Folding bikes are handy, but slow and boring for daily commutes. Singlespeeds and fixed gear bikes are quick and fun, but cumbersome and expensive to travel with. So, my aim was to build a quick, fun, compact, full size, single speed bike which I could pack:

in two suitcases;
in the back of a small car;
as a folding bike on a train (they can be taken on the train for free here);
as regular checked baggage on a plane (to avoid bike fees that exceed ticket price).

The objectives were simple but strict. The bike had to be as light, but just as rigid as it was before modification. Furthermore, one should be able to take it apart and put it together within twenty seconds (on a moving train). Also, the modification had to be done without any welding (or power tools for that matter) and for less than 100 euro's. Impossible? No. Challenging? Yes.

While it took quite a bit of time to come up with a system, the process is actually rather straightforward, requires no special skills (besides a fair bit of elbow grease), and I stayed within budget.

Why build this yourself if you can just buy these beautiful stainless S&S frame couplers?
Because they cost $400 to $600, and that doesn't include installation. Which must be done by an official retailer, which isn't available in my country. So we're looking at an investment of over $1000. For a forty year old bike :')

Disclaimer
The author of this Instructable takes no responsibility whatsoever for anyone or anything getting damaged by building, riding or even looking at a bicycle inspired by this Instructable.

Step 1: Sourcing a Bike

While it's tempting to get any old beater, I'd advise you to pick your bike carefully. An old racing bike in a decent state works best. As you’re going to reuse most of the parts, get one in an overall good condition. Rear sprockets, shifters, rear brake and most of the cables will be discarded, so they can be worn. I chose to replace the front brake and the chain as well, but that's up to you.

Make sure that the frame is straight, has horizontal drop-outs, the wheels as straight, there's not too much rust and the bearings are okay (though they're cheap to replace, so this isn't a deal breaker per se). For more info on finding a suitable bike, have a look into one of the many Instructables about converting a racing bike into a fixie/singlespeed.

I opted for a seventies ten-speed Koga Miyata, the main reason being that I got it for free. It was in good nick, though repainted badly in an ugly colour and the derailleur was stuck. (The bike in the pic is a reference, I wouldn't dare to cut that in two).

Why can't I use my oversized aluminium bike/carbon mountain bike/grandmothers wheelchair?
You probably can, but it makes the conversion exponentially more difficult. As we're doing this without a welding apparatus, there's less room to make things fit. The scaffolding couplings are available in several standard sizes. 42mm (inner diameter) and larger is mostly used in scaffolding (duh), but that's larger than most bicycle frames' outer diameters (note: the top tube and down tube often have different sizes, I'll get back on that). Plus, it's rather heavy. Different combinations might work; the key is to find a good fit between the couplings and tube size. The 27mm (about 1") "scaffolding" coupling comes close to the standard tube sizes for many classic bicycles, which makes conversion a lot easier.

This Instructable assumes a standard frame tubing size for many classic racing bikes: 28.6 (1 1/8") x 0.8 mm for the down tube and 25.4 (1") x 1.0 mm for the top tube. The catch is that you don't know the wall thickness until you cut it up, but the inner diameter is slightly less important or can be mitigated with some standard parts from the hardware store.

Found a decent bike? Now let's find the couplings!

Step 2: Sourcing the Couplings

As stated previously, we'll use 27mm scaffolding couplings. These are probably not found in the construction section of your local hardware store, but in the furniture making aisle (or online). Making furniture out of scaffolding boards and steel is totally hip, but people want them to be sufficiently refined for the living room. That's why hardware stores carry these 27mm tubes and couplings, which are probably to flimsy to build even a small platform. But they're perfect for our purpose!

A key feature of the design is that the forces on the bike are cleverly distributed in the coupling. A sleeve joint coupling (see picture) seems to be the most simple solution, but it's guaranteed to fail. The down tube, 'pulls' on the coupling under the weight of the rider. This creates considerable force, especially when you hit a bump or pedal hard. A sleeve coupling that tries to keep things together with only one clamping bolt will make sure you'll end up in the hospital. Plus, to get sufficient rigidity, the fit must be so tight that you wouldn't be able to take it apart and put it back together without a mallet (you'll see why in the next steps). And beating you bike up with a mallet would look weird, wouldn't it?

So, the way to go is a so called short swivel tee and a short piece of pipe. You can secure this coupling securely to the frame ends and it distributes the lateral (?) force nicely across a piece of sturdy steel tubing.

You should be coming back from the hardware store (or the post office) with:

Two sets of short swivel tees for 27mm 'interior scaffolding'
One short piece of 27mm tubing (mine was 25cm)
Optional: a key for the hex bolts (my bolts are imperial, all my tools are metric)
A big smile because that wasn't that expensive at all :)

Got yourself the hardware? Then let's cut!

Step 3: Preparing and Cutting the Frame

The first step is to strip the bike of all it's components. Make sure to keep the ball bearings together and keep them free from dust. (Pro-tip: labeled zip-lock bags.) If you plan to repaint the frame as well, this might be the point where you sand, clean and smooth the frame. Repainting gives you the opportunity to grind off all the lugs for cables and other stuff we don't need after conversion.

Measure the total length of the coupling when it lies flat on the table with the piece of pipe bolted in. Then unbolt and measure then maximum insertion distance for the tubes in the couplings (see picture). This is easy to do by measuring how far you can stick the piece of pipe you bought into the coupling. The total length - (2 x maximum insertion distance) gives you the size of the piece of frame you'll replace with the coupling and thus the piece you need to cut out. I ended up with about 58mm.

The coupling should be fairly close to the back of the bike to end up with two (somewhat) equally sized parts, but not too close as you want to stay away from the sprocket and the seat tube and have some space for handling you key. Now, measure and mark the location where you want to place the coupling. Measure again, and put a piece of masking tape on the frame where you make the cuts. Measure again, say a prayer, and cut your bike in two. In total you'll be making four cuts.

Congratulations, you've passed the point of no return! Now you have a disassembled bike with two gaps in its geometry and two small pieces of tube. Don't throw the pieces of tube out! We're going to use them in the next step.

Step 4: Mounting the Couplings

If you have the same frame diameters as I have, then the piece you cut out of the down tube will fit over the top tube, and the piece of top tube will fit inside the down tube. We're going to use this to reinforce the tubes before we slide on the couplings. For that, we need to cut the pieces in two, so we end up with four rings, about 25mm - 30mm wide.

If you have different diameters, you're on your own here. Luckily, there are tonnes of different steel pipes in the hardware store with different inner diameters, outer diameters and wall thicknesses. Grab a calliper, a notepad and the pieces you cut out and you'll find something to make adapter rings out of.

While the couplings are sold as 27mm couplings, their inner diameter is actually quite a bit wider. They are designed to accept 27mm pipe easily, so mine had an inner diameter of a bit over 28mm. Perfect, as our down tube is 28.6mm!

The down tube
To reinforce the thin steel of the down tube, glue the rings you made out of the top tube into the down tube (see picture). Any glue will do, as they will be compressed by the hex bolt later anyway, which keeps them in place. I used regular contact glue. Use a half round file to make the opening of the coupling round (trust me, it isn't). Press it onto on of the down tube endings every now and then, hit it with a mallet and see if it fits and where it leaves marks in the steel. File a bit more if it leaves deep marks. Don't file away too much material, as you need a very snug fit to retain rigidity. When the coupling moves about a millimetre after a hit with the mallet, you're probably good to go.

Now comes the tricky part. Put the piece of tube you bought through the eye of one of the halves of the coupling and bolt it tight. This gives you a handle to keep the coupling halve straight when you use the mallet to get it on the tube. Repeat the procedure for the other end of the down tube.

The top tube
The procedure for the top tube differs slightly, as the rings will be on the outside now. Adjust the opening of the coupling the same way you did for the down tube, for it to accept the rings. Place the rings on the workbench, put one half of the coupling over it and use the mallet to gently ease it into place. Then slide the assembled coupling halves on the top tube and align them.

Alignment and creating the middle pieces
To ensure easy (dis)assembly when it's done, make sure the two pieces of the frame align nicely and the piece of pipe is completely horizontal when you put it in. When it is, bolt the couplings to the frame (tightly!). You can slide the piece pipe over the end of the hex key to get some extra leverage. (Don't hurt yourself). Then cut two pieces off the spare piece of pipe, having the width of the assembled coupling. When you're done, it should look a bit like the last picture.

Step 5: Secure the Couplings

When the couplings are in their final position and the hex bolts are properly fastened, drill two 4mm holes through the bottom of each coupling. In the unlikely event that the hex bolts won't hold, small bolts will keep the frame from coming apart at speed.

Uncoupled, you should have just enough access to drop a (self locking) nut in there and position it over the hole with you pinky while you put the bolt through from the bottom. Then stick a spanner in there to secure it properly. It requires a bit of fiddling, but it should work. An easier method is to use a tap to thread the hole so you don't need a nut inside the tube. But I didn't have a tap, so I took the took the fiddly way.

While you might be tempted to trust the big hex bolts, picture how you will crash if one does decide to give way (spoiler: it'll be ugly). So don't skip this step.

Step 6: Painting

Ordinary spray paint will suffice to give it a nice look, although keep in mind that it is not nearly as tough as the original paint. I won't go into detail here, as there are plenty of tutorials online on how to paint a bike. There's no need to paint the coupling as they are already rust proof and the paint on it will damage anyway. Plus, they are the key feature of the bike so they should stand out!

The front fork isn't in the pictures, as I painted that before when I had to paint some other things matte black.

Tip: don't forget to mask the tread of your bottom bracket, the inside of the seat post and the inside of the headset.

Step 7: Assemble and Ride! (or Disassble and Pack)

From here on, it's all pretty straight forward. Check some other tutorials online on how to adjust the various bearings, get a straight chain line, et cetera. Secure the small pieces of pipe tightly in the couplings on one end of the frame, and use the other end to bolt and unbolt.

I rebuilt the bike with only the smaller chain wheel of the two, and I placed it on the inside of the spider (the thing that attaches the chain wheel to the crank) instead of on the outside where it originally was. This gave me just enough clearance from the frame (3mm), and a straight chain line without re-dishing the wheel.

While I originally intended on riding fixed gear, I found that free wheels are better suited for daily commutes. So the rear chain wheels were replaced by a simple BMX freewheel and I put a gold coloured single speed chain on (ya know, for blingz).

The bike was originally equipped with old centre-pull brakes that required a bracket on the stem, and they were quite worthless anyway. As the modular design allows no rear brake, I shelved out some money for a proper front brake and brake lever (both Dia Compe). Well spent money, as I would've crashed it many times by now if I hadn't.

Step 8: Costs and Experiences

While projects like this usually tend to cost more time and money than planned, in this case the latter wasn't true.

Bike (donated): 0
2x Short swivel tee: 23,98
250 x 27mm Scaffolding pipe: 3,29
Paint and sanding paper: approx 10,00
Dia-Compe 806 DP front brake: 17,99
Dia-Compe Dirt Harry brake lever: 22,95
KMC Z510S Gold chain: 9,95
Freewheel (eBay): 3,50
Ass Saver rear mudguard (I love this thing): 7,99
San Marco Rolls vintage seat (salvaged): 0

Grand total: 99,75 euro!

This doesn't include the bike, but does include the upgraded brake and some luxury items such as the mudguard and the chain. If the paint of your bike is okay, you're okay with the cable guides on the frame and the brakes aren't too shabby, you could do this conversion for about 30 euros.

As one of the commenters remarked, the original chain is made to hop off the chain wheel and a singlespeed chain is not. While I encountered no problem with chains running off the chain wheels (if the chain line is perfectly straight, that is), it is advisable to spend a few bucks on a singlespeed chain. Singlespeed chains are also more robust, last longer and come in all sorts of fancy colours.

Experiences

I've ridden the bike for a couple of months now, taken it on the train a dozen times and gave it some serious abuse. Apart from some scratches on the paint, it holds up really well. No sign of bolts loosening and the frame feels as rigid as it was before. Train conductors frown, but don't complain. Though seeing someone take a bike apart on a train is quite the conversation started for other passengers, apparently.

Feel free to post any questions in the comments!

Edit: thank you all for the wonderful comments on this project! I'm happy and humbled to see that almost twenty-five thousand people have viewed this Instructable, and to read that people are inspired by my built. I'm deeply flattered. Keep building and biking!

Introduction: Folding Fixie (without Welding)