. For future versions I think having a little more rigidity via triangulations is a must.
March 2023
I haven't updated this page in a long time, but I've been busy trying to tame the snake! A few things happened:
- The wheelbase was too long; indeed the fork tube did not really reach the beginning of the triangulation of the frame, so my whole weight was on a single tube for some length of the bike. Expectedly it failed and bent right after the fork tube ended. Not a big deal as I intended to shorten the bike anyway. (this was in January) So I cut the tube near the bend and shortened the bike; see #seat post clamp
- I tried to use handlebars by inserting a fork tube inside the seat tube; they were surprisingly very effective! (and looked quite funny) I was able to leap from 50m without falling to 500m (although my arms got incredibly tired). In the end I figured I was too reliant on my arms and that would not help improving my riding skills, so I got rid of them.
- I managed to bend the M8 threaded rod that acted as a support for the seat back as I tried to use the fork as a stroller when a friend of mine was trying to ride it.
- After that I truly understood how crucial the alignment of the seat back is for any rideability. I fixed it by welding a hub to the frame, as seen in many other projects that use this foldable seat design.
- And finally, the seat broke. I'll take it as an opportunity to add stiffness to the front part; unfortunately I will probably have to raise the seat, thus I won't be able to start pedaling with my hands still touching the ground.
I feel like in general my riding skills have decently improved: I'm able to decide where I want to go most of the times; the steering radius is still very large and going fast is still very scary (actually I still have no brakes). I think that in total I'm still under a mileage of 5km.
May 2023
Major works in April! I rebuilt the seat, added triangulation in the front part, improved the seat back and welded in place the rear fork.
Now the geometries are fixed for good, and the bike is much more sturdy, i.e., no more vertical flex. (Actually it has become quite stiff, now I see why many projects use suspensions)
After more than a month not riding it, it felt a bit weird, but I quickly regained the little skill I had been building this winter. I mean, I had to! I needed to get back home and I didn't really want to walk. It's just a couple of km, and I only fell one time, so I'm quite satisfied. I'm still terrified by cars, but this is expected; I scared a couple of pedestrians because I quietly passed near them when I wasn't exactly stable... Also, dogs bark at me :)
In May I had a couple of unwanted frame bends: I caused the first one after going against a high curb (I still had no brakes); the fork tube bent slightly. I had to expect that: it has a hole and is a central part of the frame yet it's not strong enough. To reinforce it I welded a short tube that connects the fork crown to the lower frame bend, and a thick flat steel piece placed vertically on top of it.
I also mounted the front brakes, but unfortunately the same day I borrowed the bike to a stranger eager to try it (he was also very good, he managed to make wobbly turns in less than 15 minutes) and he bent the seat stays on the front triangle, right above the welds of the square tubes that link them to the other pieces of the front assembly. I was able to bend it back into position, but I still have to reinforce it in some way.
At the end of May I took part of the Balorda, which is basically a bike carnival where you only drink wine and dress based on the theme of the year (that was La quadratissima, which in practice meant anything square or cubical). It was a lot of fun and I also won a prize! That day I pedalled for 40km, which makes it my first long-ish trip on my python (I probably had less than 40km of total travelled distance before that day, so I basically doubled my experience). I also rode offroad and at the end of the day some more structural problems showed up: the fork legs were bent upwards near the joint with the fork crown. I guess old lady bike forks are not really meant for that kind of load, so I had to reinforce it in some way. First I bent it back to being straight, after softening it with a butane torch and tying it to a pillar. Then I reinforced it with a second fork, welded to the downward bend of the frame.
June 2023
After welding the second fork I considered the frame pretty much finished, so I cut the unused bottom bracket near the pivot, to gain some ground clearance.
September 2023
I added a pivoting rack that I attached to the seat with a saddle clamp. This way I can use the rack without losing the half-hammock flexibility. I also reinforced the front triangle attachment, near the brakes as it bent when pedaling and braking at the same time.
October 2023
I finally added the kickstand: I basically welded an A shape to support the lower chain stays and spring loaded it so that it stays shut. I needed it because when I will mount the mudguards I will not be able to park the bike by folding it anymore (the minimum steering angle will not be as small, due to the mudguard touching the seat)
