I have found what i believe to be a great deal on a 2003 Co-Mo Speedster Tandem. The dimensions are 21"/18". It comes equipped with stock parts for a Co-Mo as per the Co-Mo website. I normally ride a 58cm (23") frame with an inseam length of 86cm (34"). My wife rides a 49cm single (19"). This size i am sure will be ok for my wife .. but i am just wondering if it is too small for me. Could i get "away" with a seat tube this small?? The tandem is brand new and is being sold for US$2800.00 - which i think is a great deal.
Thank you all in anticipation.
A little small, but with a long seatpost and stem, you can make almost anything fit. Santana sells a 150 mm stem but it fits a 1 & 1/4 steering tube. Not sure what the steering tube is on a Co Mo. Since you don't stand as much or stand and flail a tandem like a single, a long stem is not as much af an issue as with a single.
Hopefully, the steering tube has not been cut.
I'm not very worried about the seat tube length. When you can get a 430mm seatpost with 8" of adjustment range, lots of seat tube lengths are usable. I am a little concerned about the top tube length and height. If the top tube is shorter and the end of the steerer tube is lower, your choice of stems will be limited to the steep, long ones if anything will work. You will need to match the horizontal distance between the center of the captain seatpost and the center of the handlebars, and match the height of the handlebars above the bottom bracket, best measured by getting the heights of both off the ground plane and subtracting. Same thing with the seat tube: match the distances from the center of the bottom bracket spindle, parallel to the seat tube, to the top of the saddle.
One other thing: relative to the length of the wheelbase, you can't shift your weight fore/aft on a tandem the way you can on a single bike. Putting a big captain on an undersize frame moves the center of gravity forward, which loads the front tire more, something that may matter when cornering at the limit. This is a rather minimal concern.