Rowan

For mine, building the bike out of CF is not an issue, especially based on the process intended.

However, everyone seems to have overlooked the most important part of all -- the frame dimensions and angles. Just what design is the frame to be based on? Has the OP got himself some frame-design software, and has he banged in the numbers he is envisaging?

The lugged method is relatively outdated in terms of production CF bikes these days, but I do know that it can produce a very stiff frame, to the point of being tough on the butt to ride. The sample point for me is my Merlin C110 Works. It's not just my opinion; others have expressed it, too.

What head-tube angle will there be? Fork trail and/or rake? Dimensions of the head tube (conical, 1-1/8th) What configuration of diagonal tubing will there be? How long will the chainstays be? What curves are there to be in the seat stays? Is there a compensation to be made in any of these things because CF is being used? Can an alloy or steel frame design be applied to CF construction?

And as someone has alluded to already, be prepared to make a major mistake. The further the build goes, the more difficult it becomes to make good any mistakes, particularly in terms of geometry.

Because there are so many tubes being dealt with, there is a critical need, in my opinion, to have the lugs made to very accurate tolerances for (a) longevity of the frame and (b) so that the opposite ends of the tubes end up where they should be without having to stress them back into position (again, as someone said, you can't coldset a CF frame, even a couple of millimetres like you can a steel one).

Likewise fabricating the couplers. Plus, in my estimation, every couple introduces a potential failure point on the frame. Hence S&S' reluctance to allow anyone except fully authorised fitters to add them to frames. The couplers have to be done exactly right. On a Co-Motion tandem we saw last week, there were four to enable the bike to be broken down adequately for travel. Is that the number to be incorporated into this design? And where are they to be placed?

Expense and time will be a factor. Carbon cloth is quite expensive, much more than I thought. Epoxy resin, from my understanding, needs to be carefully chosen, with additives at least to avoid deterioration from sunlight. The couplers would, I expect, be as expensive as the rest of the bike.

Has the OP been to BikeFriday and tested ridden their steel tandem? We were there the day before yesterday, and a white tandem was sitting on the demo floor. We didn't ride it, but I know others who own BFs, and they love them. They aren't exactly the prettiest tandems in the world, but can break down pretty small, I think, without the need for fabricating couplings from scratch.

Now, having said all that, one of my ambitions is to build a CF high-wheeler recumbent. There are several sites dealing with this, and it's not all that difficult because there really is only one significant tube to deal with. Bonding the chain and seat stays seems to be the only significant issue, apart from alignment. But it's a project I am giving long and deep thought to before committing myself...

(Also note that the OP probably has thought of all these things already, but this is not evident in the postings so far. I am really interested in the progress and outcomes, too...)