David's Homebuilt Tandem
(click on images for full-size)
Here's a front/side view. Yep, this is a HOMEbuilt tandem. 
A few more pics:
The Story
A friend of mine (Chris Woodard) from college (California State University, Chico) was an Industrial Technology Master's Degree candidate (he has since graduated and is employed at NUMI in Fremont). So, he had access to all kinds of cool welding supplies, jigs, tools, etc. He made himself a tandem using warranty-returned bikes that he bought from local bike shops (broken frames mostly). He'd use the intact front triangle from one bike and the intact rear triangle from another and throw some cromoly in between, and viola! Instant tandem frame. The bike he had looked nice and the price was cheap, so I had him put a frame together for me.
My Bike
Frame:
The front triangle of my bike is from a Giant. 
The rear triangle is from a Nishiki (hard to tell, but notice
the cool unistay seat stay).
This is about as cool as a homebuilt gets (in my opinion). My
bike is not, however, a super light-weight racing bike. I had
Chris use 1 1/2 inch 4130 cromoly. Super strong, cool oversize
look, and a tad heavy. 
The middle box
section as well as the entire top tube are the inch-and-a-half
4130.
Building Up:
Most of the parts came from a '97 Specialized Rock Hopper. I
thought about buying all the parts separately, but the Rock
Hopper was cheaper as a complete bike! 
Keep in mind that
the first time I built the bike up, it wasn't painted. I built it
up to make sure it worked before I spent the money on the
excellent powder-coating you see here. After I got it working, I
took the whole thing apart, got it coated, and built it up again.
The paint is a silver vein powder-coat with a clear coat over.
These pictures do not do the paint job justice. It's gorgeous.
As you can imagine, some problems arose. First off, one parts
bike does not provide enough parts for a complete tandem. Also,
the fork size on the Rock Hopper is 1 1/8 inch and the tandem is
1 inch. I had a spare Specialized 1 inch threaded stem, so all I
had to do was find a 1 inch fork. No easy matter. I finally found
a nice cromoly fork complete with taps for a low-rider rack.
O.K. I got the front end built, so I put on the grip-shifts
that came with the Rock Hopper, and the Dia Compe linear-pull
brakes and levers (works GREAT). I even picked up a cheap pair of
Specialized bar ends to match and a Cateye Halogen copy light. 
Next came the drive train. This was the toughest part of the bike. Problems included timing chain tension (including crank-set matching), chain-ring installation, pedal threading, and cable routing.
Front Drive Train (Timing):
When I first set up the bike, I used spare cranks that I had
lying around. Two problems: the look was ugly since the cranks
didn't match (and some were quite beat up) and I needed three
right-side pedals and one left pedal. Well, I had plenty of
pedals so that wasn't a problem. For the captain, I removed the
cages on the pedals and swapped 'em around so I could get toe
clips on (pic a little fuzzy). 
I went to a bike swap in San Rafael, California and found some
matching crank arms - brand new - for five bucks a piece. They
were so cheap because the guy selling them only had the drive
side (which is all I needed! What luck!). I bought a Nashbar
cartridge type bottom bracket for the captain and slapped the
cranks on. 
Next problem was chain tension. Again, I got lucky. I had some
spare 36-tooth rings (that matched!) and used those for the
timing chain. I bought an inexpensive silver bmx chain (so it
matched the silver Hyperglide chain) and put it on. I used a
master link and removed half a link to make the tension perfect. 
Should I need a tad more tension, I expect to use the ghost chainring method (learned from the Tandem Page by Sheldon Brown - excellent site with LOTS of bicycle info).
Rear Drive Train (and cable routing):
Most of the rear drive train was simple since the rear
triangle was attached intact. The biggest problem
with the rear drive train was cable routing. The extra tubes on
the bottom of the bike made bottom routing a bit tricky. Routing
down the front down-tube, I wanted to use a bottom-bracket
routing guide, but the bottom bracket on the Giant wasn't tapped!
Bummer. I didn't want to drill the bottom-bracket, so I just put
the guide in there and didn't attach it to the bike. It floats
and works great! 
I like to run the shift cables around the head tube, so I have
to cross the cable at some point before they get to the
derailers. On single bikes, I do it at the down tube. That wasn't
gonna work on the tandem, so I crossed the cables under the
bottom tube beneath the stoker.
Again, the routing under the stoker bottom-bracket
was a bit tricky. The extra tubes didn't allow for a standard
cable guide, so I had to use some housing. A little piece of
metal and a bolt made the routing complete (pic is fuzzy). 
Stoker:
Setting up the stoker stem/bars was easy. I just picked up a 1
inch threadless stem and put it on the captain seat post. I may
pick up an adjustable tandem stem some day, but this setup works
fine for now.
If you have any questions or comments, feel free to email me at dbenj@home.com.
