If this is what you've been looking for... here are my tips, tricks, mistakes and advice.
If this doesn't seem like your cup of tea... consider it anyway! This project is a great opportunity to get creative, learn some great skills and have a blast all at the same time!
So get out your bike wrenches and roll out that welder because it's time to get dirty.
Step 1: TIME
The point is... give yourself ample time if you are planning on having an unveiling party for your tandem. That's a great idea! The second point is... you should throw an unveiling party and invite your friends over for a ride!!!
Step 2: Two Identical Bikes
I'm still kicking myself for not taking a good "before" picture of both bikes. Do yourself a favor and take lots of pictures.
Step 3: Cleaning & Disassembly
This build included two identical cruiser bikes from the late 1960's. They were neglected for several years and in rough shape so I stripped both of the bikes down completely and cleaned every part. You'll be handling the parts a lot so I recommend doing this step now. A bunch of rags, a tall can of degreaser and a nice sunny day will help move this process along.
I used two cardboard boxes to store all of the small parts from each bike separately. Storing the parts from each bike separately probably wasn't necessary but it's a good idea to find a safe place to store all of the parts until assembly.
*For simplicity, I only uploaded a few of the many photos I took.
Step 4: Design Phase: I
Try to come up with at least three different designs. Sketch them out on paper and then get a second opinion. By having a few different designs to compare, the flaws or weaknesses of each design will most likely be revealed by the others.
There is a lot to consider during this step. In no specific order:
-Front seat to rear handlebar distance
-Front head tube angle
-Rear seat tube angle
-Points of contact
-Additional support / bracing
Step 5: Design Phase: II
I loved the curves of the frames and wanted to incorporate that into the final product. I wanted the tandem to look like it was two different bikes, but also blend smoothly. Often, the rear ends of tandems are filled with diagonal support bracing and don't resemble a standard looking bike. I really wanted to avoid that so I tried to come up with designs that involved as little cutting as possible.
Overall length: I was mostly concerned with the front seat and rear handlebar spacing. I figured that I would end up pretty close to a typical tandem length.
Front seat to rear handlebar distance: I came up with two designs that had different lengths. This allowed me to see that matching the rear head tube with the front seat tube was a bad idea. I felt that would have brought the rider in rear too close to the rider in front making it uncomfortable. There was also a potential clearance problem with the front seat and the rear handlebars. The stem on the bike sets the handlebars forward of the head tube and the seat reaches out behind the seat tube. I didn't want to make the bike exceedingly long so I unbolted the stem and turn the handlebars around so that the stem faced backwards. This reduced the leg room for the rear rider, but provided clearance for the rear handlebars.
Front head tube angle & Rear seat tube angle: If you're not careful in your design, these might change drastically from their original positions and make riding/steering either really sloppy or too tight. If forced into one or the other, my intuition would say that it would be better to decrease the head tube angle and open up the steering rather than vice versa.
Since these bikes were cruisers they already had relatively low angles that would work great on a tandem so I designed the tandem to keep the angles for both virtually the same. To do this, I bought some L brackets and used two long bolts and some 2x6s to make two axle stands. This simulated the front bike standing at its natural position: both axles an equal distance off of the ground. This wasn't possible before because the bottom bracket actually sits slightly lower than the axles and without the axle stands the bike acts like a little seesaw.
For the rear bike, I just used two 2x4s to support the dropouts. This made the rear sit slightly lower and would result in slightly raising the bottom brackets once the wheels were put back on. In hind sight, because the tandem is so long compared to that difference, it didn't change the bottom bracket height by much at all. Please refer to the first picture for a better visual.
I decided to leave the fork and headset assembly on the rear bike intact and weld the forks to the seat stays of the front bike. Once I had the bikes on supports I was able to line them up and get a good feel for where I should cut the forks.
Stress points: The dynamic forces of two people riding a tandem creates an immense amount of stress on a frame. It's highly recommended to determine where the greatest stresses are to ensure your frame is strong enough to support these loads. One way to calculate a rough estimate of the forces on the frame is to draw a simplified free body diagram. Doing this would allow you to approximate the forces acting at each node (read: joint).
To really bring out your inner engineer you could use a CAD software package to model the frame. Creating a CAD model of your tandem frame would allow you to thoroughly analyze every section of the frame and make a confident determination of the frame’s strength. Unfortunately I don't currently have access to such software.
Additional support / bracing: One of my initial designs involved leaving the front frame intact and welding the cut ends of the rear fork to the dropouts of the front frame and the bottom bracket of the rear frame. However after comparing that design to my design utilizing a length of steel pipe spanning both bottom brackets it was clear that the latter would be much stronger and look better. I decided to use schedule 80 pipe for its strength.
Drive train: This consideration has its own step later on.
Chain tensioner: This also has its own step.
Step 6: Sandblasting & Future Paint
I really liked the original metallic gold color so I thought it would be cool to pay tribute to the bikes' history and match the original color, accents and logos. For future reference, I took several detailed pictures of the paint color and black accents on the forks and chain guards before they were sandblasted. I had a local shop blast my frames, forks, and chain guards.
To protect the headsets and the chrome crowns on the forks, I wrapped them in three layers of duck tape before bringing them in to get sandblasted. Had I not done this, their finish and appearance would have been ruined.
*Painting has it's own step with more details near the end.
Step 7: Fabrication - Frame Welding
We set the two bikes up in the jig and to be completely sure everything was square we took a long straight piece of scrap steel and clamped it to each bottom bracket. Doing this ensured that once the cranks and chains were on, they would be parallel and inline with each other.
Then we welded the rear forks to the front seat stays. In between was a thin steel piece that I cut to size to obtain more welding surface. The location of this union was almost exactly where the old rear brake mount was. Only after this first weld did we proceed to cut the rear end off of the front bike. If we would have cut the bike up before welding, it might have warped due to the excessive heat introduced by the welding.
Next we measured and cut a sturdy piece of schedule 80 pipe to span the bottom brackets. It took a fair amount of grinding and refitting to get it just right. Once we were happy with its fit, we welded it into place. My neighbor actually came up with the brilliant design to use the chain stays and their natural bends to join the seat stays and the bottom bracket support. The two chain stays were welded together resembling a "Y" and were large enough to slide right over the seat stays which added to the strength and allowed us to adjust the height before we welded it to the bottom bracket span.
Once we finished welding the frames together, I ground down the welds and used some metal filler paste to smooth it all out. Reminder: it's a good idea to ensure everything is square before each weld to avoid a misalignment.
Step 8: Fabrication - Drive Train
I decided to build a same-side drive train. I thought that this setup would be easier since I already had the two identical cranks original to the bikes and a spare sprocket ring. Whatever type of drive train you decide to build, it is key to have two sprockets of the same size. Without them, the two riders will be pedaling out of sync.
When the bikes were manufactured, the front sprocket was designed to line up with the freewheel in back. Now with a same-side drive on a tandem, we are using the front sprocket for the timing chain and a second sprocket to drive the freewheel. This creates an issue of misalignment. Action must be taken to keep the chains from rubbing or binding up.
To add a sprocket ring to the one-piece cranks I measured out the minimum amount of space that I thought I would need for the two chains to clear. I tried using nuts and bolts to join them, but that made it too bulky. My neighbor and I found some thin steel pieces that would space it out right so we welded the sprockets and spacers together. Then I went at the welds with a dremel grinder to clean them up.
The spacing between the two sprockets was not quite enough and now the two chains rub a bit where they cross paths. It's not enough to cause them to bind though. I settled for tearing apart the rear wheel (with an internal three speed) to try to move it over on the axle. That turned out to be a huge headache and way more than I bargained for. In retrospect, I should have allowed for more space between the sprockets. Another idea is to inspect a tandem at a local bike shop and measure the distances between its sprockets, provided it has a same-side drive.
Something else that you will want to consider is adding some spacers to the sprocket side of the cranks. I added one large washer to push the sprockets away from the frame and might need to add one more to prevent the chains from rubbing. Doing this will help to align the drive-chain.
You will probably find that the general consensus for either drive train calls for a chain tensioner on the timing chain (between the front and rear identical sprockets). I was a bit skeptical of this at first and was hoping to rely on just having the correct number of links to keep it tight. Now that my tandem is complete I can tell you that a chain tensioner is not absolutely necessary, but it is a very good idea to have one. In my anxiousness to complete my tandem and actually ride it before the end of summer, I opted to go without for now and add a tensioner later.
Step 9: Fabrication - Chain guard
With the guards installed, I was able to take some measurements and made a rough plan for joining them using a piece of thin sheet steel. First, I determined the required width of the piece (including the bend radius) and cut it. Using a 2x4 as a radius guide, I bent the filler piece at the location I marked to coincide with the original guards. Then I used a 4 inch angle grinder to cut a slot in the rear guard to allow the filler piece to slide in behind it. As it turned out, I also needed to cut a slot in the filler piece to fit around the custom “Y” section described in Step 7.
Since the three guard pieces were to be welded together, I allowed more than an inch of overlap at each end of the filler piece. The filler piece was designed to slide in behind the two original guards so that the welds joining them would be hidden from view.
The guards had embossed accents on them so I tried to extend the accent from the front guard to the filler piece, using two layered strips of sheet metal. I drilled offset holes in the bottom two layers allowing them all to be welded together. A common metal filler paste was used to smooth out the transitions. There is some minor separation between the layers of sheet metal and I’m guessing that it’s due to the heat from welding.
This step proved to be quite difficult and the faults are revealed upon close inspection but I think it looks okay from a distance. If I were to try this again, I think I would weld the edges of the accent piece together and grind it down to the desired shape and width. I would also find a thinner piece of steel to make fabrication easier.
Step 10: Painting
There’s a lot of prep work that should be done before you ever pick up a spray can. First you need a well ventilated location. A good system to hang your bike is also very beneficial. You want to hang your bike in a way that will allow you to paint every surface without ever touching it. My rig consisted of a rope slung over the roof truss, tied to a 2x4 with two holes drilled in the middle on each end. I took some strong wire and fished it through the front head tube, out the bottom of the forks and bent the wire so it wouldn’t slide out. I did the same thing inside the rear seat tube and bent the wire inside the bottom bracket. Then I took the loose ends of the wire, strung them through the holes in the 2x4 and wrapped it around twice to make sure it wouldn’t slide out.
The next task is to thoroughly clean the bike. I used some fine grit (wet/dry) sandpaper to clean off the rust that was starting to form after sandblasting. Then I sprayed the whole thing down with an air compressor to get the dust and debris off. Following that is the taping of sections not to be painted. I taped off the bottom brackets, the headsets and fork crowns with blue painter’s tape.
After all that was done, I sprayed on one coat of primer and let it dry per the directions. Then of course, the primary color. I gave it two coats, but I applied them in very thin, smooth layers. There should be no paint pooling or dripping. After the primary color was on and dry, I started prepping for the decals and accents. *Some people insist on a light sanding in-between coats with the belief that it helps the new coat adhere to the last. I don’t disagree, I just didn’t do that on this project.
To create the decals I used the pictures I had taken before the parts were sandblasted, used Photoshop to sharpen them up and print them out on paper. It took me a few tries to get the sizing just right, but once that was done I began cutting the paper images out with a utility knife. Then I laid blue painter’s tape lightly on a piece of plywood and used the cutouts to trace the designs onto the tape to create stencils. Once the image was on the tape, I cut them out with a utility knife, put the stencils in their respective places on the tandem and then firmly pressed them down. That was followed by taping off the rest of the immediate area to protect from over spray. During this step, having a steady hand and a lot of patients is crucial.
The next step was to paint the decals and I did this with the same technique as I did with the prior coats. Once the black decal paint was dry, I uncovered everything and very carefully peeled off the blue painter’s tape that was used for the stencils. As the tape came off, some of the black paint came off with it. In the end, all the decals had at least some paint peel off. I think the problem was that I applied the black paint a little too heavy and it adhered to the tape. My decals were fairly detailed and had several sharp points which seemed to pull off the most paint. I think if you apply a few extremely light coats and let them dry completely in-between applications, decals will work. However, the problem could also be due to the type of tape that I used.
Finally, it came time for the clear coat and I applied 3 solid coats and as always, I abided by the direction’s dry times. That concluded the painting portion of the project. The next step was REASSEMBLY!!!
Step 11: Reassembly & Summary
I took it for a few rides and it rode well! I’ve actually never ridden another tandem so I don’t have any baseline to compare to. I learned a great deal from this project and am extremely proud of the end result. I’ll be riding and enjoying this bike for many years to come. I hope this Instructable motivates and aids you in a tandem build of your own.
Don’t forget to throw an unveiling party for your creation!