Introduction: Starting Point...
My family was at a recent family reunion. While there, one of the cousins had a an interesting looking contraption. It was a swingbike?!!?!? A bike that had two pivot joints in it. I tried riding it while I was there, but didn't have much luck, as the terrain was rather hilly, and somewhat hard to learn on. He told me that it was something that had been around for quite some time, and the design wasn't necessarily anything new, just fun.
Upon arriving back home, I decided to do a little bit of looking around on the internet, and see what I could find out about said bikes. There was a company that produced these bikes in the early 70's, but they never really caught on. There is also a company making these today, called americasbikecompany.com. The original swingbike, and the newer swingbike design, while the same are rather different in design. The original swingbike design was almost more of a unicycle type thing where they pedals were in front of the pivot point by quite a bit. The newer design bikes have the pedals kind of below the pedals. I looked at both designs, and by luck I reckoned that it would be easier for me to build the latter type design, because it could essentially be built with one bike, and a donor bike that would provide the head tube. (NOTE: Look around on the internet for "swingbike", and you'll pretty quickly see the two types of designs that I'm talking about, and what the differences are)
Step 1: Starting Point...
These are the things that are required to build this project:
One complete bike. Can be multiple geared, or single speed. I suppose that any size bike would work for this.
One donor bike to get the front forktube, headtube and all hardware from
Heavy walled tubing, or possibly solid round bar (I used 1.25" OD x .375W tubing)
Some plate steel for making the pivot mount (I used 11GA (.120 thick))
Cut off wheel, or way to cut tubing
Die grinder (helps with shaping fishmouths)
Welder (arc, MIG or TIG)
Variety of wrenches to dis-assemble and re-assemble bike
Sharpie or marking device
My cost to build this was more or less nothing, other than the cost of welding gas and wire. I was able to scrounge for the main donor bike, and I've had the second donor bike laying around for several years. At most, I would think that you should be out less than $50 for everything.
As for difficulty, I wouldn't say that this is really a hard project, but it does require a few metal working tools that not everyone might have, or have access to. So, I put the difficulty level at the middle of the range because of that. My total time to build the bike was about 16 hours in all.
Let's get this thing started. This is the only pic that I have of the donor bike, and I don't have any pics of the second donor bike at all. My main bike is a 26" run of the mill cheapo Murray MTB. But, this isn't just any old MTB, this is an Ultra Terrain Extreme! My donor bike was a gem as well. It was a built-for-Kmart bike, and it was ALL PRO. I actually kind of felt a little bad cutting up that bike because it was a brazed together frame, from back in the day, so it was probably a pretty decent bike as far as that goes.
Psh... I got over that pretty quickly though. ;)
My first step was to disassemble my main donor bike, and get to a bare frame. After that, I cut the frame at the arrows indicated as close to the downtube as I could. I wanted the remaining tubes to be as long as possible. In this case, a slightly stretched out bike won't be a bad thing.
Before cutting it apart, I also measured the angle between the upper main tube, and the lower main tube. I needed to know what this angle was, because I am going to cut the LOWER tube at some point, and make it parallel with the top tube.
After making all of the cuts to the main bike, the resultant pieces can be seen in the third photo. Notice that the top and lower main frame tubes are now parallel to each other. The lower tube ended up being mitered very close to the front headtube, and that was fine, since I knew from the measurement I took above what the angle was. I did it this way, rather than cutting these off front head tube, because again, I wanted these tubes to be as long as possible, and this was a good way to recycle these tubes without needing to add more tubing into the project, which I didn't really have anyway.
Step 2: Cutting Angled Fishmouths for Rear Headtube...
I didn't take as many pics of this part as I thought that I did, but I thought that it was still worth sharing how I did this, in hopes that it will help someone else along the way.
Cutting fishmouths in tubes is not the easiest thing in the world. Well, actually cutting them isn't all that hard, but figuring OUT where to cut them is whole other matter. Add to that in this case, I have to cut those fishmouths on an ANGLE, and now there is another layer of complication in this whole matter.
As I was watching videos on the internet planning how I was going to do this, I saw a video of a guy who used a template to cut his fishmouths with. I thought, "Hummm. That would sure make the process of at least marking them a ton simpler." So, what I did was to accurately measure the headtube diameter, the top tube diameter and the lower tube diameter. I'm fortunate to have access to solid modeling at work, so I modeled all of these tubes in the appropriate diameters, also placing them at the appropriate angles that I measured earlier. Then, I could copy off the OUTSIDE surfaces of ALL these solid models, and I what I ended up with was surfaces that were intersecting, and the fishmouths were created automatically at the intersections of all of these surfaces. Then, I used a command that allowed me to unfold the resultant surfaces into a flat file, and I printed these flat files, which were EXACTLY the OD of the tubing.
After marking the centerline of the tubing, I used that centerline to line up the fishmouth on, and I wrapped it around the tube, and taped it on there like you see in picture 1. I did this for both the upper and lower tubes, lining them up at a predetermined spot that would also up the rake angle for the rear head tube.
After getting them both taped on there, I used a 4 1/2" grinder and air die grinder to grind out these notches until I was right on the paper. Then I would test fit the head tube and grind some more. This way, I was able to slowly sneak up on the fishmouths, and make them very tight fitting. You can see a finished fishmouth in pic 2.
My ultimate goal here was to cut these accurately enough so that I could TIG weld the joints, but my TIG welding skills are just not there yet, and after having issues with that, I resorted back to MIG, and welded all around the top and bottom to the headtube. I also added some gussets in the corners just for good measure. You can see the finished "swingarm" part of the bike in pic 3. The part to right side of the pic is the REAR half of the swingarm.
All-in-all, this method worked out very well, and it was very easy to get tight joints that were easy to MIG weld.
Step 3: Rear Pivot Joint...
In doing research for this bike, one of the things that I ran into several times was that this particular design is "weak" in the rear headtube area, and often times bends at the joint where the headtube mount attaches to the rear bike frame. The way that most people solve this problem is that they put something across the top of the rear headtube, tying it back into the rear frame. But, I had seen the commercial designs that were available out there, and they were doing it somehow without adding any extra support on the top. I liked the look of that, so I decided that there needed to be a way to accomplish that. The red arrow in pic 1 is where this joint is historically weak, and the green arrow indicates where it is historically "bridged" to the rear frame at the seat post area.
My way to accomplish that was to use a thick walled piece of tubing. I had a buddy turn that tubing down to an OD that would fit inside of the ID of the forktube that I got from my second donor bike. In pic 2, you can see the tube before we started turning on it. That is 1.25OD x .375W tubing! It IS thick walled!
Pic 3 shows the OD of the tube after turning it down.
Pic 4 shows the donor bike forktube slid over the turned down part.
Pic 5 shows some slots that I cut in the forktube so that I could weld it to the thick walled tube.
Pic 6 shows the washer that I put on the ledge that we turned in the tubing. This "ledge" will hold up the bottom bearing cup. It eventually got turned down to the same OD of the bearing cup used on the rear headtube.
Pic 7 shows some of the poor TIG welding that I was able to cobble onto the washer to get it welded onto the down tube.
Pic 8 shows the TIG welds that I added to the slots that I cut into the forktube, welding it to the downtube. This was all then carefully ground off flat with the OD of the forktube so that I could slide over the bottom bearing cup. The turned down section was up in the headtube about 5" or so, so it's all tied together pretty well.
Pic 9 is a bad shot of the inside of the forktube, after welding to the downtube. As you can see, there's still quite a bit of meat in that area as well. I think that the resultant wall thickness that was in the downtube was around .180. The joint that normally has issues should be pretty stout on this bike. In the riding that I've done so far on the bike, it shows no signs of fatiguing or bending.
Step 4: Finishing the Rear Frame Assembly...
I didn't take a lot of pics of this part, but I think that it's pretty self explanatory.
I just made some plates that I welded to the downtube, and then onto the downtube of the rear clip. For aesthetics, I added some holes, and also added a cap to the plates so that it looked a little more finished.
Pic 2 is a pic of the fully finished rear clip. You can also see in this picture how the rear headtube assembly looks when it's all put together. Notice that there is NO support on the top of this area back to the rear frame or seat post. However, if for some reason I was ever to find that the frame was bending in this location, it would be very easy to add a brace back to the seat post area.
Step 5: Final Assembly...
The final assembly is basically a bike with two pivots. ;)
One thing that I did encounter while I was building it though, is that it's kind of an unruly thing to assemble, with everything all wanting to move all over all the time. I finally got tired of fighting it, and I just laid the whole thing on the ground, and built it "flat" on the ground. That made things a lot easier.
I also have not hooked up the derailleur at this point. I basically picked a range that was comfortable to ride in, and fixed the chain at that length. I haven't decided if I'm going to add that back on there or not at this point. Might be more complication than it's worth.
One thing that I commented on in my first slide was that I got lucky with my design. The luck part of my design is that it is VERY easy to ride, which might come as a shock to some people. I have let numerous people ride it, and to date no one has fallen over on it. The reason that it's so easy to ride is that the riders weight is directly over the rear pivot point. Also, when you pivot the rear, the pedals come UP off of the ground, ever be it slightly. So, since you're weight is all over the pivot point, and the pivot point is naturally wanting to go down to a static point, when you ride the bike, it is always trying to seek this "home" position, more or less automatically, and that's where it wants to stay.
But the downside of this is that it does make it a little bit harder to actually "swing" the bike. it CAN be done, and with just a little bit of practice, I've gotten to the point that I can swing the back / front pretty easily. Sometimes holding it there takes a little bit of work, since it's always trying to find "home", but it can be overcome. I'm thinking about changing the handlebars so that they are a little bit wider. I think that this would help "push" out the front wheel.
All-in-all, I'm very happy with where this bike came out for my first attempt at building one of these. I'm now considering making the original version, which I KNOW will ride completely different than this version.
By popular demand, I've added a video of me riding on my FIRST voyage. This is a crappy cell phone video, and at this point, I was "swining" out very far, because it was only minutes old. But as I ride up the driveway, you can see that the rear wheel is not inline with the front wheel.