Originally Posted by
NOBLNG
No, the seat is not at the correct height in the pic. She has since assembled the bike, ridden it, and mentioned that the stand-over height was the biggest issue. If lowering the top tube is all that is needed it should not be difficult. Regardless, if I decide to do it, I want to do it properly. She has also given me the builder's contact info, so I will try to find out what type of tube was used and get his input.
Your bike does derive some strength from having the top tube meet the seatstays in the same place, although perhaps it wouldn't make a big difference. Some bikes are designed with the seatstays hitting the seat tube much lower than the top tube.
That seat clamp looks unique. Does it have some kind of internal wedge to catch the seat post? You still should be able to safely cut the mast down to the top of the clamp without hurting anything, giving you another 3/4" or so of seat post adjustment.
Anyway, if the seat can be positioned properly, then I would encourage the owner to get some more experience riding the bike before making any changes, then just chop the top tube if needed.
There are a lot of options. For example, you could replace the top tube with a new tube with a curve to lower the tube in the middle, but still attach it in the same place on both ends.
The "Mixte" style is to have two tubes that extend all the way from the head tube to the rear dropouts to give some extra strength with a lower top tube.
And, many women's bikes have a low top tube, although I'd worry about extra flex being introduced into the tandem. But you could also reinforce it.