Originally Posted by TheBrick
A custum frame would be expensive but you would beable to save money on the fact that you would not need tandem cranks. I don't know how much more expensive tandem cranks are as I am only just learning about tandems but I would guess quite a bit as it is a specialised part. Plus a single side drive puts less stress on the bb accroding to Mr Brown. It make sense when you consider the force and torque involved.
Good point to save money by using non-custom cranks. Yes they are expensive. You can do this on any frame - I guess what you meant was versus buying a full bike.
To my mind the main options are:
1) Cross over cranks but with swapped (and loctited) pedal spindles
+ Looks and works like a cross over drive
+ Any cranks possible, apart from those 2 piece ones with integrated b/b
- Loctite on pedals - may work loose
- Need 3 sets of pedal axles (swap pilot's axles plus extra RHS for stoker's LHS pedal)
2) Cross over cranks with helicoiled crank threads
+ Looks and works like a cross over drive
+ Helicoil strong if done by a professional (eggrings)
+ Any cranks possible, apart from those 2 piece ones with integrated b/b
- Helicoil is permanent and means your cranks won't work on a single again
3) Single sided gearing with third chainring used for connecting drive
+ Elegant solution and more efficient
+ No mucking about with swapping threads etc. and only 2 cranksets needed so cheaper
- Requires different chainrings
- No 3rd ring to get over hills
If I were you I would try option 3, then if you need the extra gears you can buy another set of cranks and try either options 1 or 2. If you're buying 3-piece Mid level campag / 105 / Ultegra level cranks (a few years old) you can get these for peanuts on ebay as everyone nowadays is 'upgrading' to compact / carbon or to get the Dura Ace / Record name (I am as guilty as anyone here!).