How to make a two person "tandem" bicycle using nothing but two dead ordinary bikes, two sticks and a roll of duck tape!
Step 1: Our Target
This is what we'd like to build - twice the power, twice the danger, twice the fun!
Step 2: Supplies
These are the kind of bikes you want - we need one good frontend and one god backend between them. We'll also want a hacksaw, an adjustable spanner (wrench), a tool for splitting chains or a few master-links, a screwdriver, a hammer AND that roll of duck tape. The two bikes need to be men's style frames (with a top tube up high) and have multiple gears on the pedals - the two smallest ones need to have the same number of teeth for this to work out.
Step 3: Ripping Things to Pieces
Next up - ripping your chosen bikes to pieces!
BACK BIKE: take the handlebars and front fork off the bike that is going to become the back of the tandem. Tear out the cables too, as we won't be needing them. Cut of the gear-changer for the pedal sprockets, as it will get in the way later. Now remove the bearing races from the head-tube (the 'grooved washers' the ball-bearings that support the front fork ran in) by hitting them with the hammer. (the bearing races are just a press-fit over the tube) Now hacksaw through this head-tube as shown in the picture. (we're trying to make a 'cup' that will cup around the seat-post tube of the front bike) That's our back half done!
FRONT BIKE: remove the rear wheel and all gear-changing equipment. Now hacksaw off the support tubes for the rear wheel as shown in the second image - we want the lower tubes left on but without the wheel brackets and we want the upper tubes completely gone.
You should end up with something looking like the third image if you put the two next to each other.
Step 4: Putting It Together
The coloured image shows the structure that we're aiming for - front bike frame in red, rear bike frame in green and a neat trick in blue!
Duck tape is very strong when you try and pull on it lengthways (tension) and pretty good if you stick two things together then try to move them past each other (shear), but it's rubbish if you try and push on it (compression) and it tears easily if you put all the load on one side (to make it easy to tear off and use).
In a bicycle two people sitting on top of it, the top tube is in compression and the bottom tube is in tension. When you start pedalling, tension in the chain adds some compression loading to the bottom tube. We need to make joints and beams strong enough to carry these loads using nothing but duck tape.
For the top this is easy - we simply bind that 'cup' on the front of the rear bike to the seatpost of the front bike with duck tape. Bind it nice and tight, the same way you'd use string and bamboo only keep the sticky side in. This lets the joint carry up-down and forward-back forces, but not twisting forces. To stop the bike twisting in the middle like a car with a trailer we use those two sticks - tie them to the top tubes of both frames with lots of duck tape then bind those two sticks to the seat-post or original joint for good measure. (you can kinda see this on the photo anyway) The height of the top tubes (and the requirement to bind two sticks to them) sets the shape of the tandem.
The bottom joint is more fun - we've nothing to tie things to AND the chain gets in the way. Check out the third and fourth pictures for what a real tandem looks like and what ours will look like. Spot the trick to joining the bottom end together? It's that spare front fork we had leftover from before!
Take everything off that spare fork and cut it down until it looks like a 'U' instead of a 'Y'. Cut two 'V' notvhes into the base of the U on the inside edge - this will help centralise the fork on the tube we're going to hook it around. Hook the fork around the seat-tube of the rear bike and match its ends to the two tubes coming from the front bike. Now look at where the chain goes - there will be a tube in the way... Remember that hammer? Take that hammer and beat seven shades of blue out of the frame and the fork until the chain can go past the tubes and the fork and frame tubes match up nicely. Now you can bind things with duck tape - start with binding the ends of the fork to the ends of the front frame (it should be like wrapping two sticks side by side). Now bind the fork to the seat-tube and for good measure bind any other part of the frame that comes close to touching (usually where the fork/two frtont frame tubes cross over the diagonal tube on the rear frame).
For the chain - you need to extend the chain from the front bike to reach the rear one. Either use a chain tool or buy two master-links and a length of chain which you can cut down to length with the hacksaw (eventually...). Chain tool is preferable as it's a reversible operation! Chances are that a set number of links won't be the right length - you'll need 37.378757332 links to make the chain the right length. Err on the side of too tight rather than too loose - the frame is flexible and if you make the chain complete then try to fit it by hooking it around one sprocket then hooking it around a sprocket tooth on the other/forcing the sprocket around, the frame will compress then spring back again and the chain will go on.
Make sure that the pedals are synchronised with each other, else you'll be forever kicking each other whilst cycling, you'll not be able to put the pedals horizontal for going around corners/up and down kerbs and you'll find it difficult to balance too.
Step 5: Riding It and Other Notes
At this point you should have a two-person bike with one gear and one brake (the front one). Buy a long cable and you can hook up the rear brake too - probably a good idea given the extra mass of the second body and the higher speeds you can get to (though we never got round to it...)
Gears are pointless on this - not being able to go too fast (we top out around 35mph as vs 45mph on a proper tandem) is probably a good idea given the crummy brakes and debateable structural integrity! Choose one that you like and leave it there (there are screws on the derailluer that you can adjust to set its 'home' position)
Handlebars can be added by taping them to the back of the front saddle, though we found it easier to ride on the back with no handlebars (if the person on the back uses the handlebars to balance against, it upsets the balance of the whole machine, whereas if they hold on to the person in front then they tend to follow the lead of the person leaning into corners etc better) Depending on who the intended passengers are you might want to leave the handlebars off as a deliberate excuse to have to hold on... ;-)
Longevity? Despite the slightly unnerving flex in the frame (you can see 5-10 degrees fo twist front-rear as you turn a corner!) this tandem was used and abused for six months by the two 182lb (13 stone) guys who built it without any indication of the joints working loose. Proof-testing to 700lb (50 stone) by cycling back from the pub with four blokes hanging off the thing led to a buckled wheel, so all indications are that the duck tape construction is sound! :-)
Else - apologies for the lack of pictures, hope there's enough here to encourage you to try this. If you do build one and wouldn't mind taking a few snaps along the way please add them to this instructable!