Originally Posted by walshclimb
I have a 2001 mountain bike that I am not using. I have considered selling it, but I am interested in buying or building a tandem. Which parts could I reasonable use from the mountain bike. I figure shifters, derailleurs, seat, pedals, and handlebars will work. What about brakes, wheels, cranks, cogs, etc? Has anyone done this? Any suggested references? Any idea where I could get a reasonably priced frame?
A lot of this would depend on your team weight and how you intend to use the tandem. The heavier your team and/or the more aggressive you'll be riding the more robust your critical components will need to be.
As you mentioned, most of the drive train is re-useable. For cranks you can either shop for a set of used tandem cross-over cranks, go for some ever increasingly hard to find inexpensive off-road tandem cross over cranks, or find two other used cranksets that would cost less than a new or used tandem cross-over crankset that you can cobble together to create your own cross-over crankset (you need three drive-side cranks and one right side -- Helicoils get installed into the three cranks that end up being installed backwards).
As for the wheelset, 32 spoked 26" wheels with box section rims should be OK for light to moderate terrain for a light to average weight team. If you're a heavier team or intend to do technical single track or fast descents on fire roads you'll need at least 36h wheels, preferably with a deep or clydesdale type rim.
V-brakes are standard fare on most all terrain and entry level mountain bikes. However, as it is with single bikes, there are some advantages to having front and/or front & rear discs.
You'll obviously need a second seatpost and saddle for the stoker as well as a stem for the stoker and their handlebars. You can opt for an adjustable, tandem specific telescoping stem or you can try to find a threadless stem with the right length and rise to work as a rigid stoker stem.
On the frame, MTBTandems.com sells a very adaptable, off-road specific frame called the Fandango. It's a bit more aggressive than the Cannondale MT frameset (higher BB height and head tube angle optimized for suspension fork). There are obviously others out there as well offered by Etailers and of course the second hand market is also a good place to look for 26" 'enduro' frames.