It is common thought that the rider's stance should be the same regardless to the frame's geometry. Or, another wards, the frame's angles shouldn't effect how the rider is positioned (seat post being set back or straight to get seat set back over the shell the same for differing seat tube angles). But as the frame tubes become more horizontal (blades included) the vertical forces are more in bending then compression. Also in the past slack angles usually were combined with longer wheelbases. And the longer the wheelbase is the further out the lever the bump starts. This is one reason why tandem stokers have seat issues,. In the attempt to keep the tandem handling quick designers try to keep the wheelbase as short as possible. And since few tandem designers ride stoker (yes, I know about Mr Santana) they don't consider the stoker with as much concern as the y do the captain and his steering the bike. Andy.