I thank you for your advices about Avid BB7 on Cannondale road tandem.
I have another question. I am 5'11" and my wife is 5'7". My race bike is 56cm c/c , 76cm from the center of bottom braket to the superior level of the saddle and 56cm from the front of the saddle to the center of the handlebar ( with a stem of 12cm in front). The race bike of my wife is 52 cm c/c , 71 cm , 52cm ( with a stem of 12cm in front).
I dont know the size of my old tandem , but it was a little short for me and short for my wife ( not so much rear room ).
I have ordered a new X/M . Sure it is a little long for me ( 60.5 cm top tube lenght ) and i must use a straight tube saddle and a shorter stem but there is more space for my wife.
Maybe it should be better L/M but in Italy this size is not imported.
What do you think about ?
One thing to look out for on your new tandem is that the captain's bottom bracket can move relative to the seat tube as it's mounted in the eccentric. Thus a slightly long captain top tube compared to a road bike can be OK.
For example, my T2000 has a 1cm longer top tube than my road bike, but this is compensated for by the forward position of the eccentric. So that my riding position is the same as my road bike I need to position the saddle about 1cm further forward than on the road bike. This also gives a bit more room for the stoker than the geometry table suggests.
So assuming your road bike fits well, a difference of 2-3cm in top tube length can be accommodated without much bother. Of course the fit may not be totally exact, but should be close enough for most people to ride the bike quite happily. If you are not part of the majority, or you find you can't get the fit to work, you will need to talk to your favourite custom builder. Rather than jumping into the custom build straight off, riding an off the shelf tandem will help you get to know what you do and don't like.